Saturday, May 30, 2026

The Law of Inertia Class 11 Physics Notes, MCQs, Questions & Answers

3. THE LAW OF INERTIA (NEET Notes – Easy Line-by-Line Explanation)

1. Galileo's Study of Motion on an Inclined Plane

Original: Galileo studied motion of objects on an inclined plane.

Easy Note: Galileo performed experiments using sloping surfaces (inclined planes) to understand how objects move.

Original: Objects moving down an inclined plane accelerate.

Easy Note: When an object rolls down a slope, its speed increases continuously. This increase in speed is called acceleration.

Original: Objects moving up retard.

Easy Note: When an object moves upward on a slope, its speed decreases gradually. This decrease in speed is called retardation (deceleration).

Original: Motion on a horizontal plane is an intermediate situation.

Easy Note: A flat (horizontal) surface is between the above two situations because the object neither gains nor loses speed due to the slope.

Original: An object moving on a frictionless horizontal plane should move with constant velocity.

Easy Note: If there is no friction, an object on a horizontal surface will keep moving forever with the same speed and direction (constant velocity).

NEET Point

Frictionless surface → No acceleration
Velocity remains constant

2. Galileo's Double Inclined Plane Experiment

Original: A ball released from rest on one plane rolls down and climbs up the other.

Easy Note: Galileo placed two slopes facing each other. A ball released from one side rolled down and then moved up the opposite slope.

Original: If the planes are smooth, the final height is nearly the same as the initial height.

Easy Note: When friction is very small, the ball reaches almost the same height from which it was released.

Original: In the ideal situation, when friction is absent, the final height equals the initial height.

Easy Note: Without friction, the ball would rise exactly to its original height.

NEET Point

No friction → Initial height = Final height

3. Effect of Reducing the Slope

Original: If the slope of the second plane is decreased, the ball still reaches the same height.

Easy Note: Making the second slope less steep does not change the height reached by the ball.

Original: The ball travels a longer distance to reach that height.

Easy Note: A gentler slope means the ball must travel farther to reach the same height.

Original: When the second plane becomes horizontal, the ball travels an infinite distance.

Easy Note: If the second plane is completely flat and frictionless, the ball will never stop moving.

NEET Point

Horizontal surface + No friction → Motion continues forever.

4. Real Situation

Original: In practice, the ball comes to a stop because of friction.

Easy Note: In real life, friction opposes motion and gradually stops the ball.

Original: Friction can never be totally eliminated.

Easy Note: Some friction is always present in practical situations.

Original: Without friction, the ball would continue with constant velocity.

Easy Note: If friction were absent, the ball would move forever with the same speed and direction.

5. Galileo's Important Conclusion

Original: State of rest and state of uniform linear motion are equivalent.

Easy Note: A body at rest and a body moving with constant velocity are both in similar conditions because neither experiences a net force.

NEET Point

Both situations have:

Net Force = 0

Body at rest → remains at rest
Body moving uniformly → continues moving uniformly

6. Common Misconception Corrected

Original: It is incorrect to assume that a net force is needed to keep a body in uniform motion.

Easy Note: A force is not required to keep an object moving with constant velocity.

Important NEET Fact

Force is needed only to:

Start motion
Stop motion
Change speed
Change direction

7. Why Do We Apply Force in Daily Life?

Original: We apply force to counter friction.

Easy Note: In daily life, friction slows objects down. Therefore, we apply force to balance friction and maintain constant speed.

Example:

A cyclist keeps pedaling to overcome friction and air resistance.

8. Inertia

Original: This property of the body is called inertia.

Easy Note: The tendency of a body to resist any change in its current state is called inertia.

Definition

Inertia = Resistance to change in the state of rest or uniform motion.

9. Law of Inertia (First Law of Motion)

Statement

A body continues to remain at rest or continues to move with uniform velocity in a straight line unless acted upon by an external unbalanced force.

NEET Shortcut

No Net Force ⇒ No Change in Motion

10. Key Points for NEET Revision

🔹 Galileo proposed the concept of inertia.

🔹 Friction is the force that stops moving objects in real life.

🔹 In the absence of friction, an object continues moving forever with constant velocity.

🔹 Rest and uniform motion are equivalent states.

🔹 Inertia means resistance to change.

🔹 Greater mass ⇒ Greater inertia.

🔹 Net external force = 0 ⇒ Velocity remains constant.

One-Line NEET Summary

Law of Inertia: A body remains at rest or continues to move with constant velocity unless an external unbalanced force acts on it. ✔️

Educational diagram showing Galileo's double inclined plane experiment explaining the law of inertia, constant velocity, friction, and Newton's First Law for Class 11 Physics students.

Galileo's double inclined plane experiment demonstrating the Law of Inertia and uniform motion.

CBSE Class 11 Physics – The Law of Inertia

Question Bank with Answers

A. MCQs (1 Mark Each)

1. Who first inferred the law of inertia?

a) Newton b) Aristotle c) Galileo d) Einstein

Answer: c) Galileo

2. In the absence of friction, a moving body on a horizontal surface will:

a) Stop immediately b) Accelerate c) Move with constant velocity d) Move in a circle

Answer: c) Move with constant velocity

3. Inertia is the property of a body to:

a) Change its state b) Resist change in its state c) Increase velocity d) Decrease velocity

Answer: b) Resist change in its state

4. The SI unit of inertia is:

a) Newton b) Joule c) No unit d) kg

Answer: c) No unit

5. Greater the mass of a body:

a) Smaller the inertia b) Greater the inertia c) No inertia d) Constant inertia

Answer: b) Greater the inertia

6. A body remains at rest or in uniform motion when:

a) Net force is maximum b) Friction is present c) Net external force is zero d) Acceleration is maximum

Answer: c) Net external force is zero

7. Which force opposes motion?

a) Gravitational force b) Frictional force c) Magnetic force d) Electrostatic force

Answer: b) Frictional force

8. Uniform motion means:

a) Constant speed in a straight line b) Changing speed c) Circular motion d) Accelerated motion

Answer: a) Constant speed in a straight line

B. Very Short Answer Questions (1 Mark)

1. What is inertia?

Answer: Inertia is the property of a body to resist any change in its state of rest or uniform motion.

2. Who proposed the concept of inertia?

Answer: Galileo.

3. Which quantity measures inertia?

Answer: Mass.

4. State Newton's First Law.

Answer: A body remains at rest or in uniform motion unless acted upon by an external unbalanced force.

5. What is meant by net force?

Answer: The vector sum of all forces acting on a body.

C. Short Answer Questions (2–3 Marks)

1. Why does a moving ball stop after some time on a horizontal surface?

Answer: The ball stops because friction acts opposite to its motion. Friction gradually reduces its speed until it comes to rest.

2. Why is force not required to maintain uniform motion?

Answer: According to the law of inertia, a body moving with constant velocity continues moving unless an external force acts on it. Therefore, no force is required to maintain uniform motion.

3. Define inertia of rest with an example.

Answer: Inertia of rest is the tendency of a body to remain at rest.

Example: Passengers fall backward when a bus starts suddenly.

4. Define inertia of motion with an example.

Answer: Inertia of motion is the tendency of a moving body to continue moving.

Example: Passengers fall forward when a moving bus stops suddenly.

D. Long Answer Questions (4–5 Marks)

1. Explain Galileo's double inclined plane experiment.

Answer:

Galileo used two inclined planes facing each other.
A ball released from one plane rolled down and climbed the opposite plane.
The ball reached nearly the same height from which it was released.
As the slope of the second plane was reduced, the ball travelled a longer distance to reach the same height.
When the second plane became horizontal, the ball would continue moving indefinitely in the absence of friction.
Galileo concluded that a body continues in its state of motion if no external force acts on it.

2. Explain the law of inertia.

Answer:

The law of inertia states that a body remains at rest or continues to move with uniform velocity in a straight line unless acted upon by an external unbalanced force.

Key points:

Inertia means resistance to change.
Rest and uniform motion are equivalent states.
No net force is needed to maintain motion.
Mass is the measure of inertia.

E. Assertion and Reason Questions

1.

Assertion (A): A force is necessary to keep a body moving with constant velocity.

Reason (R): Uniform motion is possible only when net external force is zero.

Answer: Assertion is False, Reason is True.

2.

Assertion (A): A body at rest remains at rest if no external force acts on it.

Reason (R): This property is called inertia.

Answer: Both A and R are True and R is the correct explanation.

3.

Assertion (A): Greater mass means greater inertia.

Reason (R): Mass measures the resistance of a body to change in motion.

Answer: Both A and R are True and R is the correct explanation.

4.

Assertion (A): Friction helps a moving body continue forever.

Reason (R): Friction opposes motion.

Answer: Assertion is False, Reason is True.

F. Fill in the Blanks

The law of inertia was first inferred by Galileo.
Inertia means resistance to change.
A body moving with constant velocity has zero acceleration.
Friction acts opposite to motion.
Mass is a measure of inertia.
In the absence of friction, a body moves with constant velocity.
Rest and uniform motion are equivalent states.
The net force on a body in uniform motion is zero.

G. True / False

Galileo studied motion using inclined planes. True
Friction increases the speed of a body. False
Inertia depends on mass. True
A body in motion always requires force to keep moving. False
Uniform motion means constant velocity. True

H. Case Study Questions

Case Study 1

Galileo released a ball from one inclined plane. The ball rolled down and climbed another inclined plane. When the second plane's slope was reduced, the ball travelled a longer distance but still reached the same height.

Questions

1. Who performed this experiment?

Answer: Galileo

2. What conclusion did Galileo draw?

Answer: A body continues in motion if no external force acts on it.

3. Which force prevents infinite motion in real life?

Answer: Friction

4. What happens in the absence of friction?

Answer: The body continues moving with constant velocity.

Case Study 2

A cyclist stops pedaling but the bicycle continues moving for some distance before stopping.

Questions

1. Which property keeps the bicycle moving?

Answer: Inertia of motion

2. Why does the bicycle eventually stop?

Answer: Due to friction and air resistance.

3. What would happen if friction were absent?

Answer: The bicycle would continue moving with constant velocity.

4. Which law explains this behavior?

Answer: Law of Inertia (Newton's First Law).

I. Statement-Based Questions

1. Consider the following statements:

I. Inertia is the resistance to change in state.

II. Mass is a measure of inertia.

III. Friction helps maintain motion.

Choose the correct option:

a) I only b) I and II only c) II and III only d) I, II and III

Answer: b) I and II only

2. Consider the statements:

I. Uniform motion requires zero net force.

II. Rest and uniform motion are equivalent states.

III. Galileo inferred the law of inertia.

Choose the correct answer:

a) I only b) II only c) I and III only d) I, II and III

Answer: d) I, II and III

J. Match the Columns

Column A	Column B
1. Galileo	a. Resistance to change
2. Inertia	b. Opposes motion
3. Friction	c. Double inclined plane
4. Mass	d. Measure of inertia

Answer

1 → c

2 → a

3 → b

4 → d

K. Competency-Based Questions

1. Why do passengers fall backward when a bus starts suddenly?

Answer: Due to inertia of rest, the lower part of the body moves with the bus while the upper part tends to remain at rest.

2. Why do passengers fall forward when a moving bus stops suddenly?

Answer: Due to inertia of motion, the upper body continues moving forward even after the bus stops.

One-Mark CBSE Revision Questions

Define inertia.
Name the scientist who inferred the law of inertia.
What is the measure of inertia?
State Newton's First Law.
What is uniform motion?
What is meant by net force?
Name the force opposing motion.
Can a body move without force? Explain briefly.

Answers: Inertia, Galileo, Mass, Newton's First Law, Constant velocity, Resultant force, Friction, Yes—if net external force is zero.

LAW OF INERTIA

│

├── Galileo's Observations

│ │

│ ├── Inclined Plane

│ │ ├── Moving Down → Acceleration

│ │ ├── Moving Up → Retardation

│ │ └── Horizontal Surface → Constant Velocity

│ │

│ └── Conclusion

│ └── Frictionless Horizontal Surface

│ └── Motion Continues Forever

│

├── Double Inclined Plane Experiment

│ │

│ ├── Ball Released from One Side

│ ├── Rolls Down First Plane

│ ├── Climbs Second Plane

│ │

│ ├── Smooth Planes

│ │ └── Final Height ≈ Initial Height

│ │

│ ├── No Friction (Ideal Case)

│ │ └── Final Height = Initial Height

│ │

│ └── Slope of Second Plane Reduced

│ ├── Same Height Reached

│ ├── Longer Distance Travelled

│ └── Horizontal Plane

│ └── Infinite Motion

│

├── Role of Friction

│ │

│ ├── Opposes Motion

│ ├── Causes Ball to Stop

│ └── Cannot Be Completely Eliminated

│

├── Galileo's Insight

│ │

│ ├── State of Rest

│ ├── State of Uniform Motion

│ └── Both Are Equivalent

│ └── Net Force = 0

│

├── Force and Motion

│ │

│ ├── Force Not Needed

│ │ └── To Maintain Uniform Motion

│ │

│ └── Force Needed

│ ├── Change Speed

│ ├── Change Direction

│ ├── Start Motion

│ └── Stop Motion

│

├── Inertia

│ │

│ ├── Meaning

│ │ └── Resistance to Change

│ │

│ ├── Rest Inertia

│ │ └── Resists Change from Rest

│ │

│ └── Motion Inertia

│ └── Resists Change in Motion

│

└── Law of Inertia (Newton's First Law)

│

├── Net External Force = 0

│ ├── Body at Rest → Remains at Rest

│ └── Body in Motion → Moves with Constant Velocity

│

└── External Unbalanced Force

└── Changes State of Motion

NEET KEYWORDS:

Inertia • Friction • Constant Velocity • Net Force = 0

Rest State • Uniform Motion • Galileo • Newton's First Law

Physics Constants Made Easy: Mnemonics, Tricks, Mind Maps & Memory Hacks

Learn Physics Constants Fast with Mnemonics and Visual Memory Techniques

- Dr.Sanjaykumar Pawar

🚀 Ultra-Memory Guide for Physics Constants

Instead of memorizing 20+ random numbers, group them into families, patterns, stories, and visual maps.

1. MASTER MEMORY TABLE

Constant	Value	Memory Trick
Electron Mass	9.11 × 10⁻³¹ kg	Electron = smallest → 9-11 emergency tiny particle
Proton Mass	1.67 × 10⁻²⁷ kg	Proton ≈ 1836 times heavier than electron
Neutron Mass	1.675 × 10⁻²⁷ kg	Neutron ≈ Proton
Electron Charge	1.6 × 10⁻¹⁹ C	Most important physics number
Speed of Light	3 × 10⁸ m/s	"3 followed by 8 zeros"
Planck Constant	6.626 × 10⁻³⁴	Triple 6 pattern
Gravitation Constant G	6.67 × 10⁻¹¹	G = 667
Avogadro Number	6.022 × 10²³	Mole's phone number
Boltzmann Constant	1.38 × 10⁻²³	Remember 138
Stefan Constant	5.67 × 10⁻⁸	Similar to Wien family
Wien Constant	2.898 × 10⁻³	Near 2.9
Solar Constant	1.366 × 10³	Sun sends 1366 W/m²
Sun Mass	1.989 × 10³⁰	Almost 2 × 10³⁰
Earth Mass	5.97 × 10²⁴	Almost 6 × 10²⁴
Earth Radius	6.4 × 10⁶ m	Easy approximation
Earth Density	5.5 × 10³ kg/m³	5.5
Earth Angular Velocity	7.29 × 10⁻⁵	Daily rotation
Earth-Moon Distance	3.84 × 10⁸ m	Nearly light-second
Earth Magnetic Moment	8.22 × 10²²	822
AU	1.496 × 10¹¹ m	Sun-Earth distance

Physics Constants Memory Map with Mnemonics, Tables and Visual Learning Techniques for Fast Exam Revision.

2. NUMBER FAMILY TRICK

Most physics constants belong to only a few number families:

Family A: 1.6

Think:

⚡ Charge of Electron

⚡ 1 eV

Both use:

1.6 × 10⁻¹⁹

Electron Charge
      ↓
    1.6
      ↓
    1 eV

Family B: 6.6

Think:

📌 Planck Constant

📌 Gravitational Constant

Planck → 6.626
Gravity → 6.67

Both start with 6.6

Family C: 6.0

Think:

🧪 Avogadro Number

6.022 × 10²³

Scientists love 6!

3. SUBATOMIC PARTICLE STORY

Imagine a family:

👶 Electron (tiny baby)

👨 Proton (big father)

👨‍🦳 Neutron (father's twin brother)

Electron = 9.11 ×10⁻³¹

Proton = 1.67 ×10⁻²⁷

Neutron = 1.675 ×10⁻²⁷

Memory:

"Proton and Neutron are twins."

Only remember one number:

1.67 × 10⁻²⁷

4. EARTH FAMILY TRICK

Everything about Earth revolves around 6.

Quantity	Value
Earth Mass	6 ×10²⁴
Earth Radius	6.4×10⁶
Density	5.5×10³

Mnemonic:

🌍 "Earth lives in House Number 6."

5. SUN FAMILY TRICK

Think:

☀ Sun is HUGE

Mass = 2 ×10³⁰ kg

Distance to Earth:

AU = 1.5 ×10¹¹ m

Mnemonic:

Sun weighs 2 and lives 1.5 away.

6. SPACE DISTANCE MEMORY CHAIN

Moon → Earth
3.84×10⁸ m

Sun → Earth
1.5×10¹¹ m

Light Year
9.46×10¹⁵ m

Visual ladder:

Moon
  ↓
10⁸

Sun
  ↓
10¹¹

Light Year
  ↓
10¹⁵

Every step jumps by about 10³–10⁴.

7. CONVERSION TRICKS

Conversion	Memory
1 Å	10⁻¹⁰ m
1 Fermi	10⁻¹⁵ m
1 eV	1.6×10⁻¹⁹ J
1 AU	1.5×10¹¹ m
1 Light Year	9.46×10¹⁵ m
1 HP	746 W

Tiny Length Ladder

Angstrom
10⁻¹⁰

↓ smaller

Fermi
10⁻¹⁵

Difference = 10⁵

8. SI BASE UNITS MEMORY SENTENCE

The 7 SI base units:

Quantity	Unit
Length	metre
Mass	kilogram
Time	second
Temperature	kelvin
Current	ampere
Amount	mole
Luminous Intensity	candela

Mnemonic:

"My King Takes Tea And Makes Cool Coffee"

My       → Metre
King     → Kilogram
Takes    → Time (second)
Tea      → Temperature
And      → Ampere
Makes    → Mole
Cool     → Candela
Coffee

9. QUICK EXAM CRAM SHEET

Must-Know Constants

c = 3×10⁸

e = 1.6×10⁻¹⁹

h = 6.626×10⁻³⁴

G = 6.67×10⁻¹¹

NA = 6.022×10²³

kB = 1.38×10⁻²³

Remember:

3, 1.6, 6.6, 6.0, 1.38

These five numbers solve most physics MCQs.

🧠 Mind Map

                    PHYSICS CONSTANTS
                           │
      ┌────────────────────┼────────────────────┐
      │                    │                    │
  Atomic               Universal             Earth
      │                    │                    │
 Electron             c=3×10⁸          Radius=6.4×10⁶
 Proton               h=6.626          Mass=6×10²⁴
 Neutron              G=6.67           Density=5.5×10³
 Charge=1.6           NA=6.022
                       kB=1.38
      │                    │
      └|────────────┬───────|Astronomy    Constant                        │
     Plunk constant             AU=1.5×10¹¹
Gravitation constant            Moon=3.84×10⁸
Boltzmann Constant              Sun=2×10³⁰
Stefan Constant
Wien Constant
Solar Constant                  LY=9.46×10¹⁵
               
   

🎯 30-Second Revision Formula

Remember only:

3, 1.6, 6.6, 6.0, 1.38, Earth=6, Sun=2, AU=1.5

From these anchor numbers, most of the listed constants can be reconstructed during exams.

INTERNAL LINKS

SI Units and Dimensional Analysis Explained

Complete Physics Formula Handbook

Motion and Kinematics Quick Revision Notes

Work, Energy and Power Formula Sheet

Electromagnetism Mind Maps and Memory Tricks

Modern Physics Notes for Competitive Exams

Thermodynamics Formula Cheat Sheet

Waves and Optics Revision Guide

Astronomy and Space Physics Basics

JEE Physics One-Page Revision Notes

Physics Constants & Units - Memory Dashboard

Physics Constants & Units

A comprehensive memory toolkit designed to lock these physics constants, units, and conversion factors directly into your long-term memory using memory anchors, pattern-recognition tricks, and structural maps.

1. Master Reference Table (The Pattern Identifier)

Before diving into mnemonics, let's fix a subtle typo in your source list and organize these numbers by their powers of 10 to help your brain spot structural patterns.

Note: The speed of light (c) is accurately represented here as 3 × 10⁸ m/s for standard vacuum calculations.

Constant / Unit	Symbol	Value & Unit	The Pattern / Anchor Trick
Mass of Electron	m_e	9.109 × 10^-31 kg	9-10-11 sequence (9.109 × 10^-31)
Mass of Proton	m_p	1.6726 × 10^-27 kg	1.67 is standard; Proton has 2 (1.6726)
Mass of Neutron	m_n	1.6749 × 10^-27 kg	Neutron is heavier than proton; has 4 (1.6749)
Electron Charge	e	-1.602 × 10^-19 C	Sweet 16 (1.6) and coming of age (19)
Speed of Light	c	3 × 10⁸ m/s	Classic absolute constant.
Planck's Constant	h	6.626 × 10^-34 J·s	6.6 and 3 halves to 4
Universal Gravitation	G	6.67 × 10^-11 N·m²/kg²	6, 7, 11 (Convenience store numbers)
Avogadro's Number	N_A	6.022 × 10²³ mol^-1	22nd of October (6.022 × 10²³)
Boltzmann Constant	k_B	1.38 × 10^-23 J/K	1, 2, 3 sequence hidden: 1.38 × 10^-23
Stefan's Constant	σ	5.67 × 10^-8 W·m^-2·K^-4	Sequential count: 5, 6, 7, 8
Wien's Constant	b	2.898 × 10^-3 m·K	Almost 3 × 10^-3
Solar Constant	S	1.366 × 10³ W·m^-2	1366 (A year in history style)
Mass of Sun	M_s	1.989 × 10³⁰ kg	Taylor Swift's album 1989
Mass of Earth	M_E	5.97 × 10²⁴ kg	Almost 6 × 10²⁴
Radius of Earth	R_e	6.4 × 10⁶ m (or 6371 km)	64 Lakh meters
Density of Earth	ρ	5.513 × 10³ kg/m³	Roughly 5.5 times water density (10³)
Angular Velocity (Earth)	ω	7.29 × 10^-5 rad/s	7.3 × 10^-5
Astronomical Unit	AU	1.496 × 10¹¹ m	Roughly 1.5 × 10¹¹
Earth-Moon Distance	d_em	3.844 × 10⁸ m	38-44 pattern
Earth's Mag. Moment	M	8.22 × 10²² A·m²	Triple 2s and 8s

2. High-Impact Memory Mnemonics & Tricks

The Atomic Trio (Masses & Charge)

Electron Mass (m_e = 9.1 × 10^-31): Notice the digit run. 9 point 1 × 10^-31. Think: "An electron is so light it fell down a 9 to 1 slide, all the way to -31."
Proton (1.672 × 10^-27) vs. Neutron (1.674 × 10^-27): Both share 1.67 × 10^-27.
- To remember which is heavier: Protons have 2 legs (1.672), Neutrons have 4 legs (1.674). A Neutron is always Noticeably heavier than a Proton.
Electron Charge (-1.6 × 10^-19): Remember "Sweet 16 at 19". The value is 1.6, the power is -19.

The Astrophysics Quantities

Stefan's Constant (σ = 5.67 × 10^-8): This is the easiest constant in physics because it counts sequentially: 5, 6, 7, 8.
Universal Gravitational Constant (G = 6.67 × 10^-11): Think of the convenience store chain 7-Eleven. Roll back slightly to 6.67 and couple it with -11.
Mass of the Sun (1.989 × 10³⁰): The Sun loves music. Its mass starts exactly with the year 1989 (1.989). The power is a round, hot 30.

3. Visual Concept Mapping

The Space Distance Ladder (From Smallest to Largest)

To remember distances and conversion coefficients intuitively, map them step-by-step out into space:

[ Earth ] 
    │
    ├─► Radius of Earth (Re) = 6.4 x 10⁶ m
    │
    ├─► Earth-Moon Distance = 3.84 x 10⁸ m (Same power of 10 as Speed of Light!)
    │
    ├─► 1 A.U. (Earth to Sun) = 1.496 x 10¹¹ m
    │
    └─► 1 Light Year (Interstellar) = 9.46 x 10¹⁵ m

Micro-Scale Distance Hierarchy

Use this visualization ladder to never mix up your microscopic conversions:

1 Meter ──► 1 Ångström (10⁻¹⁰ m) ──► 1 Fermi (10⁻¹⁵ m)
              [Atomic Scale]            [Nuclear Scale]

Mnemonic: A comes before F in the alphabet, just like -10 comes before -15 on the number line.

4. SI Base Units Breakdown Chat Matrix

When navigating the fundamental systems of units (CGS, FPS, MKS, MKSA), you only need to look at the letters themselves to know what they measure:

[C]entimeter   [G]ram       [S]econd
[F]oot         [P]ound      [S]econd
[M]eter        [K]ilogram   [S]econd
[M]eter        [K]ilogram   [S]econd    [A]mpere

The 7 Fundamental Pillars of SI

Base Dimension	Unit Name	Symbol	Quick Brain Anchor
Length	metre	m	Standard stride
Mass	kilogram	kg	A liter of water
Time	second	s	Heartbeat pace
Temperature	kelvin	K	No degree symbol (°) allowed!
Electric current	ampere	A	Current flow
Amount of substance	mole	mol	Chemistry's dozen
Luminous intensity	candela	cd	Brightness of 1 standard candle

5. Instant Association Flash-Tricks

1 Horsepower = 746 Watts: Think of a mythical mechanical horse running down Route 746.
1 eV = 1.6 × 10^-19 J: You do not need to memorize this separately! It is exactly identical to the magnitude of the charge of an electron.
Boltzmann Constant (k_B = 1.38 × 10^-23): Notice the pattern: 1, 2, 3. 1.38 × 10^-23. The digits 1, 2, and 3 are all present in the structural layout!

Aristotle's Fallacy Explained | Laws of Motion Class 11 Physics

Aristotle's Fallacy Class 11 Physics Notes, MCQs & Questions Answers

- Dr.Sanjaykumar Pawar

4.2 ARISTOTLE'S FALLACY – Easy Line-by-Line Notes (NEET Level)

Introduction

One important question in physics is: "Is a force required to keep a body moving?"
Today the answer seems simple.
But scientists took many centuries to find the correct answer.
The correct explanation was given by in the 17th century.
Galileo's ideas became the foundation of Newtonian Mechanics.
These ideas marked the beginning of modern science.

Aristotle's View About Motion

was a Greek philosopher who lived from 384 BC to 322 BC.
Aristotle believed that:

"A moving object needs a continuous external force to keep moving."
According to him, if the force is removed, the object should stop moving.

Example Given by Aristotle

Consider an arrow shot from a bow.
Aristotle thought the arrow continues moving because:
- Air behind the arrow keeps pushing it forward.
Therefore, he concluded that motion needs a continuous force.

Aristotle's Law of Motion

Aristotle's idea can be written as:

Aristotle's Law

"An external force is required to keep a body in motion."

Why Did Aristotle Think So?

Aristotle's conclusion came from everyday observations.
In daily life, moving objects eventually stop.

Examples

A toy car stops after some time.
A bicycle slows down if we stop pedaling.
A rolling ball finally comes to rest.
Therefore, it appeared that:
- Continuous force is necessary for continuous motion.
Even a small child notices this fact.

Example: Toy Car

A child pulls a toy car using a string.
The toy car moves only while the child keeps pulling.
When the child stops pulling:
- The toy car eventually stops.
This observation seems to support Aristotle's idea.

What Was the Mistake in Aristotle's Argument?

Aristotle ignored an important force:

Friction
Friction always acts opposite to the direction of motion.
Friction tries to slow down moving objects.

Example

When a toy car moves on the floor:
- Friction acts between the wheels and the floor.
- Friction opposes motion.
- Because of friction, the car slows down and stops.

Role of the Child's Force

To keep the toy car moving:
- The child must continuously pull it.
The pulling force balances the frictional force.

During Uniform Motion

When the toy car moves with constant speed:

Child's Pulling Force = Frictional Force

Therefore,

Net Force = 0

The forces cancel each other.
The car continues moving with constant speed.

What If Friction Were Absent?

Imagine a perfectly smooth floor.

Then:

No friction would act on the toy car.
The child would give one pull.
After that, no force would be needed.
The toy car would continue moving with constant speed.

Important Conclusion

Force is not required to maintain uniform motion.

Force is required only to change motion.

Friction and Viscous Force

In the real world, opposing forces are almost always present.

In Solids

Opposing force = Friction

Examples:

Ball rolling on the ground
Toy car moving on a floor

In Fluids (Liquids and Gases)

Opposing force = Viscous Force (Drag)

Examples:

Boat moving in water
Air resistance on a moving car

Why Aristotle Was Wrong

Aristotle observed real-life motion correctly.
But he misunderstood the reason objects stop.
He thought objects stop because force is absent.
Actually, objects stop because friction opposes motion.

His Mistake

He ignored frictional forces.

Galileo's Great Contribution

Galileo imagined an ideal world without friction.
He asked:

"What would happen if no opposing force existed?"
He concluded:

Galileo's Conclusion

A body in uniform motion will continue moving uniformly unless an external force changes its motion.

This idea later became:
- Law of Inertia
- Newton's First Law of Motion

NEET Quick Facts

Aristotle Said

❌ Force is needed to keep a body moving.

Galileo Said

✅ Force is needed only to change the state of motion.

Real Reason Objects Stop

✅ Friction

If Friction Were Zero

✅ A moving body would continue moving with constant velocity.

Foundation of Newton's First Law

✅ Galileo's concept of inertia

One-Line Revision

Aristotle believed continuous force is required for continuous motion.
His idea was based on everyday observations.
He ignored the effect of friction.
Friction opposes motion and causes objects to stop.
Galileo showed that without friction, a body would keep moving uniformly.
This led to the Law of Inertia and Newton's First Law of Motion. ✅

Educational illustration showing Aristotle, Galileo, a toy car experiencing friction, and a frictionless motion concept explaining Aristotle's Fallacy in Class 11 Physics.

Aristotle's Fallacy Explained: How Galileo Corrected the Ancient Theory of Motion and Laid the Foundation of Modern Physics.

Class 11 Physics – Aristotle's Fallacy Question Bank (CBSE & NEET Level)

A. Multiple Choice Questions (MCQs)

1. Who proposed the idea that a force is required to keep a body in motion?

a) Newton
b) Galileo
c) Aristotle
d) Einstein

Answer: (c) Aristotle

2. Aristotle lived during:

a) 1642–1727 AD b) 384 BC–322 BC c) 1564–1642 AD d) 1879–1955 AD

Answer: (b) 384 BC–322 BC

3. According to Aristotle, a moving body:

a) Moves forever without force b) Needs a continuous external force to keep moving c) Stops immediately d) Accelerates continuously

Answer: (b) Needs a continuous external force to keep moving

4. What was the major flaw in Aristotle's argument?

a) Ignoring gravity b) Ignoring friction c) Ignoring velocity d) Ignoring mass

Answer: (b) Ignoring friction

5. The correct explanation of motion without friction was given by:

a) Aristotle b) Archimedes c) Galileo d) Kepler

Answer: (c) Galileo

6. A toy car comes to rest mainly because of:

a) Gravity b) Magnetism c) Friction d) Tension

Answer: (c) Friction

7. Friction acts:

a) Along the motion b) Opposite to motion c) Vertically upward d) Vertically downward

Answer: (b) Opposite to motion

8. Galileo imagined a world:

a) Without gravity b) Without mass c) Without friction d) Without motion

Answer: (c) Without friction

B. Very Short Answer Questions (1 Mark)

1. Who gave the correct answer to the problem of motion?

Answer: Galileo.

2. State Aristotle's law of motion.

Answer: An external force is required to keep a body in motion.

3. What force did Aristotle ignore?

Answer: Frictional force.

4. Which force opposes motion?

Answer: Frictional force.

5. What is viscous force?

Answer: The force that opposes motion in fluids.

6. Name the scientist who laid the foundation of Newtonian mechanics.

Answer: Galileo.

C. Short Answer Questions (2–3 Marks)

1. Why did Aristotle think force is needed to keep a body moving?

Answer: Aristotle observed that moving objects eventually come to rest. He concluded that continuous force is needed to maintain motion. He did not consider the effect of friction.

2. Explain the toy car example used to describe Aristotle's fallacy.

Answer: A toy car moves when pulled by a child. When the child stops pulling, the car stops because friction acts opposite to motion. Aristotle incorrectly thought the car stopped due to the absence of force.

3. What is the role of friction in Aristotle's fallacy?

Answer: Friction opposes the motion of objects and slows them down. Aristotle ignored friction and wrongly concluded that force is necessary to maintain motion.

4. What happens if friction is absent?

Answer: In the absence of friction, a moving body would continue moving with constant velocity without requiring any external force.

D. Long Answer Questions (5 Marks)

1. Explain Aristotle's fallacy and Galileo's correction.

Answer:

Aristotle believed that a body requires continuous external force to keep moving. He based this idea on everyday observations where moving objects eventually stop.

For example, a toy car stops when the child stops pulling it.

The flaw in Aristotle's argument was that he ignored friction. Friction acts opposite to motion and causes moving objects to stop.

Galileo imagined an ideal world without friction. He concluded that a moving body would continue moving with constant velocity even without any external force.

This idea became the foundation of the Law of Inertia and Newton's First Law of Motion.

2. Explain how friction led Aristotle to an incorrect conclusion.

Answer:

In daily life, all moving objects experience friction. Because of friction, they gradually slow down and stop.

Aristotle observed this behavior and concluded that motion requires continuous force.

However, the actual reason for stopping is friction, not the absence of force.

When friction is removed, a body continues moving uniformly. Thus Aristotle's conclusion was incorrect because he ignored friction.

E. Assertion and Reason Questions

1.

Assertion (A): Aristotle believed that a force is required to keep a body moving.

Reason (R): He ignored the effect of friction.

Answer: ✔ Both A and R are true and R is the correct explanation of A.

2.

Assertion (A): A toy car stops when the child releases the string.

Reason (R): Friction acts opposite to the direction of motion.

Answer: ✔ Both A and R are true and R correctly explains A.

3.

Assertion (A): Galileo imagined a frictionless world.

Reason (R): He wanted to understand the true law of motion.

Answer: ✔ Both A and R are true and R correctly explains A.

4.

Assertion (A): Force is necessary to maintain uniform motion.

Reason (R): Friction always acts on moving bodies.

Answer: ✘ Assertion is false but Reason is true.

F. Fill in the Blanks

1.

Aristotle was a ______ philosopher.

Answer: Greek

2.

According to Aristotle, a body requires ______ force to keep moving.

Answer: external

3.

The force that opposes motion is called ______.

Answer: friction

4.

Galileo imagined motion without ______.

Answer: friction

5.

Motion in fluids is opposed by ______ force.

Answer: viscous

6.

Galileo's work laid the foundation of ______ mechanics.

Answer: Newtonian

G. True or False

1.

Aristotle correctly explained the cause of motion.

Answer: False

2.

Friction acts opposite to motion.

Answer: True

3.

Galileo ignored friction.

Answer: False

4.

A moving body can continue moving without force if friction is absent.

Answer: True

5.

Viscous force acts in fluids.

Answer: True

H. Statement-Based Questions

Statement I:

Aristotle believed that force is needed to maintain motion.

Statement II:

He ignored the effect of friction.

a) Both statements are true and II explains I. b) Both statements are true but II does not explain I. c) I is true, II is false. d) I is false, II is true.

Answer: (a)

Statement I:

Galileo imagined a frictionless world.

Statement II:

Without friction, uniform motion can continue without force.

Answer: Both statements are true and II explains I.

I. Match the Columns

Column A	Column B
A. Aristotle	1. Frictionless world
B. Galileo	2. Opposes motion
C. Friction	3. Force needed to keep motion
D. Viscous Force	4. Acts in fluids

Answers

A → 3

B → 1

C → 2

D → 4

J. Case Study Questions

Case Study

A child pulls a toy car using a string. The car moves with constant speed while the child keeps pulling. When the child releases the string, the car gradually slows down and stops.

Questions

1. Why does the toy car stop?

Answer: Due to friction between the car and the floor.

2. Which philosopher would say that force is required to keep the toy car moving?

Answer: Aristotle.

3. Which scientist corrected this idea?

Answer: Galileo.

4. What force opposes the motion of the toy car?

Answer: Frictional force.

5. What would happen if friction were absent?

Answer: The toy car would continue moving with constant velocity.

K. Higher Order Thinking Skills (HOTS)

1.

A hockey puck slides on a perfectly smooth ice surface. No force acts on it after it is hit. What will happen?

Answer: The puck will continue moving with constant velocity because no friction acts on it.

2.

Why was Aristotle's theory accepted for many centuries?

Answer: Because it matched everyday observations where moving bodies stop due to friction. The role of friction was not understood at that time.

Important CBSE Exam Questions

1 Mark

Who proposed Aristotle's law of motion?
What force opposes motion?
Define viscous force.

2 Marks

State Aristotle's law of motion.
Why does a toy car stop after being released?

3 Marks

Explain the flaw in Aristotle's argument.
Discuss the role of friction in motion.

5 Marks

Explain Aristotle's fallacy with examples.
Describe Galileo's contribution in correcting Aristotle's views on motion.

Competency-Based Question

A student says, "A moving bicycle stops because no force acts on it." Is the statement correct? Give a reason.

Answer: No. The bicycle stops because friction and air resistance oppose its motion, not because force is absent. Force is required only to change motion, not to maintain uniform motion.

ARISTOTLE'S FALLACY

│

├── Main Question

│ │

│ └── Is force required to keep a body moving?

│

├── Historical Importance

│ │

│ ├── Answer took centuries

│ ├── Correct answer given by Galileo

│ ├── Foundation of Newtonian Mechanics

│ └── Beginning of Modern Science

│

├── Aristotle's View

│ │

│ ├── Greek Philosopher (384 BC – 322 BC)

│ ├── Studied motion of bodies

│ └── Believed:

│ "Continuous force is needed to keep a body moving"

│

├── Aristotle's Example

│ │

│ └── Arrow shot from a bow

│ │

│ └── Air behind arrow keeps pushing it forward

│

├── Aristotle's Law of Motion

│ │

│ └── External force is required

│ to keep a body in motion

│

├── Why Aristotle's Idea Seemed Correct

│ │

│ ├── Based on everyday observations

│ ├── Moving objects eventually stop

│ ├── Bicycle stops when pedalling stops

│ ├── Ball stops rolling

│ └── Toy car comes to rest

│

├── Toy Car Example

│ │

│ ├── Child pulls toy car with string

│ ├── Car moves while pulling

│ ├── Child releases string

│ └── Car eventually stops

│

├── Aristotle's Mistake

│ │

│ └── Ignored friction

│

├── Friction

│ │

│ ├── Opposes motion

│ ├── Acts opposite to velocity

│ ├── Slows moving bodies

│ └── Causes objects to stop

│

├── During Uniform Motion

│ │

│ ├── Child's Pulling Force →

│ ├── Frictional Force ←

│ ├── Both are equal

│ └── Net Force = 0

│

├── If Friction Were Absent

│ │

│ ├── No opposing force

│ ├── No continuous pulling required

│ ├── Body keeps moving

│ └── Uniform motion continues

│

├── Opposing Forces in Nature

│ │

│ ├── Solids

│ │ └── Friction

│ │

│ └── Fluids

│ └── Viscous Force (Drag)

│

├── Galileo's Contribution

│ │

│ ├── Imagined a frictionless world

│ ├── Analyzed ideal motion

│ └── Reached correct conclusion

│

├── Galileo's Conclusion

│ │

│ └── Force is not needed

│ to maintain uniform motion

│

├── Foundation For

│ │

│ ├── Law of Inertia

│ ├── Newton's First Law

│ └── Newtonian Mechanics

│

└── Exam Corner

│

├── Aristotle:

│ Force needed to keep motion

│

├── Galileo:

│ Force needed only to change motion

│

├── Real Reason Objects Stop:

│ Friction

│

└── Without Friction:

Uniform motion continues forever

INTERNAL LINKS

Laws of Motion Introduction Notes

Law of Inertia Explained

Newton's First Law of Motion

Newton's Second Law of Motion

Newton's Third Law of Motion

Friction Class 11 Physics Notes

Motion in a Straight Line Notes

Uniform and Non-Uniform Motion

Galileo's Contributions to Physics

Conservation of Momentum Notes

Circular Motion Notes

CBSE Class 11 Physics Important Questions

NEET Physics Chapter-wise MCQs

NCERT Solutions for Laws of Motion

Physics Assertion and Reason Questions Collection

Laws of Motion Introduction Class 11 Physics Notes CBSE & NEET

Chapter 4: Laws of Motion – Introduction (NEET Level Easy Notes)

- Dr.Sanjaykumar pawar

1. What was studied in the previous chapter?

In the previous chapter, we studied motion of particles.
We learned how to describe motion using:
- Velocity → tells how fast and in which direction an object moves.
- Acceleration → tells how velocity changes with time.
We could describe motion mathematically.
But one important question remained:

Question:

What causes a body to move or change its motion?

This question is answered in the chapter Laws of Motion.

2. Need for Force in Daily Life

From our everyday experience, we observe that an object at rest does not start moving on its own.
Some external action is needed.

Examples:

A football starts moving only when someone kicks it.
A stone moves upward only when someone throws it.
Tree branches swing because of wind.
A boat moves because of the flowing river.

Conclusion:

➡️ To move an object from rest, an external force is required.

3. Force Can Also Stop Motion

Force is not only used to start motion.
It can also slow down or stop a moving object.

Example:

A ball rolling down a slope can be stopped by applying force in the opposite direction.

Conclusion:

➡️ Force can:

Start motion.
Stop motion.
Change speed.
Change direction.

4. Contact Forces

In many cases, force is applied through physical contact.

Examples:

Hand kicking a football.
Hand throwing a stone.
Wind pushing tree branches.
Water pushing a boat.

Definition:

Contact Force: A force that acts when two objects touch each other.

Examples:

Muscular force
Friction force
Normal reaction force

5. Non-Contact Forces

Force can also act without physical contact.

Example 1: Gravitational Force

A stone dropped from a building falls toward Earth.
Earth attracts the stone due to gravity.

Example 2: Magnetic Force

A magnet attracts an iron nail from a distance.

Conclusion:

➡️ Physical contact is not always necessary for force to act.

Definition:

Non-Contact Force: A force that acts without direct contact.

Examples:

Gravitational force
Magnetic force
Electrostatic force

6. Important Definition of Force

Force

A force is a push or pull that can:

Start motion,
Stop motion,
Change speed,
Change direction,
Change shape of an object.

SI Unit:

Newton (N)

7. External Agency

The source that applies force is called an external agency.

Examples:

External Agency	Force Produced
Hand	Muscular force
Wind	Air force
Flowing water	Water force
Earth	Gravitational force
Magnet	Magnetic force

Key Point:

An external agency may be:
- In contact with the object.
- Away from the object.

8. Important Question Raised in the Chapter

Consider a skater moving on a smooth ice surface with constant speed.

Question:

Does he need a continuous force to keep moving?

At first glance, the answer seems "Yes."

But this idea is actually incorrect.

This misconception was given by Aristotle, and later corrected by Galileo and Newton.

This leads to:

Aristotle's Fallacy
Law of Inertia
Newton's First Law of Motion

which are discussed in the next sections.

NEET Quick Facts

Force is needed to:

✅ Start motion

✅ Stop motion

✅ Change speed

✅ Change direction

✅ Change shape

Types of Forces

1. Contact Forces

Muscular force
Friction
Normal force

2. Non-Contact Forces

Gravitational force
Magnetic force
Electrostatic force

Most Important NEET Concept

A force is required to change the state of motion, but not necessarily to maintain uniform motion.

This idea forms the basis of Newton's First Law of Motion (Law of Inertia).

One-Line Revision

Velocity describes motion.
Acceleration describes change in motion.
Force explains why motion changes.
Force can start, stop, speed up, slow down, or change direction.
Force may be contact or non-contact.
Earth attracts objects through gravity.
Magnets attract iron without contact.
The major question of this chapter is: "Is force required to keep a body moving uniformly?"
The answer leads to Newton's Laws of Motion. ✅

Illustration showing force, motion, football kick, gravitational force, magnetic force, and Class 11 Physics Laws of Motion concepts for CBSE and NEET students.

Class 11 Physics Laws of Motion Introduction – Easy Notes, MCQs, and Exam Questions for CBSE and NEET Preparation.

Class 11 Physics – Laws of Motion (Introduction) Question Bank with Answers

(CBSE + NEET Foundation Level)

A. Multiple Choice Questions (MCQs)

1. Which quantity is sufficient to describe uniform motion?

a) Acceleration b) Velocity c) Force d) Momentum

Answer: (b) Velocity

2. Which quantity is additionally required to describe non-uniform motion?

a) Mass b) Velocity c) Acceleration d) Density

Answer: (c) Acceleration

3. A football at rest starts moving when:

a) Gravity acts b) It is kicked c) Air acts d) It moves itself

Answer: (b) It is kicked

4. Which of the following is a non-contact force?

a) Friction b) Muscular force c) Magnetic force d) Tension

Answer: (c) Magnetic force

5. A stone falls toward Earth due to:

a) Friction b) Magnetic force c) Gravitational force d) Tension

Answer: (c) Gravitational force

6. The SI unit of force is:

a) Joule b) Newton c) Watt d) Pascal

Answer: (b) Newton

7. Which of the following is a contact force?

a) Gravity b) Magnetism c) Friction d) Electrostatic force

Answer: (c) Friction

8. Force can change:

a) Speed b) Direction c) State of motion d) All of these

Answer: (d) All of these

B. Very Short Answer Questions (1 Mark)

1. What is force?

Answer: Force is a push or pull that can change the state of motion of a body.

2. Name one contact force.

Answer: Frictional force.

3. Name one non-contact force.

Answer: Gravitational force.

4. What is the SI unit of force?

Answer: Newton (N).

5. Give one example of gravitational force.

Answer: A stone falling toward Earth.

6. Give one example of magnetic force.

Answer: A magnet attracting an iron nail.

C. Short Answer Questions (2–3 Marks)

1. Why is force required to move a stationary object?

Answer: A stationary object remains at rest unless an external force acts on it. Force changes the state of rest and starts motion.

2. Differentiate between contact and non-contact forces.

Contact Force	Non-Contact Force
Requires physical contact	Does not require contact
Example: Friction	Example: Gravity

3. Give three examples where force starts motion.

Answer:

Kicking a football.
Throwing a stone.
Wind moving tree branches.

4. Give two examples where force acts from a distance.

Answer:

Earth attracting a stone.
Magnet attracting an iron nail.

D. Long Answer Questions (5 Marks)

1. Explain the need for force with suitable examples.

Answer:

Force is a push or pull that changes the state of motion of a body.

Force is required to:

Start motion
- Example: Kicking a football.
Stop motion
- Example: Stopping a rolling ball.
Change speed
- Example: Accelerating a bicycle.
Change direction
- Example: Hitting a moving cricket ball.

Force may act through contact or from a distance. Examples include gravitational and magnetic forces.

Thus, force is responsible for changing the state of motion of objects.

2. Explain contact and non-contact forces with examples.

Answer:

Contact Forces

These forces act only when objects are in physical contact.

Examples:

Friction
Muscular force
Tension

Non-Contact Forces

These forces act without physical contact.

Examples:

Gravitational force
Magnetic force
Electrostatic force

Thus, forces can act both through contact and at a distance.

E. Assertion and Reason Questions

1.

Assertion (A): A magnet can attract an iron nail from a distance.

Reason (R): Magnetic force is a non-contact force.

Answer: ✔ Both A and R are true and R is the correct explanation of A.

2.

Assertion (A): A football at rest starts moving when kicked.

Reason (R): Force changes the state of motion.

Answer: ✔ Both A and R are true and R correctly explains A.

3.

Assertion (A): A falling stone accelerates toward Earth.

Reason (R): Earth exerts gravitational force on the stone.

Answer: ✔ Both A and R are true and R correctly explains A.

F. Fill in the Blanks

1.

A ______ is needed to change the state of motion of a body.

Answer: force

2.

A force acting without contact is called a ______ force.

Answer: non-contact

3.

The SI unit of force is ______.

Answer: Newton

4.

A magnet attracts an iron nail due to ______ force.

Answer: magnetic

5.

A stone falls toward Earth due to ______ force.

Answer: gravitational

G. True/False Questions

1.

Force is required to change the state of motion.

Answer: True

2.

Gravitational force is a contact force.

Answer: False

3.

A magnet can attract iron without touching it.

Answer: True

4.

Velocity alone describes non-uniform motion.

Answer: False

5.

Wind can exert force on tree branches.

Answer: True

H. Statement-Based Questions

Statement I:

A force can act without physical contact.

Statement II:

Gravity and magnetic force are examples of non-contact forces.

a) Both statements are true. b) Both statements are false. c) Statement I is true but II is false. d) Statement I is false but II is true.

Answer: (a) Both statements are true.

Statement I:

Force is needed to stop a moving object.

Statement II:

Force cannot change speed.

Answer: Statement I is true but Statement II is false.

I. Match the Columns

Column A	Column B
A. Gravity	1. Contact force
B. Friction	2. Iron nail attraction
C. Magnet	3. Earth pulls stone
D. Hand push	4. Muscular force

Answers

A → 3

B → 1

C → 2

D → 4

J. Case Study Questions

Case Study

A student drops a stone from the roof of a building. The stone starts moving downward and gains speed. Nearby, a magnet attracts an iron nail placed on a table.

Questions

1. Why does the stone fall?

Answer: Due to gravitational force.

2. Is gravity a contact force?

Answer: No, it is a non-contact force.

3. Why does the nail move toward the magnet?

Answer: Due to magnetic force.

4. What type of force is magnetic force?

Answer: Non-contact force.

5. Name two non-contact forces involved in this case.

Answer:

Gravitational force
Magnetic force

Important CBSE Exam Questions

1 Mark

Define force.
State SI unit of force.
Give one example of non-contact force.

2 Marks

Differentiate between contact and non-contact forces.
Explain why force is required to stop a moving body.

3 Marks

Explain three effects of force.
Give examples of contact and non-contact forces.

5 Marks

Explain the need for force in daily life with examples.
Discuss contact and non-contact forces with suitable examples.

HOTS Question

A skater moves on a smooth ice surface with constant speed. Is force necessary to keep him moving? Explain.

Answer: No. A force is required only to change the state of motion. Uniform motion can continue without an external force if no opposing force acts.

LAWS OF MOTION (Introduction)

│

├── Previous Chapter

│ │

│ ├── Motion of Particle

│ ├── Velocity

│ │ └── Describes uniform motion

│ └── Acceleration

│ └── Describes non-uniform motion

│

├── Main Question

│ │

│ └── What governs the motion of bodies?

│

├── Force

│ │

│ ├── Push or Pull

│ ├── Causes motion

│ ├── Stops motion

│ ├── Changes speed

│ ├── Changes direction

│ └── Changes state of motion

│

├── Force Needed to Start Motion

│ │

│ ├── Kick a football

│ ├── Throw a stone upward

│ ├── Wind moves branches

│ └── River flow moves boat

│

├── Force Needed to Stop Motion

│ │

│ └── Stop a rolling ball by applying force opposite to motion

│

├── External Agency

│ │

│ ├── Hand

│ ├── Wind

│ ├── Flowing water

│ ├── Earth

│ └── Magnet

│

├── Types of Force

│ │

│ ├── Contact Force

│ │ │

│ │ ├── Hand pushing object

│ │ ├── Wind on branches

│ │ └── Water on boat

│ │

│ └── Non-Contact Force

│ │

│ ├── Gravitational Force

│ │ └── Stone falls toward Earth

│ │

│ └── Magnetic Force

│ └── Magnet attracts iron nail

│

├── Important Conclusion

│ │

│ ├── Force may act through contact

│ └── Force may act from a distance

│

└── Key Question of Chapter

│

├── Is force required to keep a body moving uniformly?

│

├── Example:

│ └── Skater moving with constant speed on ice

│

└── Leads To

│

├── Aristotle's Fallacy

├── Law of Inertia

├── Newton's First Law

├── Newton's Second Law

├── Newton's Third Law

└── Conservation of Momentum

INTERNAL LINKS

Motion in a Straight Line Class 11 Notes

Units and Measurements Notes

Vectors and Scalars Explained

Laws of Motion Complete Chapter Notes

Newton's First Law of Motion

Newton's Second Law of Motion

Newton's Third Law of Motion

Conservation of Momentum Notes

Friction Complete Notes

Circular Motion Class 11 Physics

Class 11 Physics MCQ Collection

NEET Physics Important Questions

CBSE Class 11 Physics Previous Year Questions

Work Energy and Power Notes

Mechanical Properties of Solids Notes

Friday, May 29, 2026

Example 3.9 Uniform Circular Motion Solution Step by Step

Angular Speed and Linear Speed Numerical with Solution

Dr.Sanjaykumar pawar

Educational diagram of uniform circular motion illustrating an insect moving along a circular groove with velocity tangent to the path and centripetal acceleration directed toward the center.

Uniform circular motion showing angular speed, linear speed and centripetal acceleration for an insect moving in a circular path.

Internal Links

Introduction to Uniform Circular Motion

Angular Velocity Formula and Numerical Problems

Linear Velocity in Circular Motion Explained

Centripetal Force and Centripetal Acceleration

Motion in a Plane Class 11 Notes

NCERT Physics Class 11 Solved Examples

Circular Motion Important Formulas PDF

Difference Between Speed and Velocity

Uniform vs Non-Uniform Circular Motion

Class 11 Physics Chapter-Wise Numerical Solutions

Example 3.9 - Uniform Circular Motion

Example 3.9 – Uniform Circular Motion

Example 3.9 An insect trapped in a circular groove of radius 12 cm moves along the groove steadily and completes 7 revolutions in 100 s. (a) What is the angular speed, and the linear speed of the motion? (b) Is the acceleration vector a constant vector ? What is its magnitude

Given Data

Radius of groove, R = 12 cm
Number of revolutions = 7
Total time taken = 100 s

(a) Find Angular Speed (ω) and Linear Speed (v)

Step 1: Find Time Period (T)

Time period is the time taken to complete one revolution.

T = Total Time / Number of Revolutions

T = 100 / 7

T = 14.3 s

Step 2: Calculate Angular Speed (ω)

ω = 2π / T

ω = (2 × 3.14) / 14.3

ω = 0.44 rad s⁻¹

Angular Speed (ω) = 0.44 rad s⁻¹

Step 3: Calculate Linear Speed (v)

v = ωR

v = 0.44 × 12

v = 5.28 cm s⁻¹

v ≈ 5.3 cm s⁻¹

Linear Speed (v) = 5.3 cm s⁻¹

(b) Is the Acceleration Vector Constant?

Step 1: Direction of Velocity

In circular motion, velocity always acts along the tangent to the circle. As the insect moves, the tangent changes continuously. Therefore, the direction of velocity changes continuously.

Step 2: Direction of Acceleration

The acceleration is always directed towards the centre of the circle. This is called centripetal acceleration.

As the insect moves around the circle, the direction towards the centre keeps changing. Therefore, the acceleration vector is not constant.

Acceleration Vector = Not Constant

Step 3: Find Magnitude of Acceleration

a = ω²R

a = (0.44)² × 12

a = 0.1936 × 12

a = 2.32 cm s⁻²

a ≈ 2.3 cm s⁻²

Magnitude of Acceleration = 2.3 cm s⁻²

Final Answers

(a) Angular Speed = 0.44 rad s⁻¹
(a) Linear Speed = 5.3 cm s⁻¹

(b) Acceleration vector is NOT constant because its direction changes continuously.
Magnitude of acceleration = 2.3 cm s⁻²

Quick Exam Notes

Velocity is always tangent to the circle.
Centripetal acceleration is always towards the centre.
Speed remains constant in uniform circular motion.
Velocity changes because its direction changes.
Acceleration vector is not constant.
Magnitude of acceleration remains constant.