Laws of Motion: Complete JEE Notes

Newton’s Three Laws

1. First Law (Inertia) — a body remains at rest or in uniform motion unless acted upon by a net external force.
2. Second Law — F = dp/dt, which reduces to F = ma for constant mass.
3. Third Law — every action has an equal and opposite reaction; the two forces act on different bodies.

Friction

Static friction opposes the tendency of relative motion, while kinetic friction opposes actual relative motion. The maximum static friction is generally greater than kinetic friction:

f_s(max) = μ_s N
f_k = μ_k N

Circular Motion & Pseudo-Forces

In a rotating (non-inertial) frame, a centrifugal pseudo-force of magnitude mω²r must be added, directed outward, for Newton’s laws to hold.

JEE Problem-Solving Strategy

• Always draw a free body diagram (FBD) first.
• Resolve forces along and perpendicular to the direction of motion.
• For connected bodies, use the constraint relation before writing equations.
• Check units and limiting cases (e.g., μ → 0, θ → 0) to validate your answer.

Common Mistakes

• Forgetting the pseudo-force in accelerating (non-inertial) reference frames.
• Mixing up maximum static friction with kinetic friction in the same problem.
• Ignoring normal force changes on inclined planes with applied forces.