You already know a force is just a push or a pull. But here's the real question: once you push or pull something, what actually happens to it?
It turns out a force can only ever do four jobs. Every push and every pull you have ever felt does one of these — and nothing else:
Learn these four and you understand what forces are for. Press play and watch a single ball get two of them at once — a push that speeds it up, then a push that turns it.
Line up a football on the grass. On its own it will sit there all afternoon — a still thing stays still until something touches it. Then you kick it. Your foot gives the ball a hard, quick push, and in that instant it goes from not moving at all to zooming across the field.
The bigger the push, the faster it goes. A gentle tap rolls it a little way; a mighty boot rockets it over the fence. And this same job speeds up things that are already moving: pedal harder on your bike and it picks up speed, blow harder on a toy sailboat and it scoots away quicker. A force pushing the way something is already headed always makes it faster.
A moving thing doesn't stop by itself either — something has to push against it. When a goalkeeper catches a screaming shot, her hands push back on the ball exactly as hard as it takes to kill all its speed. One moment it's a blur; the next it's sitting dead still in her gloves. The force was pointing the opposite way to the ball's motion, so it drained the speed away instead of adding it.
You use this job constantly. Squeeze your bike brakes and a rubber pad pushes on the wheel until you slow to a walk. Drag your shoes in the sand to stop your scooter. Even catching a beanbag is a force gently taking its speed away. Same idea as job one — but the push points backwards.
Watch a good cricket fielder catch a ball hurtling at them and you'll spot a secret: as the ball smacks their hands, they pull their hands back with it. Why? Stopping the ball over a longer, gentler journey means their hands only ever push back softly — so it doesn't sting, and doesn't bounce out. Stop it too suddenly with stiff hands and the ball punches straight back off them. The slowing-down force is the same either way; spreading it out just makes it kinder on your fingers.
Here's the sneaky one. A force can change which way something is going without making it any faster or slower at all. Roll a marble at a wall and it bounces off sideways. Its speed hardly changes — but the wall's quick push has sent it off in a completely different direction.
You steer with this job. Turn your bike's handlebars and a force curves you round the corner. A goalkeeper doesn't have to stop a ball to save it — one fingertip can be enough to nudge a rocketing shot just wide of the post. Same speed, brand new direction, all from a single touch.
A tennis player faces a ball flying at them and, with one swing, sends it flying back the other way. Think about how strange that is: for a heartbeat the racket has to grab the ball's motion, wipe it out, and fling it off in almost the opposite direction — all in the blink of an eye. That is a force changing direction, taken to the extreme. The very best players can also make the ball curve and dip in the air, bending its path like magic, just by how they brush the racket across it.
The first three jobs all change how something moves. The last one is different: a force can change the very shape of a thing. Press your thumb into a lump of playdough and it squashes flat. The harder you press, the flatter it goes — the push and the squash grow together.
Pushing squashes things smaller; pulling stretches them longer. Sit on a cushion and it squashes; tug the ends of a rubber band and it stretches. Below is a soft ball of clay. Slide How hard? to press or pull harder, and flip between press and pull to watch the same lump squash flat or stretch long.
Look closely at a bouncing ball with a slow-motion camera and you'll see it squash against the ground into a flat blob — then spring back to a round ball and leap up again. All the bounce is stored in that little squash and release. A trampoline does the same with its stretchy sheet: your weight stretches it down, and the moment it snaps back to shape it flings you into the air. Every springy thing — a spring, a diving board, an elastic band — works by changing shape and then pinging back.
These three catch almost everyone out: