Forces

You push the pedals, and the bike speeds up. You squeeze a sponge, and it changes shape. You brake, and the scooter slows down. Behind all of this is the same thing: a force. You cannot see forces, but you always recognise them by their effect.

What forces do

A force can do two things: it can change the motion of an object, making it faster, slowing it down or changing its direction. Or it can change its shape, like the squeezed sponge. Force is measured in newtons (N), named after Isaac Newton. To get a feel for it: a bar of chocolate weighs about 100 g and pushes down on your hand with about 1 N. Importantly, a force has not only a strength but also a direction. That is why forces are drawn as arrows: the length shows the strength, the arrowhead the direction.

Forces add up or cancel out

Usually more than one force is acting. In a tug of war, two teams pull on the same rope, but in opposite directions. If both pull equally hard, the forces cancel out and the rope does not move. If one team pulls with 500 N and the other with 400 N, a force of 100 N remains, and the rope slips towards the stronger side. If two forces act in the same direction, they add up: if two friends push a car together with 200 N each, 400 N act in total. One force is almost always in play: friction. It acts against the motion and slows it down, for example when you push a box across the floor.

Equilibrium

When all the forces on an object cancel each other out, it is in equilibrium. That does not mean no forces are acting. Right now gravity pulls you down, and your chair pushes up just as hard. Because the two cancel, you stay sitting calmly. So equilibrium means: the motion does not change.