What can a force do?
Change the speed, change the direction, change the shape. Then drive a car and see all three.
When you drive, the engine, the brakes, the steering and a bump in the road all use forces. What is each one actually doing to the car?
A force acts in the same direction as an object’s velocity. The object…
A satellite moves in a circle at constant speed. The force on it is…
A 2.0 kg block has an acceleration of 1.0 m s⁻². The net force on it is…
A car goes round a bend at a steady speed. The sideways friction force from the tyres changes the car’s…
A spring with k = 200 N m⁻¹ is pulled with a force of 10 N. The extension is…
A speed breaker pushes a wheel upwards and squeezes the suspension spring. Which effect of a force is this?
A car goes round a bend at a steady 10 m s⁻¹. Explain, using what a force can do, why the driver still needs a force although the speed does not change.
For constant mass, F = …
If F_net = 0 the velocity…
Key points
- • A force can change the speed (along v), the direction (at 90° to v) or the shape of an object.
- • Speed: F = ma. Direction: F = mv² / r. Shape: F = kx.
- • Engine force is with the velocity, braking force is against it, tyre friction is across it.
Formulas
- F = ma
- F = mv² / r
- F = kx
Key terms
- Centripetal force :
- the force towards the centre that keeps an object moving in a circle
- Hooke’s law :
- extension is proportional to the force, F = kx, within the limit of proportionality
Common errors
- • Thinking a force is needed to keep an object moving at constant speed.
- • Thinking a force at 90° to the velocity speeds the object up.
- • Forgetting that Hooke’s law only holds up to the limit of proportionality.