ZIMSEC O Level · Physics · Form 3
A force is a push or pull measured in newtons (N). Forces can change an object's speed, direction or shape.
Newton's laws
- An object stays at rest or moves at constant velocity unless a resultant force acts on it.
- Resultant force = mass × acceleration (F = ma).
- When A exerts a force on B, B exerts an equal and opposite force on A.
Mass and weight
Mass is the amount of matter (kg), the same everywhere. Weight is the gravitational force (N): W = mg.
Friction
Opposes motion. Useful for walking and braking; reduced by lubrication and smooth surfaces.
Stretching
Hooke's law: extension is proportional to load, up to the limit of proportionality (F = kx).
Worked example: A resultant force of 600 N acts on a 1 200 kg car. Find its acceleration.
a = F ÷ m = 600 ÷ 1 200 = 0.5 m/s²
Momentum (extension)
Momentum = mass × velocity; in a collision, total momentum is conserved.
Exam tip: Always use the RESULTANT force in F = ma – subtract friction from the driving force first.
Check yourself
- State Newton's first law.
- Find the weight of a 70 kg man on the Moon, where g = 1.6 N/kg.
- A spring stretches 2 cm with a 4 N load. Find the extension with 10 N (within the limit).
Show answers
- An object remains at rest or at constant velocity unless acted on by a resultant force.
- 112 N
- 5 cm
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