Forces change motion — Physics, 11–13 years
See forces as pushes and pulls that can start, stop, speed up, slow down or turn an object.
Pushes and pulls
A force is an interaction that can change an object’s motion or shape. Kicking a ball pushes it, a brake slows a wheel, and gravity pulls objects towards Earth. Forces have a size and a direction, so two forces can work together or oppose each other.
Why study forces?
People needed to explain why carts moved, stopped or tipped, instead of treating motion as mysterious. Describing the pushes and pulls makes it possible to predict what will happen and design safer bridges, bicycles and playground equipment.
Opposing forces
A box is pushed right with 10 newtons while friction pushes left with 4 newtons. Choose right as positive, then calculate 10 − 4 = 6 newtons to the right. The forces do not cancel because they are unequal, so the box’s motion changes to the right.
The trap: force is not motion
It feels natural to think that a moving object must have a force pushing it forwards all the time. In fact, a force is needed to change motion; a puck can keep sliding while friction is small. Everyday surfaces make this mistake reasonable because friction usually slows things down.
Forces in design
Tyres use friction to grip the road, seat belts provide a force that stops passengers, and cranes balance forces while lifting loads. Designers calculate these interactions so objects move, hold together or stop without causing avoidable harm.
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