Mechanics
Mechanics is the LC Physics domain that tests straight-line motion, Newton’s laws, momentum, energy, moments and circular motion. Questions usually combine a calculation with a statement of the relevant principle, so show the equation, substitute with units and state the direction or conservation condition.
Core rules
- Use the suvat equations only when acceleration is constant. Choose a positive direction and keep it for displacement, velocity and acceleration.
- Newton’s second law is F_net = ma; weight is W = mg. Action–reaction pairs act on different bodies.
- Momentum is a vector: p = mv. In a closed system total momentum is conserved.
- Work done is W = Fs cos θ; kinetic energy is ½mv²; gravitational potential energy near Earth is mgh.
- For uniform circular motion, a = v²/r towards the centre and F = mv²/r.
- A moment is Fd, where d is the perpendicular distance from the pivot.
Worked example
A 1200 kg car travelling at 20 m/s brakes to rest over 50 m. Find the acceleration and the average braking force. Using v² = u² + 2as: 0 = 20² + 2a(50), so 100a = −400 and a = −4.0 m/s². The negative sign means the acceleration is opposite the motion. Then F = ma = 1200(−4.0) = −4800 N; the magnitude is 4.8 × 10³ N. The time follows from v = u + at: 0 = 20 − 4t, so t = 5.0 s.
Traps
Do not use suvat for changing acceleration. Do not cancel momentum without direction. Do not treat an action–reaction pair as balanced on one object. Convert cm to m before moments. Keep significant figures consistent with the data.