Forces change motion — Physical education, 11–13 years
A force is a push or a pull. Forces can start, stop, speed up, slow down or turn objects, and several forces can act at once.
Pushes and pulls
A force is an interaction that can change an object’s motion or shape. A football is pushed by a foot, while a stretched elastic band pulls back. Forces have a size and a direction, so two equal forces can cancel when they point opposite ways.
Why talk about forces?
Saying that an object moved is not enough to explain what happened. We want to know what made it start, stop or turn. The idea of force gives us a way to describe these changes and to design pushes, brakes and supports deliberately.
A tug of war
One team pulls a rope with 300 newtons to the left. The other pulls with 250 newtons to the right. The forces oppose each other, so subtract: 300 − 250 = 50 newtons to the left. The rope’s resultant force is 50 newtons leftwards, not 550 newtons.
Forces do not always add
It is tempting to add every force you see, because adding is what we often do with amounts. But direction matters: opposite forces partly cancel, while forces in the same direction combine. Draw arrows first; then decide whether to add or subtract.
Forces in everyday objects
Brakes use friction to oppose a bicycle’s motion, and seat belts provide a force that stops a person moving forward suddenly. Bridges use forces through their beams and cables. Designers study these forces so objects work without bending or breaking.
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