# Unbalanced Forces

## Key Stage 3

### Meaning

Unbalanced Forces are Forces acting in opposite directions with different magnitudes.

When there are unbalanced forces on an object the object will accelerate.
 The 4N up and the 7N down mean the object will accelerate downwards due to a resultant force of 3N. The 4N left and the 3N right mean the object will accelerate to the left due to a resultant force of 1N. The 6N up is balanced by the 6N down but the 6N left and the 4N right mean the object will accelerate to the left due to a resultant force of 2N.

## Key Stage 4

### Meaning

Unbalanced forces are force vectors which add together to make a (non-zero) resultant force.

When forces on an object are unbalanced it will accelerate (change speed or direction).
Unbalanced forces are related to Newton's First Law of motion:
An object in motion will tend to stay in motion in a straight line and an object at rest will tend to stay at rest unless an unbalanced force acts upon it.
If the resultant force on an object is not zero, then the forces are unbalanced.

### Examples

 Choose which directions are positive. Up is positive. Calculate the Resultant Force There is 29N up and 37N down. FR = (+29N) + (-37N) FR = -8N upwards The forces are unbalanced. Choose which directions are positive. Right is positive. Calculate the Resultant Force There is 114N right and 105N left. FR = (+114N) + (-105N) FR = 9N right The forces are unbalanced. Choose which directions are positive. Right is positive. Up is positive. Calculate the Resultant Force There is 38N up and 38N down. FR = (+38N) + (-38N) FR = 0N upwards The vertical forces are balanced. Calculate the Resultant Force There is 50N right and 40N left. FR = (+50N) + (-40N) FR = 10N right The horizontal forces are unbalanced.

### References

#### AQA

Unbalanced forces, pages 118-119, GCSE Physics; Third Edition, Oxford University Press, AQA
Unbalanced forces, pages 146, 156, 180-1, GCSE Physics; Student Book, Collins, AQA

#### Edexcel

Unbalanced forces, page 13, GCSE Physics, Pearson Edexcel