Chapter 1: Newton's Laws of Motion
Newton's First Law defines inertia — the property of matter by which it resists any change in its state of rest or uniform motion. No net force means no acceleration. This law also implicitly defines inertial reference frames, which are the frames in which all three laws hold.
Newton's Second Law is the quantitative workhorse: F_net = dp/dt = ma (for constant mass). Force, momentum, and impulse all share the same dimensional structure at their core. Impulse J = F = equals the area under the F–t graph. Momentum is conserved in isolated systems (F_ext = 0): the total momentum before and after any interaction is equal.
Newton's Third Law states that action and reaction forces are equal in magnitude, opposite in direction, and act on different bodies. This "different bodies" condition is the NEET examiner's favourite trap. Normal force and weight both act on the same body — they are balanced forces resulting from Newton's Second Law (a = 0), not a Third Law pair.
Chapter 2: Apparent Weight and Lift Problems
The "apparent weight" of a person in a lift is the normal force the lift floor exerts — what a weighing scale reads. Applying Newton's Second Law to the person: N − mg = ma (taking up as positive). Therefore N = m(g + a) when accelerating upward and N = m(g − a) when accelerating downward. The four key cases — at rest, accelerating up, accelerating down, and free fall — each have a specific formula and a "feel" associated with them.
Chapter 3: Atwood Machine
The Atwood machine is a two-mass pulley system that converts the weight difference into a net force, accelerating both masses. The derivation requires writing separate FBD equations for each mass, adding them to eliminate tension, solving for acceleration, then back-substituting for tension. The tension in the connecting string is always less than the heavier weight and greater than the lighter weight.
Chapter 4: Friction
Static friction is the adhesive force between surfaces that prevents relative sliding. It is self-adjusting — it provides exactly the force needed to prevent motion up to a maximum of μ_s N (limiting friction). Once the applied force exceeds μ_s N, the body begins to slide and kinetic friction f_k = μ_k N takes over (with μ_k < μ_s). Rolling friction is much smaller than kinetic friction, which is why wheels drastically reduce energy loss in transport. The angle of repose (tan θ = μ_s) is the maximum incline angle for which a body remains stationary.
Chapter 5: Circular Motion Dynamics
Any body moving in a circle requires a centripetal force (inward) = /r. This force is always provided by existing forces in the system: friction on level roads, the horizontal component of normal force on banked roads, tension for a stone in circular motion, gravity for satellites. Banking a road at angle θ = arctan(/rg) allows vehicles to negotiate curves at speed v without any friction, which is why highways have banked curves.