Laws of MotionNEET Physics · Class 11 · NCERT Chapter 4

You can expect 3 to 5 questions from Laws of Motion in NEET 2027. The chapter has very high PYQ frequency. Friction on an inclined plane, the Atwood machine, conservation of momentum and banking of roads are the most heavily tested concepts.

Almost every problem in mechanics, rotational motion, gravitation and even fluid mechanics applies Newton's laws somewhere. Once you can draw a clean free body diagram and write Newton's second law correctly, half the problem is already done.

First law (inertia): an object stays at rest or in uniform motion unless a net force acts on it. Second law: F equals dp by dt, which simplifies to F equals m a for constant mass. Third law: every action has an equal and opposite reaction. The action and reaction act on different bodies.

Static friction acts when surfaces are not yet sliding and adjusts itself up to a maximum f_s = mu_s N. Kinetic friction acts during sliding and equals f_k = mu_k N (a constant). Always mu_s is greater than mu_k for the same surfaces, which is why it takes more force to start moving an object than to keep it moving.

Resolve gravity along and perpendicular to the incline: m g sin theta along, m g cos theta perpendicular. Normal force N = m g cos theta. Maximum static friction = mu_s m g cos theta. Compare m g sin theta with maximum static friction: if larger, the block slides and net force = m g sin theta minus mu_k m g cos theta. Acceleration follows from F = m a.

The angle of repose is the maximum angle of an inclined surface at which a block does not slide. At this critical angle, m g sin theta exactly equals the maximum static friction mu_s m g cos theta, which gives tan theta = mu_s. This is one of the most-tested NEET facts in this chapter.

For a road banked at angle theta with no friction, the maximum safe speed is v = square root of (r g tan theta). With friction (coefficient mu), the safe speed range is from v_min equals square root of (r g (tan theta minus mu) over (1 plus mu tan theta)) up to v_max equals square root of (r g (tan theta plus mu) over (1 minus mu tan theta)).