Laws of Motion
What this domain tests
Laws of Motion covers inertia, Newton's three laws, linear momentum and impulse, friction, circular-motion dynamics, banking of tracks and apparent weight in lifts.
Core rules
- Newton's second law: F_net = dp/dt = ma when mass is constant.
- Impulse J = FΔt = Δp.
- Static friction f_s ≤ μ_sN; kinetic friction f_k = μ_kN, usually with μ_k < μ_s.
- For a banked road without friction, tanθ = v²/(rg).
- In a lift: N = m(g ± a), using + for upward acceleration and − for downward acceleration.
Worked example
A 60 kg person stands in a lift accelerating upward at 2 m/s². Taking g = 9.8 m/s², N − mg = ma, so N = m(g + a) = 60 × 11.8 = 708 N. The scale reads about 708 N, not the true weight of 588 N. If the lift accelerated downward at 2 m/s², N = 60 × 7.8 = 468 N.
Traps
Treating action and reaction as forces on the same body; adding a separate 'centripetal force' alongside tension or friction; reversing the direction of kinetic friction; using μ_sN as the actual friction when the body is not at the verge of slipping; forgetting that apparent weight changes while true weight does not.