PREPWISER.
Practise online

Friction revision notes

Every idea in the Friction chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 22 and download the PDF.

Sign in to read all 22Download PDF

Read the first part free. Sign in free with Google to read the rest and download the PDF: 10 A4 pages.10 A4 pages, ready to print.

How we count: 7× asked means 7 past JEE Main questions in the PrepWiser bank needed that concept, and "last" is the latest year one appeared. ★ marks ideas asked 8 or more times.

Static and kinetic friction

Static friction and limiting friction
7× asked, last 2023
0≤fs≤fmax⁡=μsN0\le f_s\le f_{\max}=\mu_s N
Static friction is the force, up to μsN\mu_s N, that stops slip. At rest on a fixed surface, it cancels the other forces along it.
applied FFfriction ffμsN\mu_sNμsN\mu_sNμkN\mu_kNlimitingstatic: f=Ff=Fkinetic
Static friction matches the push up to μsN\mu_sN, then drops to μkN\mu_kN once sliding starts
  • "Limiting equilibrium" or "just about to move": friction is exactly μsN\mu_s N and balances the other forces along the surface, e.g. two charges qq of mass mm each, LL apart: kq2/L2=μsmgkq^2/L^2=\mu_s mg.
  • Level floor, horizontal push: friction against applied force is a 45∘45^\circ line (f=Ff=F) up to μsN\mu_sN, then a drop to μkN\mu_kN. A push F=ktF=kt starts the block at t0=μsmg/kt_0=\mu_smg/k.
  • Friction against applied force: a 45∘45^\circ line (f=Ff=F) up to μsN\mu_sN, then a drop to μkN\mu_kN. A push F=ktF=kt starts the block at t0=μsmg/kt_0=\mu_smg/k.
Kinetic friction
7× asked, last 2024
fk=μkN,μk<μsf_k=\mu_k N,\qquad \mu_k<\mu_s
Acts once the surfaces slide; nearly constant and independent of speed. Opposes velocity relative to the surface.
  • Not g=10g=10 by habit. Use the gg the question gives (often 9.8), because the options usually include μmg\mu mg worked out with both values.
Laws of friction
1× asked, last 2024
  • Limiting friction is proportional to NN and depends on the materials and state of the surfaces. μ\mu has no unit; usually μk<μs\mu_k<\mu_s.
  • Not dependent on area of contact. Friction comes from the real contact area, which grows with NN, not with the apparent area. Both static and kinetic friction are independent of area.
Sliding to rest on level ground
4× asked, last 2023
a=μkg,s=u22μkg,t=uμkga=\mu_k g,\qquad s=\frac{u^2}{2\mu_k g},\qquad t=\frac{u}{\mu_k g}
Only friction acts along the floor, so the mass cancels. aa here is the retardation.
  • Applied force FF along a level floor: F−μkmg=maF-\mu_k mg=ma. Get aa from s=ut+at2/2s=ut+at^2/2 or v=u+atv=u+at, then solve for μk\mu_k.
  • Bag dropped gently on a belt moving at vv: friction speeds it up at μg\mu g until it reaches vv. It slips for t=v/μgt=v/\mu g, covers v2/2μgv^2/2\mu g on the ground, and slips the same v2/2μgv^2/2\mu g relative to the belt.
Work done against friction
3× asked, last 2024
Wf=fk d=μkNdW_f=f_k\,d=\mu_k N d
N=mgN=mg on a level floor, mgcos⁡θmg\cos\theta on an incline (if no other force presses). Work by friction on the body is −Wf-W_f.
  • Not the work of the applied force. Work against friction is only μkNd\mu_k Nd, because the rest of the push goes into potential and kinetic energy.
  • Quick method. Rest to rest over a path with smooth and rough parts: loss of PE =μkN×=\mu_k N\times(rough length only). This skips finding the speed at each join.
Rolling vs sliding friction
1× asked, last 2021
frolling≪fk<fmax⁡f_{\text{rolling}}\ll f_k<f_{\max}
Rolling friction is far smaller than sliding friction; hence wheels, ball bearings and lubricants.
  • Rolling without slipping needs only static friction, anywhere up to μsN\mu_s N, and on a fixed surface it does no work. If the set-up forces the contact point to slide, friction is kinetic: μkN\mu_k N.

Practise Friction questions as you read. Free, and you can start without signing up.

Practise Friction

16 more concepts in 5 topics

Angle of friction and angle of repose; Forces at an angle, walls and minimum force; Motion on a rough incline; and Two blocks, accelerating frames and chains, plus ideas from Newton's laws. Read the first part free. Sign in free with Google to read the rest and download the PDF.

Sign in to read the restYou come straight back to this spot.Download the PDF

Angle of friction and angle of repose

  • Angle of friction2×
  • Angle of repose5×
  • Resting on a curved rough surface3×

Forces at an angle, walls and minimum force

  • Pulling or pushing at an angle3×
  • Force to just start motion2×
  • Minimum force to move a block1×
  • Block pressed against a wall2×
  • Sliding or toppling1×
  • Also remember1

Motion on a rough incline

  • Sliding down a rough incline7×
  • Moving up a rough incline2×
  • Up and back over the same distance2×
  • Rough vs smooth incline: time ratio3×
  • Force along the incline: push up or hold3×

Two blocks, accelerating frames and chains

  • Two blocks: together or slipping3×
  • Box on an accelerating truck
  • Chain hanging off a rough table1×

Also asked here: Newton's laws

  • Key points1

Practise Friction next

About 45 past JEE Main questions from this chapter, with instant answer checking and step-by-step solutions where available. Free, and you can start without signing up.

Practise Friction

Also free on PrepWiser

Spotted a mistake? Write to support@prepwiser.in and we will fix it in the next version.

Version 1, October 2026. Free to share for study, as it is. Please don't sell it or remove the PrepWiser name.

PractiseSign in to read all 22Download PDF
Free with Google, takes a few seconds10 pages, free