Mechanics
This section covers Mechanics. The exam tests kinematics, Newton's laws, momentum, work, energy and power, often through multistage motion or force problems.
Core rules
- Use suvat equations only for constant acceleration. Identify u, v, a, s and t, choose the equation with one unknown, and keep a sign convention.
- On displacement-time graphs, gradient is velocity; on velocity-time graphs, gradient is acceleration and area is displacement.
- Newton's second law: resultant force = mass × acceleration. Friction opposes relative motion. Weight = mg, while mass is invariant.
- Momentum p = mv is a vector. In a closed system total momentum is conserved; collisions are elastic if kinetic energy is also conserved.
- Work done = force × displacement in the force direction. Power = work/time. Mechanical energy is conserved without non-conservative work.
Worked example
A 2.0 kg block starts from rest and is pushed by a constant net force of 6.0 N for 3.0 s. Acceleration a = F/m = 6.0/2.0 = 3.0 m/s². Final velocity v = u + at = 0 + 3.0 × 3.0 = 9.0 m/s. Displacement s = ut + ½at² = 0 + ½ × 3.0 × 3.0² = 13.5 m. Net work = Fs = 6.0 × 13.5 = 81 J, equal to final kinetic energy ½mv² = ½ × 2.0 × 9.0² = 81 J.
Traps
Use resultant force, not a single applied force, when finding acceleration. Do not treat weight as mass. Include direction signs for velocity, acceleration and momentum. In inelastic collisions momentum is conserved but kinetic energy is not. The area under a velocity-time graph gives displacement, not distance, unless you account for sign.