Mechanics and Motion
What the exam tests
Mechanics and Motion covers kinematics in one and two dimensions, Newton's three laws, friction, moments, work, energy, power, momentum and circular motion. Questions typically ask you to choose the correct equation of motion, resolve vectors, draw free-body diagrams and account for energy losses.
Core rules
- Equations of motion apply only for constant acceleration: v = u + at, s = ut + ½at², v² = u² + 2as.
- Weight W = mg uses g = 9.8 m/s²; mass is in kg and does not change with location.
- Newton's second law: net force F = ma; the resultant, not any single force, causes acceleration.
- Momentum p = mv is conserved in any collision or explosion when no external force acts.
- For a body in equilibrium the sum of forces and the sum of moments about any point are zero; moment = force × perpendicular distance.
- Circular motion needs a centripetal force F = mv²/r directed towards the centre.
Worked example
A car accelerates uniformly from rest to 24 m/s in 8 s. Find its acceleration and distance. a = (v − u)/t = (24 − 0)/8 = 3.0 m/s². s = ut + ½at² = 0 + ½(3.0)(64) = 96 m. Check with v² = u² + 2as: 576 = 0 + 2(3.0)s, so s = 96 m. The two routes agreeing is a useful check.
Traps
- Mixing up u and v, or forgetting that deceleration is negative.
- Using g = 9.81 when the paper expects 9.8.
- Treating Newton's third-law pairs as acting on the same body; they never cancel.
- Giving a scalar answer when direction is asked.
- Forgetting to convert cm to m or g to kg before substituting.