CBSE Class 11 Physics Question Bank: Rolling Friction (With Answers)

 Rolling Friction Question Bank – CBSE 11 & NEET Foundation

Rolling Friction – Easy NEET Notes 

Surface deformation at the contact area is the primary mechanism that generates rolling resistance (friction).

- Dr.Sanjaykumar Pawar 

1. Rolling Without Slipping

• When a ring, wheel, or sphere rolls on a horizontal surface without slipping, ideally there should be no frictional force.
• At any instant, only one point of the body touches the ground.
• This contact point is momentarily at rest with respect to the ground.
• Since there is no relative motion at the contact point, kinetic friction does not act.
• In the ideal case, static friction is also zero.
• Therefore, the body should continue moving with constant velocity according to Newton's First Law.

NEET Point

Pure rolling without slipping on an ideal surface requires no friction force.

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2. Why Does a Rolling Body Stop in Real Life?

• In practical situations, a rolling wheel or ball eventually slows down and stops.
• This shows that some resistance to motion is acting.
• This resistance is called rolling friction or rolling resistance.
• To keep the object rolling at constant speed, an external force must be applied.

Definition

Rolling friction is the force that opposes the motion of a rolling body.

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3. Magnitude of Rolling Friction

• Rolling friction is much smaller than static friction.
• Rolling friction is also much smaller than sliding (kinetic) friction.
• It may be 100 to 1000 times smaller (2–3 orders of magnitude).

Relation

F_{\text{rolling}} < F_{\text{sliding}} < F_{\text{static(max)}}

NEET Point

Rolling friction is the least among common types of friction.

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4. Importance of the Wheel

• Because rolling friction is very small, moving objects on wheels requires much less force.
• This is why the invention of the wheel became one of the greatest achievements in human history.
• Vehicles, carts, and machines can move efficiently because of low rolling friction.

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5. Cause of Rolling Friction

• Rolling friction arises due to the deformation of surfaces in contact.
• When a wheel rolls, both the wheel and the surface become slightly compressed.
• Therefore, contact is not at a single point but over a small finite area.
• Because of this deformation, the contact force becomes slightly tilted.
• The horizontal component of this contact force opposes the motion.
• This opposing force is called rolling friction.

NEET Point

Rolling friction mainly occurs due to deformation of the wheel and the surface.

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6. Why Friction Is Often Undesirable

• Friction opposes relative motion.
• It converts useful mechanical energy into heat energy.
• This leads to power loss in machines.
• Continuous friction also causes wear and tear of machine parts.

Effects of Friction

• Energy loss
• Heating
• Reduced efficiency
• Damage to surfaces

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7. Methods to Reduce Friction

(a) Lubrication

• Lubricants such as oil and grease are used between moving surfaces.
• They reduce direct contact between surfaces.
• Hence, friction decreases.

(b) Ball Bearings

• Ball bearings are placed between moving machine parts.
• They convert sliding friction into rolling friction.
• Since rolling friction is much smaller, power loss decreases.

NEET Point

Ball bearings reduce friction by replacing sliding motion with rolling motion.

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(c) Air Cushion

• A thin layer of air can be maintained between moving surfaces.
• The surfaces then do not touch each other directly.
• Friction becomes very small.

Example

• Air hockey table
• Air-cushion transport systems

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8. Useful Effects of Friction

Although friction causes energy loss, it is extremely important in daily life.

(a) Walking

• While walking, the foot pushes the ground backward.
• Static friction from the ground pushes us forward.
• Without friction, walking is impossible.

(b) Brakes

• Brakes use friction to stop vehicles.
• Friction converts kinetic energy into heat.
• Thus, the vehicle slows down and stops.

(c) Motion of Cars

• A car moves because of friction between tyres and the road.
• The tyres push the road backward.
• The road exerts a forward frictional force on the tyres.
• This forward friction accelerates the car.

Example

On an icy road, friction is very small, so vehicles may skid and cannot accelerate properly.

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Quick NEET Revision

Rolling Friction

• Opposes rolling motion.
• Smallest type of friction.
• Caused by deformation of surfaces.
• Much smaller than sliding friction.

Order of Friction

F_{\text{rolling}} < F_{\text{sliding}} < F_{\text{static(max)}}

Applications

• Wheels
• Ball bearings
• Vehicles
• Machinery

Important Facts

• Pure rolling on an ideal surface → friction ≈ 0.
• Real surfaces deform → rolling friction appears.
• Ball bearings reduce friction by converting sliding into rolling.
• Walking, braking, and vehicle motion require friction.

NEET One-Liner

Rolling friction is a small resistive force produced due to deformation of the rolling body and the surface in contact, and it is much smaller than sliding or static friction.

ROLLING FRICTION

• Definition
  • Friction opposing rolling motion
  • Also called rolling resistance
  • Acts opposite to motion
• Ideal Rolling (Without Slipping)
  • One point touches the ground
  • Contact point is at rest relative to ground
  • No kinetic friction
  • No static friction required
  • Body moves with constant velocity
• Real Situation
  • Rolling body slows down
  • Resistance acts on motion
  • Called rolling friction
  • External force needed to maintain motion
• Cause of Rolling Friction
  • Deformation of wheel
  • Deformation of surface
  • Contact area becomes finite
  • Contact force develops a component opposing motion
• Magnitude
  • Very small compared to other frictions
  • Less than sliding friction
  • Less than static friction
  • 100–1000 times smaller
• Order of Friction
  • Rolling Friction < Sliding Friction < Static Friction (Maximum)
• Importance of Wheel
  • Reduces friction greatly
  • Makes transportation easier
  • Major milestone in human history
• Disadvantages of Friction
  • Opposes motion
  • Produces heat
  • Causes energy loss
  • Reduces efficiency
  • Causes wear and tear
• Methods to Reduce Friction
  • Lubrication
    • Oil
    • Grease
  • Ball Bearings
    • Convert sliding friction into rolling friction
    • Reduce power loss
  • Air Cushion
    • Thin layer of air between surfaces
    • Very small friction
• Useful Applications of Friction
  • Walking
    • Static friction helps us move forward
  • Brakes
    • Friction stops vehicles
  • Cars
    • Tyres push road backward
    • Road friction pushes car forward
• NEET Key Points
  • Rolling friction is the smallest type of friction
  • Caused by deformation of surfaces
  • Ball bearings reduce friction
  • Pure rolling ideally requires no friction
  • Walking and vehicle motion require friction

CBSE Class 11 Physics Question Bank
Topic: Rolling Friction (School Exams, Unit Tests & NEET Foundation)

1. Multiple Choice Questions (MCQs)

Q1. Rolling friction is also called:

• (a) Static friction
• (b) Kinetic friction
• (c) Rolling resistance
• (d) Limiting friction

Answer: (c) Rolling resistance

Q2. Rolling friction is generally:

• (a) Greater than static friction
• (b) Greater than sliding friction
• (c) Smaller than sliding friction
• (d) Equal to static friction

Answer: (c) Smaller than sliding friction

Q3. The main cause of rolling friction is:

• (a) Surface deformation
• (b) Gravity
• (c) Air pressure
• (d) Magnetic force

Answer: (a) Surface deformation

Q4. Ball bearings reduce friction by converting:

• (a) Rolling friction into static friction
• (b) Sliding friction into rolling friction
• (c) Static friction into kinetic friction
• (d) Rolling friction into sliding friction

Answer: (b) Sliding friction into rolling friction

Q5. In ideal pure rolling without slipping:

• (a) Kinetic friction acts
• (b) Static friction acts
• (c) Friction is zero
• (d) Friction is maximum

Answer: (c) Friction is zero

Q6. Which friction is the smallest?

• (a) Static friction
• (b) Sliding friction
• (c) Rolling friction
• (d) Limiting friction

Answer: (c) Rolling friction

2. Very Short Answer Questions (1 Mark)

Q1. What is rolling friction?

Answer: The frictional force that opposes the motion of a rolling body.

Q2. What is another name for rolling friction?

Answer: Rolling resistance.

Q3. Which is smaller: rolling friction or sliding friction?

Answer: Rolling friction.

Q4. Why are wheels used in vehicles?

Answer: Wheels reduce friction by converting sliding motion into rolling motion.

Q5. Name one device that reduces friction in machines.

Answer: Ball bearing.

3. Short Answer Questions (2–3 Marks)

Q1. Why is rolling friction less than sliding friction?

Answer:

• In rolling motion, there is very little relative motion between surfaces.

• Only slight deformation occurs at the contact area.

• Therefore, rolling friction is much smaller than sliding friction.

Q2. What causes rolling friction?

Answer:

• During rolling, the wheel and surface deform slightly.

• The contact area becomes finite instead of a point.

• This produces a force component opposing motion.

• This opposing force is called rolling friction.

Q3. How do ball bearings reduce friction?

Answer:

• Ball bearings are placed between moving parts.

• They replace sliding friction with rolling friction.

• Since rolling friction is smaller, energy loss decreases.

Q4. State two disadvantages of friction.

Answer:

1. Causes wear and tear of machine parts.

2. Produces heat and energy loss.

4. Long Answer Questions (4–5 Marks)

Q1. Explain rolling friction and its causes.

Answer: Rolling friction is the frictional force that opposes the motion of a rolling object.

Causes:

• The rolling body and surface deform slightly.

• Contact occurs over a finite area instead of a point.

• The contact force develops a component opposite to motion.

• This component opposes rolling and produces rolling friction.

Rolling friction is much smaller than static and sliding friction and helps vehicles move efficiently.

Q2. Explain the importance of friction in daily life.

Answer:

• Walking is possible due to static friction.

• Vehicles move because friction acts between tyres and roads.

• Brakes use friction to stop vehicles.

• Writing on paper is possible due to friction.

• Holding objects firmly requires friction.

Thus, friction is essential despite causing energy losses.

5. Fill in the Blanks

1. Rolling friction is also called __________.

Answer: rolling resistance

2. Rolling friction is __________ than sliding friction.

Answer: smaller

3. Ball bearings reduce __________ friction.

Answer: sliding

4. Friction converts mechanical energy into __________ energy.

Answer: heat

5. Walking is possible due to __________ friction.

Answer: static

6. The invention of the __________ reduced friction in transportation.

Answer: wheel

6. Assertion and Reason Questions

Q1.
Assertion (A): Rolling friction is smaller than sliding friction.
Reason (R): Rolling surfaces undergo less relative motion than sliding surfaces.

• (a) Both A and R are true and R is the correct explanation.
• (b) Both A and R are true but R is not the correct explanation.
• (c) A is true but R is false.
• (d) A is false but R is true.

Answer: (a) Both A and R are true and R is the correct explanation.

Q2.
Assertion (A): Ball bearings are used in machines.
Reason (R): They increase sliding friction.

Answer: (c) A is true but R is false.

Q3.
Assertion (A): Friction is always harmful.
Reason (R): Friction causes wear and tear.

Answer: (d) A is false but R is true.

7. Statement-Based Questions

Q1.
Statement I: Rolling friction is the smallest type of friction.
Statement II: Rolling friction is caused mainly by surface deformation.

Choose the correct option:

• (a) Both statements are true.
• (b) Both statements are false.
• (c) Statement I is true, Statement II is false.
• (d) Statement I is false, Statement II is true.

Answer: (a) Both statements are true.

Q2.
Statement I: Walking is possible without friction.
Statement II: Static friction helps us walk.

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

8. Match the Columns

Column A	Column B
A. Ball Bearing	1. Walking
B. Static Friction	2. Rolling Friction
C. Wheel	3. Reduces Friction
D. Rolling Motion	4. Easier Transportation

Answer: A → 3, B → 1, C → 4, D → 2

9. Case Study-Based Questions

Case Study:
A bicycle moves smoothly on a road because its wheels roll over the surface. The rolling friction between the tyres and road is very small compared to sliding friction. Ball bearings are used in the wheel assembly to reduce friction further.

Q1. What type of friction acts on the bicycle wheels?

Answer: Rolling friction.

Q2. Why is rolling friction preferred over sliding friction?

Answer: It is much smaller and reduces energy loss.

Q3. What is the function of ball bearings?

Answer: They convert sliding friction into rolling friction.

Q4. What is the main cause of rolling friction?

Answer: Deformation of surfaces in contact.

10. Competency-Based Questions

Q1. A heavy box requires 300 N force to slide but only 20 N force when placed on rollers.

(a) Which friction is acting in the second case?
Answer: Rolling friction.

(b) Why is the required force smaller?
Answer: Rolling friction is much smaller than sliding friction.

Q2. Why do suitcases have wheels attached to them?

Answer: Wheels convert sliding motion into rolling motion. Since rolling friction is very small, less force is needed to move the suitcase.

Important CBSE Exam One-Liners

• Rolling friction is the smallest type of friction.
• Rolling friction is caused by deformation of surfaces.
• Ball bearings reduce friction by converting sliding into rolling.
• Walking requires static friction.
• Friction is both useful and harmful.
• Wheels reduce friction and make transportation easier.

Friction Explained Simply | NEET Physics Notes for Beginners

Friction in Physics: Static & Kinetic Friction Easy Notes for NEET 

- Dr.Sanjaykumar pawar

Basic idea of forces on a body

• A body of mass m is kept on a horizontal table.
• Two vertical forces act on it:
  • Weight = mg (downward)
  • Normal reaction = N (upward)
• These two forces cancel each other.
• So, net vertical force = 0 → no vertical motion.

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When horizontal force is applied

• Now a horizontal force F is applied on the body.
• We expect the body to move.
• But sometimes it does not move immediately.

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Why the body may not move

• If only force F acted, acceleration would be:
  • a = F/m
• But body stays at rest → contradiction.
• So, another force must be acting opposite to F.

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Introduction of friction

• A force appears between surfaces in contact.
• This force opposes motion.
• It acts parallel to surface.
• This force is called frictional force (f).

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Types of friction

• Static friction (fₛ): when body is not moving.
• Kinetic friction (fₖ): when body is moving.

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Static friction (fₛ)

Meaning

• Acts when body is at rest.
• Opposes impending motion (motion that is about to happen).

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Important points

• Static friction exists only when force is applied.
• If no force is applied → fₛ = 0
• It increases as applied force increases.
• It always adjusts itself to balance applied force.

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Condition

• Until a limit:
  • fₛ = F (equal and opposite)
• So net force = 0 → body remains at rest.

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Limiting static friction

• Maximum value of static friction is called limiting friction.
• Formula:
  • (fₛ)max = μₛ N
• Where:
  • μₛ = coefficient of static friction
  • depends on nature of surfaces
  • N = normal reaction

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Law of static friction

• fₛ ≤ μₛ N
• Means static friction can vary from 0 to maximum value.

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Kinetic friction (fₖ)

Meaning

• Acts when body is already moving.
• Opposes actual motion.

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Important points

• Always acts opposite to motion.
• Independent of contact area.
• Almost independent of speed.

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Formula

• fₖ = μₖ N
• Where:
  • μₖ = coefficient of kinetic friction

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Key comparison

• μₖ < μₛ
• So kinetic friction is less than maximum static friction.

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Motion after overcoming friction

• If applied force F > (fₛ)max → body starts moving.

• During motion:

  • Net force = F − fₖ
  • Acceleration = (F − fₖ)/m

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If force is removed

• Only kinetic friction acts opposite motion.
• Acceleration becomes negative:
  • a = −fₖ/m
• Body slows down and eventually stops.

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Nature of friction laws

• These laws are not fundamental laws.
• They are experimental (empirical).
• They are approximate but very useful in physics problems.

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Important concept

• Friction acts on contact surfaces.
• It is a component of contact force parallel to surface.
• It opposes relative motion, not absolute motion.

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Real-life example: Train and box

• A train accelerates forward.
• A box is kept inside it.

Without friction:

• Box would stay at rest (due to inertia).
• Train would move ahead.
• Box would hit back wall.

With friction:

• Static friction acts on box.
• It pulls box forward with train.
• So box accelerates with train.
• Hence, box stays at rest relative to train.

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Final NEET summary

• Friction is a contact force opposing relative motion.
• Two types:
  • Static friction (before motion)
  • Kinetic friction (during motion)
• Key formulas:
  • fₛ ≤ μₛ N
  • fₖ = μₖ N
• Always: μₖ < μₛ

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Internal Links

1. Newton's Laws of Motion Explained

2. Contact and Non-Contact Forces

3. Free Body Diagrams in Physics

4. Force and Acceleration Relationship

5. Circular Motion Fundamentals

6. Work, Energy and Power

7. Applications of Newton's Second Law

8. Momentum and Impulse

9. Laws of Motion Class 11 Notes

10. Coefficient of Friction Numerical Problems

Friction - Mind Map (NEET Level)

FRICTION
|
|-- Definition
|     |-- Force opposing relative motion between surfaces
|     |-- Acts parallel to surface of contact
|
|-- Types of Friction
|     |
|     |-- 1. Static Friction (fs)
|     |       |-- Acts when body is at rest
|     |       |-- Opposes impending motion
|     |       |-- Adjusts with applied force
|     |       |-- Range: 0 ≤ fs ≤ μs N
|     |
|     |-- 2. Kinetic Friction (fk)
|             |-- Acts when body is in motion
|             |-- Opposes actual motion
|             |-- fk = μk N
|             |-- μk < μs
|
|-- Laws of Friction
|     |-- Independent of area of contact
|     |-- Depends on nature of surfaces
|     |-- Proportional to normal reaction (N)
|
|-- Coefficients
|     |-- μs → coefficient of static friction
|     |-- μk → coefficient of kinetic friction
|
|-- Limiting Friction
|     |-- Maximum static friction
|     |-- (fs)max = μs N
|
|-- Motion Cases
|     |
|     |-- F ≤ (fs)max → body at rest
|     |
|     |-- F > (fs)max → motion starts
|     |       |-- Acceleration = (F - fk)/m
|     |
|     |-- Force removed
|             |-- Retarding force = fk
|             |-- Body stops eventually
|
|-- Important Concept
|     |-- Friction opposes relative motion, not absolute motion
|
|-- Example: Train and Box
      |-- Train accelerates
      |-- Box moves due to static friction
      |-- Without friction → box slips backward

Friction — Complete Question Bank
Class 11 Physics (CBSE / NEET Level)

✅ 1. Very Short Answer Questions (1 Mark)

Q1. What is friction?

Ans: Friction is the force that opposes the relative motion or the tendency of relative motion between two surfaces in contact.

Q2. Write the formula of limiting friction.

Ans: f_s(max) = μ_sR (or μ_sN), where μ_s is the coefficient of static friction and R (or N) is the normal reaction.

Q3. Which is greater: μ_s or μ_k?

Ans: μ_s (coefficient of static friction) is greater than μ_k (coefficient of kinetic friction).

Q4. What is kinetic friction?

Ans: Kinetic friction is the opposing force that comes into play when there is actual relative motion between two surfaces in contact.

Q5. What is the direction of friction?

Ans: It acts tangential to the surfaces in contact, opposite to the direction of relative motion or impending relative motion.

✅ 2. Short Answer Questions (2–3 Marks)

Q1. Why does friction arise?

Ans: Friction arises due to two primary causes:

• Interlocking of surface irregularities: No surface is perfectly smooth; microscopic hills and valleys interlock when surfaces press together.
• Molecular Adhesion: Highly localized chemical bonding/attractive forces established at the actual contact points between the molecules of the two surfaces.

Q2. Differentiate between static and kinetic friction.

Ans:

• Static friction operates when the body is at rest relative to the surface; Kinetic friction operates when the body is in relative motion.
• Static friction is a self-adjusting variable force (0 ≤ f_s} ≤ f_s(max)), whereas kinetic friction is nearly constant for a given pair of surfaces.
• The coefficient of static friction (μ_s) is always greater than the coefficient of kinetic friction (μ_k).

Q3. State the laws of limiting friction.

Ans:

1. The magnitude of limiting friction depends entirely on the nature and roughness of the surfaces in contact.
2. It acts tangentially and opposite to the direction of impending motion.
3. The magnitude of limiting friction is directly proportional to the normal reaction (f_s(max) ∝ R).
4. It is independent of the apparent area of contact between the surfaces, as long as the normal reaction remains constant.

Q4. What is limiting friction?

Ans: Limiting friction is the maximum values of static frictional force that comes into action just before a body slides or begins to move over the surface of another body.

✅ 3. Long Answer Questions (5 Marks)

Q1. Explain static and kinetic friction with the help of a suitable graph.

Ans:

When an external force is applied to a body resting on a rough surface, the static friction increases linearly with the applied force to balance it (f = F_applied). This continues up to a threshold limit called limiting friction.

Once the applied force crosses this threshold value, the molecular bonds break, the interlocking is partially overcome, and the body begins to slide. At this point, the friction drops slightly below the limiting value to a steady value called kinetic friction. Further increase in the applied force does not change the kinetic friction value.

(Graph Note: A plot of Friction Force vs. Applied Force shows a straight line at 45° representing the static region, peaks at the limiting friction value, takes a minor downward dip, and transitions into a flat horizontal line representing constant kinetic friction.)

Q2. Derive the mathematical expression for limiting friction.

Ans:

By experimental observation, the limiting friction force (f_s(max)) is found directly proportional to the normal reaction force (R) pressing the surfaces together.

Mathematically:
f_s(max) ∝ R

To eliminate the proportionality sign, we introduce a constant:

f_s(max) = μ_s · R

Where μ_s is the dimensionless constant called the coefficient of static friction. It depends purely on the materials, temperature, and roughness conditions of the touching surfaces.

Q3. Explain why a box placed on the floor of an accelerating train moves forward along with the train. Identify the force responsible.

Ans:

When the train accelerates forward with an acceleration a, an observer inside the non-inertial frame views a pseudo force acting on the box in the backward direction. Relative to the floor of the train, the box has a tendency to slide backward due to inertia.

Because of this impending backward relative motion, a static frictional force acts on the box in the forward direction (tangential to the floor). If this static friction is large enough (f_s = ma) and does not exceed the maximum limiting value (μ_smg), it prevents relative slipping. Therefore, static friction acts as the accelerating force that moves the box forward alongside the train.

✅ 4. Multiple Choice Questions (1 Mark Each)

Q1. Friction always acts:

• A) In the direction of motion
• B) Opposite to the direction of relative motion
• C) Perpendicular to the surface
• D) In a random direction

Ans: B

Q2. Limiting friction is:

• A) Minimum friction
• B) Maximum value of static friction
• C) Kinetic friction
• D) Zero friction

Ans: B

Q3. The coefficient of kinetic friction (μ_k) is generally:

• A) Greater than μ_s
• B) Equal to μ_s
• C) Less than μ_s
• D) Independent of the nature of surfaces

Ans: C

Q4. Frictional force between two solid surfaces depends directly on:

• A) Apparent area of contact
• B) Normal reaction
• C) Speed of sliding only
• D) Mass of the earth

Ans: B

Q5. Kinetic friction acts when:

• A) The body is at rest
• B) There is relative motion between surfaces
• C) No external force is applied
• D) The body moves through deep space

Ans: B

✅ 5. Assertion and Reason Questions

Directions: Choose Option (A) if both Assertion and Reason are true and Reason is correct explanation; Option (B) if both are true but Reason is not correct explanation; Option (C) if Assertion is true but Reason is false; Option (D) if Assertion is false but Reason is false.

Q1.
Assertion (A): Static friction is a self-adjusting force.
Reason (R): It changes its magnitude and direction according to the applied external force up to its maximum threshold limit.

Ans: Both A and R are true, and R is the correct explanation of A.

Q2.
Assertion (A): Kinetic friction is greater than static friction.
Reason (R): Mechanical interlocking between surface irregularities increases once the relative motion starts.

Ans: Both A and R are false. (Kinetic friction is less than static friction, and interlocking decreases during motion).

Q3.
Assertion (A): Friction always opposes the relative motion between surfaces.
Reason (R): Friction always acts opposite to the absolute velocity vector of the body.

Ans: A is true but R is false. (Friction opposes *relative* motion, not necessarily the actual direction of velocity—e.g., in walking or an accelerating train box).

✅ 6. Fill in the Blanks

Q1. Frictional force acts _________ to the surfaces in contact.

Ans: parallel (or tangentially)

Q2. Limiting friction value is given by = _________ × Normal Reaction.

Ans: μ_s (coefficient of static friction)

Q3. For any two given surfaces, μ_k is always _________ μ_s.

Ans: less than

Q4. Friction opposes _________ motion between contact points.

Ans: relative

Q5. Kinetic friction is also broadly referred to as _________ friction when a body slides.

Ans: sliding

✅ 7. Match the Column

Column A	Column B
(1) Static friction	(A) Ratio of limiting friction to normal reaction
(2) Kinetic friction	(B) Force perpendicular to contact plane
(3) μs	(C) Operates under relative motion conditions
(4) μk	(D) Operates under relative rest conditions
(5) N (or R)	(E) Ratio of sliding friction to normal reaction

Correct Match Answers:

(1) → D
(2) → C
(3) → A
(4) → E
(5) → B

✅ 8. Case Study Based Question

Case Background: A heavy block of mass 5 kg is kept stationary on a rough horizontal track surface. A pulling horizontal force is applied to it gradually. It is observed that the block refuses to shift initially, but just starts moving the instant the applied force crosses exactly 20 N.

Q1. What specific term is given to this threshold value of 20 N?

Ans: Limiting friction (f_s(max)).

Q2. What is the value of the frictional force acting when the applied force is only 12 N?

Ans: 12 N. (Before motion starts, static friction is self-adjusting and perfectly balances the applied force).

Q3. What kind of frictional force acts once the block starts moving across the track?

Ans: Kinetic (sliding) friction.

Q4. If the block is in steady sliding motion, will the required force to maintain velocity be less than, equal to, or greater than 20 N?

Ans: Less than 20 N (since kinetic friction is slightly less than limiting static friction).

✅ 9. Statement Based Questions

Q1.
Statement I: Frictional force depends heavily on the visible apparent area of contact.
Statement II: This area dependency law holds true for all macroscopically rigid engineering surfaces.

Ans: Both Statement I and Statement II are false. (Friction is independent of apparent area).

Q2.
Statement I: Friction is a necessary evil that allows humans to walk safely on ground platforms.
Statement II: Without any friction force components acting, walking on a surface is completely impossible.

Ans: Both Statement I and Statement II are true.