Newton's three laws of motion form the bedrock of classical mechanics and together account for 3–4 questions per NEET paper. Understanding them at a deep conceptual level, not merely formula-level, is the key to handling both direct and tricky questions.
Newton's First Law — Law of Inertia: A body at rest remains at rest, and a body in uniform motion continues in the same straight line, unless compelled to change by a net external force. Inertia is the inherent resistance to any change in state of motion, and it is directly proportional to mass. This law defines what an inertial reference frame is: one in which bodies obey F = 0 → a = 0. Common NEET misconception: "force is needed to maintain motion." No — force is needed only to CHANGE motion.
Newton's Second Law: The net external force on a body equals the rate of change of its linear momentum: F = dp/dt. For constant mass, this reduces to F = ma. The dimensional formula of force is [M^{1}$$L^{1}$$T^{-2}] and its SI unit is Newton (N). Linear momentum p = mv has dimension [M^{1}$$L^{1}$$T^{-1}] and is measured in kg·m/s. Impulse J = F· = represents the change in momentum produced by a force acting over a time interval; it equals the area under a force-time graph. Momentum is conserved in a system whenever the net external force on the system is zero: m_{1}u_{1} + m_{2}u_{2} = m_{1}v_{1} + m_{2}v_{2}.
Newton's Third Law: For every action, there is an equal and opposite reaction acting on a different body: F_AB = −F_BA. The critical qualifier "different bodies" is the most-tested point. The normal force N and the weight mg acting on the same block are NOT an action-reaction pair — they are balanced forces on the same body. The true action-reaction pair for the book's weight is: Earth pulling book down (mg) and book pulling Earth up (mg). NEET trap: option (a) N and mg → wrong; option (b) mg and gravitational pull of book on Earth → correct.
Apparent Weight in a Lift: A person of mass m in a lift experiences a normal force (apparent weight W') from the floor that depends on the lift's acceleration. Taking upward as positive: when the lift accelerates upward at a, W' = m(g + a) — person feels heavier. When it accelerates downward at a, W' = m(g − a) — person feels lighter. In free fall (a = g downward), W' = 0 — complete weightlessness. A weighing machine in the lift reads apparent weight, not true weight mg.
Atwood Machine: Two masses m_{1} > m_{2} connected by a light string over a frictionless, massless pulley. Free body diagrams for each mass give two equations that are solved simultaneously. Acceleration: a = (m_{1} − m_{2})g / (m_{1} + m_{2}). Tension: T = 2m_{1}m_{2}g / (m_{1} + m_{2}). The tension always satisfies m_{2}g < T < m_{1}g — it is between the two individual weights. If m_{1} = m_{2}, then a = 0 and T = mg: the system stays still with string bearing full weight of each mass.
Friction: Three types exist in order of decreasing magnitude. Static friction f_s is self-adjusting: it ranges from 0 to μ_s N (limiting friction) and precisely matches any applied force below the limit. It does NOT always equal μ_s N — this is the most-tested friction trap on NEET. Kinetic friction f_k = μ_k N is constant once sliding begins. Rolling friction f_r = μ_r N is much smaller still. The inequality μ_s > μ_k > μ_r always holds. On a rough inclined plane, the normal force is N = mg cos θ (not mg), and the weight component along the plane is mg sin θ. The angle of repose θ_r satisfies tan θ_r = μ_s — at this angle, limiting static friction exactly balances the component of weight along the plane.
Circular Motion: For a body in circular motion, Newton's Second Law provides a centripetal acceleration a_c = /r directed toward the centre. The centripetal force F_c = m/r is NOT a separate force — it is provided by real existing forces. On a level road, friction provides centripetal force: μmg = m/r, giving maximum safe speed v_max = √(μrg). For a banked road (no friction), horizontal component of normal force provides centripetal force, giving tan θ = /(rg). With friction, both N and f contribute, and the maximum and minimum safe speeds can be calculated.
Free Body Diagram (FBD) Method: The universal problem-solving strategy for all Newton's law problems. Step 1: Isolate each body. Step 2: Draw all forces (weight, normal force, tension, friction, applied forces). Step 3: Choose axes (often align one axis with direction of motion or acceleration). Step 4: Write ΣF = ma for each axis. Step 5: Solve simultaneous equations. Never apply F = ma before completing the FBD.