ZIMSEC O Level · Combined Science · Form 1, Form 2, Form 3
Measurement and SI units
Length – metre (m); mass – kilogram (kg); time – second (s); temperature – kelvin (K) or °C; current – ampere (A).
Density = mass ÷ volume (units g/cm³ or kg/m³). Find the volume of an irregular solid by displacement of water.
Forces
A force is a push or pull, measured in newtons (N) with a spring balance.
- Weight = mass × gravitational field strength (W = mg, with g ≈ 10 N/kg on Earth).
- Friction opposes motion; it is useful (brakes, walking) but also causes wear and heat.
- A turning effect (moment) = force × perpendicular distance from the pivot. In balance, clockwise moments = anticlockwise moments.
Motion
- Speed = distance ÷ time.
- Acceleration = change in velocity ÷ time taken (m/s²).
- On a distance–time graph the gradient is speed; on a velocity–time graph the gradient is acceleration and the area under the graph is the distance.
Worked example: A stone has a mass of 60 g and a volume of 24 cm³. Find its density.
Density = 60 ÷ 24 = 2.5 g/cm³
Worked example: A car speeds up from 10 m/s to 30 m/s in 5 s. Find its acceleration.
a = (30 − 10) ÷ 5 = 4 m/s²
Exam tip: Mass (kg) and weight (N) are different. Your mass is the same on the Moon; your weight is less.
Check yourself
- Find the weight of a 50 kg learner (g = 10 N/kg).
- A runner covers 400 m in 80 s. Find her average speed.
- A 2 N force acts 0.3 m from a pivot. Find its moment.
Show answers
- 500 N
- 5 m/s
- 0.6 N m
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