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. ✔️
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| 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
