JEE Main Physics Laws of Motion 2027: Friction, Banking & Pulleys Made Simple

Ninety percent of Laws of Motion mistakes happen before any calculation starts: a missing force, a wrong direction, a forgotten friction arrow. The fix is one habit. Draw the free-body diagram first. This chapter feeds Work-Energy, Rotation and Electrostatics, so a strong base here lifts your whole Physics score.

If you can draw the forces correctly, the equation almost writes itself.

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Chapter at a Glance

Priority reflects past-paper trends, not an official NTA weightage.

What the NTA Syllabus Covers

• Force and inertia, Newton's first, second and third laws, momentum and impulse
• Conservation of linear momentum and its applications, equilibrium of concurrent forces
• Static and kinetic friction, laws of friction, rolling friction
• Dynamics of uniform circular motion, centripetal force, vehicle on a level road and on a banked road

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Master These Topics

1. Newton's Second Law and the Atwood Machine

The real form of the second law is F = dp/dt, which reduces to F = ma for constant mass. Impulse equals change in momentum: J = FΔt = Δp.

Worked example (Atwood machine): Masses m₁ = 3 kg and m₂ = 2 kg hang over a light frictionless pulley (g = 10).

• Acceleration: a = (m₁ − m₂)g / (m₁ + m₂) = 1 × 10 / 5 = 2 m/s²
• Tension: T = 2m₁m₂g / (m₁ + m₂) = 2 × 3 × 2 × 10 / 5 = 24 N

Check: for the heavier block, T = m₁(g − a) = 3 × 8 = 24 N. Always verify with the second block.

Lift problems: apparent weight is N = m(g + a) when the lift accelerates upward and m(g − a) when it accelerates downward. A 60 kg person in a lift accelerating up at 2 m/s² feels 60 × 12 = 720 N.

2. Friction: Static, Kinetic and the Angle of Repose

Static friction adjusts itself up to a limit: f_s ≤ μ_s N. Once sliding starts, kinetic friction is f_k = μ_k N, usually smaller than the static limit.

• On a rough incline, a block just starts sliding when tanθ = μ_s. That angle is the angle of repose.
• Friction is not always opposite to motion. It opposes relative motion (or the tendency for it) between surfaces.

Trap: Never write f = μN when the block is not on the verge of slipping. Static friction equals whatever value keeps the block at rest, up to the maximum.

3. Circular Motion: Level Roads and Banked Roads

Friction supplies the centripetal force on a level road, so the maximum safe speed is v = √(μrg). With μ = 0.4 and r = 40 m: v = √(0.4 × 10 × 40) = √160 ≈ 12.6 m/s.

On a banked road of angle θ with no friction needed, v = √(rg tanθ). With friction, the limiting speed is v² = rg (tanθ + μ) / (1 − μ tanθ). Banking lets vehicles turn at higher speeds without depending on tyre grip alone.

4. Conservation of Linear Momentum

With no external force on a system, total momentum stays constant. This explains gun recoil: a 2 kg gun firing a 20 g bullet at 300 m/s recoils at 0.02 × 300 / 2 = 3 m/s.

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Common Traps to Avoid

• Skipping the free-body diagram and losing a force.
• Applying f = μN when friction is static but below its limit.
• Forgetting the pseudo force when working in an accelerating frame such as a lift.
• Mixing up the angle of banking with the angle of repose.

60-Second Revision Sheet

• F = dp/dt, impulse J = Δp
• Atwood: a = (m₁ − m₂)g/(m₁ + m₂), T = 2m₁m₂g/(m₁ + m₂)
• Angle of repose: tanθ = μ_s
• Level road: v_max = √(μrg); banked (no friction): v = √(rg tanθ)

Your Study Plan

1. Day 1: free-body diagrams for 15 mixed problems, no calculation.
2. Day 2: pulleys and connected bodies, including wedge and incline setups.
3. Day 3: friction on inclines and block-on-block problems.
4. Day 4: circular motion, banking and pseudo-force problems in a timed set.

Practice Laws of Motion Questions Free → (opens in a new tab)

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Continue Your Physics Journey

• Previous chapter: Kinematics
• Next chapter: Work, Energy & Power
• All 20 JEE Main Physics chapters
• Complete JEE Main Syllabus 2027 guide

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