Friction — Prelims Strategy

Prelims Strategy

To ace friction questions in NEET, a systematic approach is essential. Firstly, always draw a clear Free-Body Diagram (FBD) for each object involved, showing all forces acting on it, including gravitational force ($mg$), normal force ($N$), applied forces, and friction ($f$).

Secondly, correctly identify the type of friction acting: static or kinetic. If the object is at rest or on the verge of motion, use static friction ($f_s \le \mu_s N$). If it's already sliding, use kinetic friction ($f_k = \mu_k N$).

Remember that $\mu_k < \mu_s$.

For inclined plane problems, resolve forces into components parallel and perpendicular to the incline. This simplifies calculations significantly. The normal force is always perpendicular to the surface. For problems involving circular motion, recognize that friction often provides the necessary centripetal force ($F_c = mv^2/r$ or $m r \omega^2$).

Pay close attention to the direction of friction: it always opposes the *relative motion* or *tendency of relative motion* between the surfaces. Don't assume it always opposes the overall motion of the object.

Practice problems involving multiple blocks, where friction might act on one or both, and where tension in strings also plays a role. Be careful with units and conversions, especially when angular velocity is involved.

Lastly, review the concepts of angle of friction and angle of repose, as they provide quick ways to determine $\mu_s$ for inclined planes.