# Newton's First Law

## Key Stage 4

### Meaning

Newton's First Law states that "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."

Newton's First Law describes the fact that:
If the resultant force on an object is zero then it will continue to move at a constant velocity.
If the resultant force on an object is not zero then an object will accelerate (change speed or direction).
This means that if an object is observed changing speed or direction then there must be an unbalanced force acting on it.

### Examples

Stationary Object - The object remains at rest because the forces and balanced.

Uniform Velocity - The object remains at a constant speed and continues in a straight line because the forces acting on the object are balanced.

Uniform Speed but Changing Velocity - The object is changing direction because there are unbalanced forces acting on it.

Changing Speed - The object is changing speed because there are unbalanced forces acting on it.