Friction: PYQ Walkthrough (1)

Question

A block of mass $5$ kg rests on a horizontal floor. The coefficient of static friction is $\mu_s = 0.4$ and kinetic friction is $\mu_k = 0.3$. A horizontal force of (a) $15$ N, (b) $25$ N is applied. Find the friction force in each case and the acceleration of the block. (Take $g = 10$ m/s².) NEET 2022 pattern.

Solution — Step by Step

Why This Works

The two-faced nature of friction trips students up. Static friction is a self-adjusting force — it equals whatever you push with, up to a maximum. Once you exceed that maximum, the block breaks free, and friction drops to the (usually smaller) kinetic value.

That’s why pushing a heavy box feels hardest at the very start — you’re fighting $\mu_s N$. Once it slides, it slides easily because $\mu_k < \mu_s$.

Alternative Method

Always start by computing $f_{s,\max}$. Then compare the applied force:

• If $F \leq f_{s,\max}$: block stationary, friction $= F$, acceleration $= 0$.
• If $F > f_{s,\max}$: block moves, friction $= \mu_k N$, acceleration $= (F - \mu_k N)/m$.

This decision tree handles every introductory friction problem.