Young YOU: Forces that push and pull

Have you ever been about to take a bite of food when you accidentally dropped it and it fell to the floor? If so, you've experienced force.

We're constantly surrounded by forces. There are push and pull forces that make objects move, that increase or decrease their speed or make them stay in one place. Forces make objects move.

Think of a weightlifter swiftly picking up a heavy weight and holding it above his head. He uses his muscles to push the weight up above his head, while Earth's gravity pulls the weight towards the ground.

For a few seconds he balances the weight, holding it in one place above his head, until his muscles tire and he drops it to the ground.

Let's find out more about the various types of forces and how they work.

TWO TYPES OF FORCE

There are two main types of force: contact and non-contact force. Both can push or pull.

• Contact force This occurs when two objects physically touch each other, such as when you kick a ball or drag your school bag on the ground. Other examples of contact force include when you use sandpaper to sand a piece of wood, when a crosswind almost blows you off your bike, or if you tie a rubber band around your lunchbox to keep the lid on. You can exert contact force in different ways on the same object, for example if you throw, bounce, roll or drop a ball.

• Non-contact force (action-at-a-distance) This is when two objects exert force on each other without physically touching. This type of force can make objects push or pull each other from a distance. One example of a non-contact force is gravity. When an orange falls from a tree, Earth's gravity is pulling the orange to the ground. Magnetism is another example of a non-contact force - more on that later.

CONTACT FORCE

What does contact force do?

MAKES OBJECTS MOVES

When you play putt-putt, you move the ball by striking it with the club. The club exerts force on the ball, making it move forward. If you miss the shot, the ball stays where it is because there's no force to make it move.

INCREASES SPEED

The force of the wind against a wind turbine's blades make them turn faster. The stronger the wind, the faster the blades turn. Air is made up of invisible particles. If there's wind, these particles move in a certain direction. If an object - such as a wind turbine - is in the wind's way, the particles push against it and make the blades move.

The same happens with flowing water. In the olden days, millers would erect a watermill in a flowing river. The moving water particles push on the watermill's paddles, which sets the mill in motion. This enabled the miller to grind wheat into flour.

REDUCES SPEED

If you're riding your bicycle and need to stop, you pull on the brake lever to activate the brakes. This action is relayed via the brake cable to the brake pads, which contract and exert a force upon the wheel so the bike reduces speed.

Another example is race cars with jet engines at a drag race that accelerate super-fast to see who can cross the finish line first. To stop the car, the driver has to deploy a special parachute (called a drag parachute) to break the car's speed so it can stop. The force of the air particles against the parachute is called windage.

CHANGES AN OBJECT'S SHAPE

When you're playing tennis, the racket provides the force to change the movement of the ball. As you hit the ball, the racket squashes the ball. The racket's strings also bend and stretch as a result of the ball's force. When the ball bounces away from the racket, the ball and the strings return to their original shape and size.

MAKES OBJECTS CHANGE DIRECTION

The purpose of a racket or a cricket bat is to change the direction in which the ball is moving.

The force exerted by the cricket bat when it hits a cricket ball makes the ball change direction and speed up.

NON-CONTACT FORCE

Let's look at the two examples of non-contact force that we've already mentioned.

Gravity

Gravity is the force by which planet Earth, the moon and other massive objects in the universe attract other objects. Every object on our planet experiences gravity (also called gravitational force), which is directed downward towards Earth's centre or core.

Gravity keeps the planets in their orbit around the sun and also ensures the moon orbits Earth. Thanks to gravity, our atmosphere - which is a blend of gases - stays in place. And it also prevents us from floating off into space!

Magnetism

A magnet is a rock or piece of metal that attracts certain other metals. Like gravity and electricity, magnetic force - or magnetism - is a fundamental force of nature.

Magnetism works across a distance. This means the magnet doesn't have to touch an object to attract (pull) or push it away. But the magnetic force is strongest when the magnets touch. The force is also strongest at the ends, or poles (north and south), of the magnet. If you bring the same poles of two magnets together, they'll push away from each other. But two different poles (north and south) will attract each other if you bring them close together.

There are two types of magnets: permanent magnets and electromagnets.

Permanent magnets

These never lose their magnetic force. They're made by heating magnetic material such as iron to a high temperature and then leaving it to cool down in the presence of a strong magnetic field.

This causes some of the particles in the magnet to face the same direction. The material is then made magnetic by placing it in another strong magnetic field. This magnetises all the particles that are already facing the same way.

Each of these tiny particles now exerts its own magnetic field. You can also make ordinary objects such as a needle magnetic by rubbing it against a bar magnet.

Electromagnets

These are made by using electricity and a magnetic material such as iron.

When electricity travels through a copper wire, it creates a magnetic field around the wire. If you coil an insulated copper wire around an iron core, you can expand the magnetic field, thereby creating an electromagnet.

Unlike permanent magnets, you can disable the magnetism of an electromagnet by removing the electricity.

A bar magnet exerts non-contact force to attract iron filings on a piece of paper.