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.
A cyclist travelling at has a forward force of but is acted on by friction and air resistance of which push against her.
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 .
'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'.
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 ( ) from the rocket engines is greater than the weight ( ) of the rocket system. Because force is greater than the force , the effect of one force does not cancel that of the other. The forces acting are unbalanced.