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Turning needs a force

Forces and motion · level 4, lesson 5 of 6

A question to start

You swing a ball on a string round your head, then let go. Does it fly straight out from the middle, or off along the circle, or somewhere else? And what was the string doing before you let go?

The idea

Anything moving keeps going in a straight line unless a force acts on it. So to make something go round a curve, something must keep pulling or pushing it sideways, towards the inside of the curve.

  • The string pulls the swinging ball inwards.
  • Friction between the tyres and the road pushes a turning car inwards.
  • Gravity pulls the Moon towards the Earth and keeps it going round.

If that inward force stops, the thing carries straight on, along the direction it was going at that moment. The ball you let go of flies off along the circle's edge, not straight outwards.

string pulls inlet go: straight on

The faster you go round, or the tighter the curve, the bigger the inward force needed. That is why a car must slow down for a sharp bend: the tyres can only grip so hard.

On Mars, gravity is weaker, so a vehicle presses less on the ground and its tyres grip less. It has to take bends more slowly than it could on Earth. A Mars rover, at 4 cm/s, never has this problem.

Worked examples

1. Mud from a bike wheel. Mud flies off a spinning wheel along the edge, at the point where it loses grip.

2. A hammer thrower. The athlete pulls the hammer inwards on its wire as it spins. On release, it flies off along the line it was moving.

3. A satellite. If gravity could be switched off, a satellite in orbit would fly off in a straight line.

Try it

  1. What pulls a swinging ball inwards?
  2. What makes a car able to go round a bend?
  3. You let go of the string. Which way does the ball go?
  4. Why do cars slow down for sharp bends?
Answers
  1. The string.
  2. Friction between the tyres and the road, pushing inwards.
  3. Straight on, along the edge of the circle at that point.
  4. A sharp bend at high speed needs a bigger inward force than the tyres can grip.

Common mistake

"Something going round is thrown outwards by a force." There is no outward force on the ball. It tries to go straight; the string pulls it in. The "outward" feeling in a turning car is your body carrying on straight while the car turns under you.

Where it leads