O Level / IGCSE Physics · Motion, forces and energy. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.
Physics starts with measuring. Choose the instrument that suits the size: a tape for many metres, a ruler for lengths up to a metre (it reads to 1 mm), and a micrometer for very small lengths such as the diameter of a wire (it reads to 0.01 mm). A measuring cylinder gives the volume of a liquid, and the volume of a solid from how far the water level rises.
A very small length or a very short time is hard to measure once. Measure many together, then divide. This makes the error in each one much smaller.
A scalar has size only. A vector has size and direction.
When timing a pendulum, students say "one" as they start the stopwatch, so they time one swing fewer than they think. Start the count at zero.
Answer: 13 N (not 17 N: forces at right angles cannot simply be added).
Try each one, then open the answer.
Answer: B. A metre rule is marked in millimetres, so readings are given to 0.1 cm. Extra decimal places claim a precision the rule does not have.
Answer: B. Saying "1" at the start means no swing has been completed yet, so "20" marks 19 complete oscillations. Period = 19.0 ÷ 19 = 1.0 s. Dividing by 20 gives the wrong 0.95 s.
Answer: B. Period = time for one swing = 30.0 ÷ 20 = 1.5 s. Timing many swings reduces the effect of reaction time.