Balanced and unbalanced forces

'Unchanging motion' is when the body is or is moving with a steady speed in a straight line. 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 .

Example

Question

A cyclist travelling at $$10\, m\,s^{-1}$$ has a forward force of $$200 N$$ but is acted on by friction and of $$200 N$$ 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 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 that cause 'changing motion'.

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 ($$T$$) from the engines is greater than the ($$W$$) 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.