Describe forces as pushes and pulls, identify common forces such as friction, weight and air resistance, and explain what forces can do to an object

What this dot point is asking

This dot point wants you to describe what a force is, name some common forces, and explain what forces can do to an object. The big idea is that a force is simply a push or a pull. Forces are all around you, and they can change how things move and even change their shape. You should be able to name forces like friction, weight and air resistance, and say what each one does.

The answer

What a force is

A force is a push or a pull on an object. You cannot usually see a force, but you can see what it does. Force is measured in a unit called the newton (N), and it is measured with a device called a newton meter (a spring balance).

A force always has a direction as well as a size. A push to the left and a push to the right are different, even if they are the same size.

Common forces to know

You should know these common forces:

• Weight: the downward pull of gravity on an object. Gravity pulls everything toward the centre of the Earth, which is why things fall. Your weight is the force of gravity pulling you down.
• Friction: a force that acts when two surfaces rub against each other. Friction always acts in the opposite direction to the movement, so it slows things down. Rougher surfaces give more friction.
• Air resistance: a kind of friction from the air. When something moves through the air, the air pushes back against it and slows it down. A parachute uses a lot of air resistance to slow a fall.
• Push and pull forces: forces from your hand, a motor or a spring, such as pushing a door or pulling a trolley.

What forces can do

A force can do several things to an object:

• Make a still object start to move. A push gets a stationary trolley rolling.
• Make a moving object speed up, slow down, or stop. Pushing harder speeds a bike up; friction and brakes slow it down.
• Change the direction of a moving object. A sideways push changes the way a ball is rolling.
• Change the shape of an object. Squashing a sponge or stretching a rubber band changes its shape.

So in short, forces can change the movement of an object or change its shape.

Friction: helpful and unhelpful

Friction can be useful or a nuisance. It is useful when it grips: friction lets your shoes grip the floor so you do not slip, and lets brakes stop a bike. It is unhelpful when it slows machines down and wears parts out. We add oil or use wheels to reduce friction when we do not want it, and we use rough surfaces or rubber to increase it when we do.

Examples in context

Example 1. Walking on a wet floor. You can walk safely because friction between your shoes and the floor grips and stops you slipping. On a wet or polished floor there is less friction, so your shoes slip and you can fall. This is why "wet floor" signs warn you, and why shoes have rough rubber soles to increase friction.

Example 2. A parachute slowing a skydiver. A falling skydiver speeds up because of their weight pulling them down. When the parachute opens, its large surface catches a lot of air, giving a big air resistance force pushing up against the motion. This backward force slows the skydiver to a safe speed for landing.

Try this

• Cue. Name the force that makes an apple fall from a tree. The downward force is the apple's weight, caused by gravity pulling it toward the Earth.

• Cue. State two things a force can do to a football. A force can make the still ball move (a kick), make it speed up, slow down, stop or change direction, or change its shape when kicked hard.

• Cue. Explain why oil is put on a squeaky door hinge. Oil reduces the friction between the moving parts of the hinge, so it moves more easily and stops squeaking.