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.