JEE Physics Laws of Motion Friction Static Kinetic Rolling

Laws of Motion — Friction: Static, Kinetic, Rolling

What This Is and Why It Matters for JEE

Friction is a crucial topic in Physics that appears in 2-3 questions every year in JEE Main and Advanced. It's a moderate-level topic that requires a good understanding of the concepts. Friction is more important for JEE Advanced, where it can be a part of multi-stage problems.

Prerequisites

• Force and Motion: Understand the basics of force, motion, and Newton's laws.
• Work and Energy: Familiarity with work, energy, and power concepts.
• Vector Algebra: Basic understanding of vector operations and components.

Quick Revision Path

If you're not familiar with these topics, quickly revise them using your class notes, textbooks, or online resources.

Core Concepts (Exam-Focused)

• Static Friction: opposes motion between two surfaces in contact. Formula: F_s = μ_s N, where F_s is static friction, μ_s is static friction coefficient, and N is normal force.
• Kinetic Friction: opposes motion between two surfaces in contact when they are already moving. Formula: F_k = μ_k N, where F_k is kinetic friction, μ_k is kinetic friction coefficient, and N is normal force.
• Rolling Friction: opposes rolling motion between two surfaces in contact. Formula: F_r = μ_r N, where F_r is rolling friction, μ_r is rolling friction coefficient, and N is normal force.
• Coefficient of Friction: a dimensionless quantity that depends on the surfaces in contact. Formula: μ = F / N, where μ is coefficient of friction, F is frictional force, and N is normal force.

Step-by-Step Problem-Solving Strategy

1. Identify the type of friction involved (static, kinetic, or rolling).
2. Determine the normal force (N) acting on the object.
3. Use the appropriate friction formula to calculate the frictional force (F_s, F_k, or F_r).
4. Check for any special conditions or assumptions in the problem.
5. Verify the unit of the answer using dimensional analysis.

⚠️ Common mistake: Forgetting to check the unit of the answer.

Important Graphs / Diagrams

• Friction vs. Normal Force Graph: a graph showing how frictional force changes with normal force.
• Coefficient of Friction vs. Surface Roughness Graph: a graph showing how coefficient of friction changes with surface roughness.

Typical JEE Question Patterns

• Find minimum value of...: Find the minimum value of kinetic energy or potential energy.
  • Go-to method: Use the formula for kinetic energy (KE = ½ mv^2) or potential energy (PE = mgh).
• Compare time periods...: Compare the time periods for two different objects or scenarios.
  • Go-to method: Use the formula for time period (T = 2π √(m/k)) or the equation of motion.

Common Mistakes & Exam Traps

• The mistake: Forgetting to consider the direction of frictional force.
• Why it happens: Misunderstanding the concept of friction or rushing through the problem.
• How to avoid it: Always consider the direction of frictional force and draw a free-body diagram.

• Exam board insight: The examiners may penalize you for not considering the direction of frictional force.

• The mistake: Using the wrong formula for friction.

• Why it happens: Misreading the problem or not checking the unit of the answer.
• How to avoid it: Read the problem carefully and check the unit of the answer using dimensional analysis.

• Exam board insight: The examiners may penalize you for using the wrong formula or not checking the unit of the answer.

• The mistake: Forgetting to consider the coefficient of friction.

• Why it happens: Misunderstanding the concept of friction or not considering the surface roughness.
• How to avoid it: Always consider the coefficient of friction and the surface roughness.
• Exam board insight: The examiners may penalize you for not considering the coefficient of friction.

Time-Saving Shortcuts

• Use the formula for friction: Instead of deriving the formula for friction, use the given formula and plug in the values.
• Check the unit of the answer: Use dimensional analysis to check the unit of the answer and avoid errors.

Practice MCQs (Exam-Style)

Question 1: A block of mass 5 kg is placed on a rough surface with a coefficient of friction μ = 0.5. What is the maximum force that can be applied to the block without moving it?

A) 10 N B) 15 N C) 25 N
D) 30 N

Answer: C) 25 N
Solution: F_s = μ_s N = 0.5 × 5 × 9.8 = 24.5 N Common Wrong Answer: A) 10 N, because the student forgot to consider the coefficient of friction.

Question 2: A car of mass 1500 kg is moving on a road with a coefficient of friction μ = 0.2. What is the force of friction acting on the car when it is moving at a speed of 20 m/s?

A) 300 N B) 400 N C) 500 N D) 600 N

Answer: D) 600 N
Solution: F_k = μ_k N = 0.2 × 1500 × 9.8 = 294 N Common Wrong Answer: A) 300 N, because the student forgot to consider the coefficient of friction.

Question 3: A wheel of radius 0.5 m is rolling on a rough surface with a coefficient of friction μ = 0.1. What is the force of friction acting on the wheel when it is rolling at a speed of 10 m/s?

A) 2 N B) 3 N C) 4 N D) 5 N

Answer: D) 5 N
Solution: F_r = μ_r N = 0.1 × 9.8 = 0.98 N Common Wrong Answer: A) 2 N, because the student forgot to consider the coefficient of friction.

Quick Revision Card (60-Second Summary)

• Static Friction Formula: F_s = μ_s N
• Kinetic Friction Formula: F_k = μ_k N
• Rolling Friction Formula: F_r = μ_r N
• Coefficient of Friction: μ = F / N
• Dimensional Analysis: Check the unit of the answer using dimensional analysis.

If You Get Stuck in Exam

• Write partial marks: If you're unsure of the answer, write down the partial marks you can get.
• Eliminate distractors: Eliminate the options that are clearly incorrect.
• Skip and return: If you're stuck, skip the question and return to it later.

Related JEE Topics

• Work and Energy: Friction is related to work and energy, as it can convert kinetic energy into heat energy.
• Motion in a Straight Line: Friction is related to motion in a straight line, as it can affect the motion of an object.
• Circular Motion: Friction is related to circular motion, as it can affect the motion of an object on a circular path.