Mechanics and Motion
Mechanics and Motion on ORT Physics tests how fluently you move between describing motion (kinematics) and explaining it through forces, momentum and energy. Questions tend to be short calculations with conceptual distractors rather than long derivations.
Core rules
- For constant acceleration use v = u + at, s = ut + ½at² and v² = u² + 2as. Fix a positive direction once and keep it through the whole solution.
- Newton's second law uses the resultant force: F = ma. Weight W = mg acts at the centre of gravity; tension and normal reaction are contact forces that adjust to the constraints.
- Momentum p = mv is conserved in collisions and explosions when external forces are negligible. Kinetic energy is conserved only in elastic collisions.
- Work done W = Fs cosθ; gravitational potential energy ΔEp = mgΔh; kinetic energy Ek = ½mv². Friction transfers mechanical energy to thermal stores.
- Circular motion needs a centripetal resultant F = mv²/r directed towards the centre. It is a resultant, not an extra force to add to a free-body diagram.
Worked example
A ball is thrown horizontally at 5.0 m/s from a cliff 20 m high. Take g = 9.8 m/s² and ignore air resistance. Vertically: 20 = ½ × 9.8 × t², so t = √(40/9.8) = 2.02 s. Horizontal velocity stays 5.0 m/s, so range = 5.0 × 2.02 = 10.1 m. At impact the vertical velocity is v_y = gt = 19.8 m/s; speed = √(5.0² + 19.8²) = 20.4 m/s, at about 76° below the horizontal. The independence of horizontal and vertical motion is the key idea.
Traps
- Applying suvat during a phase where acceleration changes, for example after a string slackens.
- Writing friction on an incline as μmg rather than μmg cosθ.
- Treating centripetal force and weight as separate forces in a vertical circle.
- Sign errors when an object reverses direction.
- Assuming tension equals weight in an accelerating lift or pulley system.