Mechanics and Motion
Core concepts
A candidate should be fluent with kinematics in one and two dimensions, Newton's three laws, free-body diagrams, friction, circular motion, work–energy and conservation of momentum. Distinguish scalar (speed, distance, mass, energy) from vector (velocity, displacement, force, momentum) quantities, and know that acceleration is the slope of a velocity–time graph while displacement is the area under it.
Common traps
- Mass vs weight: weight is mg and changes with g; mass does not.
- Net force: a body moving at constant velocity has zero net force, not a net force in the direction of motion.
- Projectiles: horizontal velocity is constant (no air resistance) while vertical motion accelerates at g; the two components are independent.
- Action–reaction pairs act on different bodies and never cancel on one object.
- Energy conservation fails if friction is present unless you account for the heat generated.
Worked example
A 2 kg block slides from rest down a frictionless 5 m incline at 30°. Height dropped = 5 sin30° = 2.5 m. Using energy: v = √(2gh) = √(2 × 9.8 × 2.5) ≈ 7.0 m/s. Note the mass cancels - a classic distractor is to give mass and tempt a force calculation.
Rule of thumb
Always draw the free-body diagram before writing equations; resolve forces along and perpendicular to the motion, and check units (N = kg·m/s²).