Balanced and unbalanced forces

'Unchanging motion' is when the body is at rest or is moving with a steady speed in a straight line. Balanced forces are responsible for unchanging motion.

Balanced forces are forces where the effect of one force is cancelled out by another. A tug of war, where each team is pulling equally on the rope, is an example of balanced forces. The forces exerted on the rope are equal in size and opposite in direction. The rope will have an acceleration of zero under the action of these balanced forces. It will therefore remain stationary.

Two men pulling on opposite ends of a rope in a tug of war

Example

Question

A cyclist travelling at 10\, m\,s^{-1} has a forward force of 200 N but is acted on by friction and air resistance of 200 N which push against her.

A cyclist travels at 10m/s-1.

What happens to the cyclist's motion?

The forces are balanced, cancelling each other out, so there is NO CHANGE in the cyclist's motion – the cyclist continues at a constant speed of 10\, m\,s^{-1}.

Balanced and unbalanced forces

'Changing motion' includes movement where the body is speeding up or slowing down. It also includes motion where the direction in which the body moves is changing, ie the body is moving in a curved path. It is unbalanced forces that cause 'changing motion'.

Space shuttle at launch: the thrust (T) from the rocket engines is greater than the weight (W) of the shuttle.Space rocket launch

The lift-off of a space rocket is an example of an unbalanced force in action. The space rocket accelerates upwards from its launch pad. The thrust ( T) from the rocket engines is greater than the weight ( W) of the rocket system. Because force T is greater than the force W, the effect of one force does not cancel that of the other. The forces acting are unbalanced.