Grade 12 physics has four units: mechanics, current electricity, electromagnetism and modern physics. In every unit, the first decision is which law applies. These three original practice problems show how to make that decision and check the result.
Example 1: a perfectly inelastic collision
A 2 kg trolley moving at 3 m/s hits a 1 kg trolley at rest, and they stick together. Find their speed and the kinetic energy lost. Law: conservation of momentum (always true in collisions). 2 × 3 = (2 + 1) × v, so v = 2 m/s. Kinetic energy before: ½ × 2 × 3² = 9 J; after: ½ × 3 × 2² = 6 J; lost: 3 J.
Common mistake: using conservation of kinetic energy. In an inelastic collision momentum is conserved but kinetic energy is not.
Example 2: torque at an angle
A 10 N force acts 0.5 m from a pivot. Find the torque when the force is perpendicular to the arm, and when it makes 30° with the arm. Torque = F × r × sin θ. Perpendicular: 10 × 0.5 × 1 = 5 N·m. At 30°: 10 × 0.5 × 0.5 = 2.5 N·m.
Common mistake: forgetting sin θ, or using the angle with the wrong line. θ is the angle between the force and the arm.
Example 3: half-life
A sample of 80 g has a half-life of 5 days. How much is left after 15 days? 15 ÷ 5 = 3 half-lives: 80 → 40 → 20 → 10 g.
Common mistake: subtracting a fixed amount each period. Each half-life halves what is left, so the amount never reaches zero in a whole number of steps.
Before calculating, name the law: conservation of momentum, torque, Ohm's law, Lenz's law, half-life. The law tells you which quantities matter.
How EXAMEV helps
The EXAMEV grade 12 physics bank has 1,254 questions across all four units, plus a final-exam style section, with explanations. The examples here are EXAMEV practice.