Physics
Fox

### Summary

• The force on an object is a measure of how much it is pushed or pulled.

• The SI unit of force is Newtons:

• Examples of forces:

• A force always happens between two objects.

The force on an object is a measure of how much it is pushed or pulled. This might sound a bit vague, but it's genuinely the best way to think about it.

The unit of force is Newtons: A small force might be 10N (e.g. the force used to push a door closed). A large force might be 500N (e.g. the force used to lift weights at the gym, bro.)

Forces can occur because of many things, not just biceps. Let's have a look at some of them…

Let's explore some examples of forces. What could cause a push or pull on an object?

• Gravity The gravitational force pulls a football towards the ground Gravity (aka. the gravitational force) pulls things towards the centre of the Earth. We'll investigate gravity in more detail soon.

• Drag (& air resistance) Air resistance pulls a parachute upwards (opposite to the direction it is moving) This force drags things backwards when they move though a liquid or gas. This slows the object down. Drag through air is called air resistance.

• Normal force The normal force prevents this mug from falling through the table. When an object is in contact with a surface (e.g. a floor or a table), there is a force along the normal of the surface. This is called the normal force. It's what stops you from sinking into your chair!

• Friction Friction is the reason a mug slows down when you slide it on a table. Friction is a similar force to drag: When an object is moving along a surface, friction pulls it backwards. This slows the object down. Without friction, objects would slide on surfaces forever.

• Upthrust Upthrust acting on a boat When an object is submerged in a fluid (such as water), there is a force that pushes the object upwards. This force is known as upthrust (or the buoyancy force). Without upthrust, all boats would be doomed to sink.

• Thrust Recoil on a gun. If you shoot a bullet from a gun, the gun pushes away from the bullet. This is known as recoil. If a gun were to continually shoot bullets, it would keep moving backwards. This is the idea behind thrust — generating force by pushing other objects away. A gun shoots bullets to the left. This causes a repeating recoil, moving it to the right.
The other objects don't need be bullets of course! A propeller pushes water or air away from an object to generate thrust, as used in boats and planes. A propeller pushes water to the left. This causes thrust to the right. Rockets also use thrust to move upwards. The thrust of a rocket comes from the ejection or hot gases.
• Traction Traction pushes the car forwards Many people think that thrust is what moves cars forwards, as if the exhaust pipe of the car works like a jet engine. This isn't true — otherwise you could easily accelerate a car that's on ice. A car on ice struggles to start moving, even though the tyres are spinning.
It is actually the friction between car tyres and the road that pushes the car forwards. This is called traction.

Note that this means friction isn't what pushes cars back; this is purely due to air resistance.

• Lift We have previously thought of drag as a force that pulls things backwards (i.e. away from the direction the object is moving in). However, if we're clever we can use drag to pull things upwards as well. The shape of aeroplane wings are specially designed to generate this upwards force from drag. This upwards force is called lift.

• Elastic force If you stretch an object (such as an elastic band), it usually wants to pull back to its original shape. This pulling force is known as the elastic force. (The force that is doing the stretching is known as tension.)

• Electric force To be discovered in the next chapter.

• Magnetic force To be discovered in the chapter after the next chapter.

In many works of science fiction there are additional forces to the ones we have in the real world. The most notorious of these is The Force from Star Wars.

There are certain forces which hold a special place in physics, known as the fundamental forces.

Examples of fundamental forces include the gravitational force, the electric force and the magnetic force. The latter two are so important that the next two chapters are all about them!

There are only two more fundamental forces, which describe what happens in the nuclei of atoms (called the strong and weak nuclear forces). These are a bit complicated, so we won't cover them yet.

Every other force (e.g. friction, air resistance, upthrust, normal force) is simply a bi-product of the fundamental forces. That's what makes them fundamental!

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