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Unbalanced forces

Forces and motion · level 3, lesson 3 of 6

A question to start

In the tug of war, one team gives an extra heave, and the flag starts moving their way. What changed?

The idea

When the forces on something do not cancel, they are unbalanced. Then there is an overall force, also called the resultant or net force, and it changes how the thing moves:

  • in the direction it is already going, it speeds up;
  • against the way it is going, it slows down;
  • from the side, it turns.

To find the overall force along a line, add up the forces going one way and take away the forces going the other way.

push 300friction 100overall: 200 forwards

The bigger the overall force, the faster the speed changes. And, as in level 1, the more mass something has, the more slowly the same force changes its speed.

On Mars, a lander falling towards the ground has unbalanced forces: gravity pulls it down harder than the thin air pushes up, so it speeds up. To land softly it must make the forces unbalanced the other way, with rockets pushing up harder than gravity pulls, so it slows down.

Worked examples

1. The cart. Push 300 forwards, friction 100 backwards. Overall: \(300 - 100 = 200\) forwards. The cart speeds up.

2. A braking bike. The rider stops pedalling and brakes: friction backwards 150, nothing forwards. Overall: 150 backwards. The bike slows down.

3. Two pushes the same way. Two children push a box with 40 and 60. Friction is 70. Overall: \(40 + 60 - 70 = 30\) forwards.

Try it

  1. A box is pushed with 50 to the right; friction is 20 to the left. What is the overall force?
  2. Does the box speed up or slow down?
  3. A car has a forward push of 2,000 and air and friction of 2,000 backwards. What happens to its speed?
  4. A lander's rockets push up with 8,000; its weight on Mars is 6,000. Is it speeding up or slowing down as it falls?
Answers
  1. 30 to the right.
  2. It speeds up (if it is moving to the right).
  3. It stays the same: the forces balance.
  4. Slowing down: the overall force is 2,000 upwards, against its fall.

Common mistake

"An overall force backwards means it moves backwards." A force against the motion first slows the thing down. It only moves backwards if the force goes on after it has stopped.

Where it leads