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Exam level3 past questions

Friction and its applications

Study the types of friction (static and kinetic), their effects on motion, and how friction interacts with other forces in different scenarios.

Why this shows up in the exam

Questions often require you to calculate or reason about frictional forces in real and idealized problems.

How NEET tests this

Numerical · 3 QsApplicationStatement analysis

Learn the idea

Friction is the contact force parallel to a surface that resists relative motion. The decisive insight is that static friction can vary up to a maximum μ_s N, so you compare the needed force with this limit to decide if slipping occurs.

🧠 Memory hook: Static friction is like a rubber band that stretches up to μ_s N; kinetic friction is the snapped band that stays at μ_k N.

Get this one clearly and it pays off every single time it shows up in the paper. 🎯

Formulas & facts to keep ready

  • Friction acts parallel to the contact surface and opposite to the direction of relative motion or impending motion
  • Maximum static friction f_s(max)=μ_s N
  • Kinetic friction f_k=μ_k N (constant magnitude)
  • On a horizontal plane the normal reaction N equals mg
  • Static friction adjusts itself up to its limiting value; kinetic friction does not

How to approach it

  1. 1Identify whether the body is at rest relative to the surface → use static friction
  2. 2Write the horizontal force required for the motion (ma)
  3. 3Set ma ≤ μ_s mg; for the limiting case use equality and solve for a
  4. 4If slipping is assumed, replace μ_s by μ_k and use f_k=μ_k mg

Worked example — watch it click

A box of mass 15 kg is kept on the floor of a stationary trolley. The coefficient of static friction between the box and the trolley is 0.12. Keeping the box stationary state over the trolley, the maximum acceleration with which the trolley can be moved horizontally in m s⁻² is: (g = 10 m/s²)

  • A)1.8
  • ✅1.2
  • C)1.5
  • D)2.1

The concept behind this problem

The worked example forces you to equate the required horizontal force (ma) to the maximum static friction μ_s mg, revealing the highest acceleration the trolley can have without the box slipping.

Step by step

  1. 1For the box to remain stationary on the accelerating trolley, the friction force must provide the required force: f = ma.
  2. 2Maximum static friction: f_max = μ_s × N = μ_s × mg.
  3. 3For maximum acceleration without slipping: ma_max = μ_s × mg, so a_max = μ_s × g = 0.12 × 10 = 1.2 m/s².

Watch out

A common mistake is to plug μ_k or treat friction as μ N without first matching the required force to the static‑friction limit.

Common slip-ups that cost marks

  • •Using the kinetic coefficient when the body is still stationary (static case)
  • •Forgetting that N=mg on a horizontal surface and thus missing the factor of g
  • •Assuming friction always equals μ N even when the required force is smaller than the limiting value

🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.

Practise it

These are real questions from past NEET papers that test this exact idea.

Question 1 of 3

Which one of the following statements is incorrect?

Push further

More challenging

9 harder questions built from the past papers above — a step up in difficulty, with distractors designed so you can't get there by elimination. Written and checked by our reviewers, not from a real paper.

Question 1 of 9

A small coin is placed on a horizontal rotating turntable. The coefficient of static friction between the coin and the turntable is 0.3. If the turntable is rotating at 30 revolutions per minute, what is the maximum distance from the center at which the coin can be placed without slipping? (Take g = 10 m/s² and π = 3.14)

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