Force and simple machines

When you kick a ball, open a door or lift your school bag, you use a force. A force is a push or a pull. We cannot see a force, but we can see what it does.

What a force can do

• It can make a still object move: you push a swing.
• It can stop a moving object: a goalkeeper catches a ball.
• It can change the speed or direction of a moving object: a batsman hits a ball.
• It can change the shape of an object: you press a lump of dough.

Some forces around us

Muscular force comes from our muscles. Gravity is the pull of the earth that makes things fall down. Friction is the force that slows things when two surfaces rub; it lets us walk without slipping. Magnetic force is the pull of a magnet on iron.

Simple machines

A machine is a tool that makes work easier. It does not do the work for us; it lets us use a smaller force, or push in a more convenient direction. There are six simple machines, and every complicated machine, from a bicycle to a crane, is built from them.

1. Lever: a rigid bar that turns about a fixed point called the fulcrum. A see-saw, a pair of scissors, a bottle opener and a crowbar are levers. Pushing down on the long end lifts a heavy load on the short end.
2. Pulley: a wheel with a groove for a rope. It changes the direction of the force: you pull the rope down and the bucket comes up, as at a well or on a flagpole.
3. Inclined plane: a slope. It is easier to push a heavy box up a ramp than to lift it straight up. Hill roads and the ramp of a truck are inclined planes.
4. Wheel and axle: a wheel fixed to a rod. Turning the big wheel turns the rod with greater force. A door knob, a steering wheel and the pedals of a bicycle work this way.
5. Wedge: two slopes joined to make a sharp edge. It splits or cuts things. A knife, an axe and a nail are wedges.
6. Screw: an inclined plane wound around a rod. It holds things together or lifts them. Screws, bottle caps and a car jack use it.

Why machines help

With a machine we usually move our hand through a longer distance and, in return, need less force. A ramp is longer than a straight lift, but each step is easier. Machines save effort, not work.

Machines must be looked after. Oil reduces friction between moving parts, and paint protects iron from rust.