Contact forces — Science, 11–13
A force is a push or a pull. Contact forces, such as friction and air resistance, act when objects touch and can change motion.
Pushes and pulls change motion
A force can start an object moving, stop it, speed it up, slow it down or turn it. Friction is a contact force that acts between touching surfaces, while air resistance acts between moving objects and the air. Forces are not visible, but their effects on motion are measurable.
Why include friction?
The problem is that objects do not move in a perfectly empty, smooth world. A toy car slows on the floor even after you stop pushing, and a bicycle brake must slow a wheel. Friction explains these effects and helps us predict when surfaces will grip, slide, heat up or wear away.
A box on the floor
You push a 4 kg box to the right with 12 N. Friction pushes left with 5 N because the floor resists the sliding. The forces oppose each other, so the resultant force is 12 − 5 = 7 N to the right. If the push stops, friction keeps acting and the box slows until it stops.
Motion does not need a forward force
A common mistake is to think that a moving object must have a forward force acting on it. This seems sensible because our pushes usually stop when the object stops, but friction is often the hidden reason. Once friction is small, such as on ice, an object can keep moving even when nobody is pushing it.
Shoes, tyres and brakes
Shoe soles and bicycle tyres need enough friction to grip the ground, especially when it is wet. Brakes use friction to transfer movement into heat and slow wheels. Engineers reduce unwanted friction with oil or smooth bearings, but they cannot remove it completely: without some grip, walking and steering would fail.
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