Mechanics
Mechanics is the HKDSE Physics domain that tests how you describe motion, identify forces, and apply conservation laws to particles and simple systems. Questions often combine a graph, a free-body diagram, or a multi-stage motion, so the examiner is checking whether you can choose the right principle rather than just substitute numbers.
Core rules and concepts
- Distinguish scalars from vectors. Resolve forces into perpendicular components and draw a free-body diagram before writing equations.
- Use the equations of uniformly accelerated motion only when acceleration is constant: v = u + at, s = ut + ½at², v² = u² + 2as.
- Apply Newton’s second law to the net force. Centripetal force is the net inward force in circular motion, not an extra force to add.
- Work done is Fs cos θ; gravitational potential energy near Earth is mgh; power is work done per unit time. With friction, mechanical energy is not conserved, but total energy is.
- Momentum is a vector and is conserved in collisions and explosions when external forces are negligible. Impulse equals change in momentum.
Worked example
A 1200 kg car accelerates uniformly from 10 m/s to 25 m/s in 6.0 s. Find the acceleration, net force, distance travelled, and net work done.
Acceleration: a = (25 − 10)/6.0 = 2.5 m/s². Net force: F = ma = 1200 × 2.5 = 3000 N. Distance: s = (u + v)t/2 = (10 + 25)(6.0)/2 = 105 m. Net work: W = Fs = 3000 × 105 = 3.15 × 10⁵ J. The same result comes from the change in kinetic energy, ½ × 1200 × (25² − 10²), which confirms the reasoning.
Traps that cost marks
- Using v = u + at when acceleration changes; split the motion into stages.
- Forgetting that weight acts vertically while normal reaction is perpendicular to the surface.
- Treating centripetal force as a separate outward or inward force in addition to tension, friction, or gravity.
- Losing direction in momentum problems; assign a positive direction and keep signs consistent.
- Mixing work, power, and energy units, especially kW·h and J.