The force that pulls things to the ground on Earth (and other planets) is called gravity.
Gravity also holds Earth and the other planets in their orbits around the Sun.
Gravity is a very useful force – It keeps us on the Earth, and keeps the Earth and the other planets revolving around the Sun. It holds everything together, which is why it has been called ‘The Universal Glue’.
Although the force of gravity also exists on the Moon it is not as strong as it is here on Earth. This is because the Moon is much smaller than the Earth it is not as heavy as the Earth, and so gravity is much weaker there.
In the past people thought that heavier things fell faster than light things. Galileo, an Italian scientist from the 1600’s, conducted some experiments and found that things with different weight fell at approximately the same speed.
Friction is a force between two surfaces when they are moving or sliding across each other. For example, when you try to push your hand across a table. Friction will make this difficult.
Friction works in the opposite direction to which the object is moving. Friction always slows a moving object down, and may even stop an object from moving.
The amount of friction depends the two surfaces.. The rougher the surface, the more friction, the smoother the surface, less friction.
Friction also produces heat. If you rub your hands together quickly, can you feel them get warmer?
The ways friction can help include:
preventing our shoes from slipping on the footpath when we walk
stopping car tyres from skidding on the road
allowing bicycle brakes to grip and slow or stop a turning wheel
Sometimes we want to reduce friction. For example, we use oil to reduce the friction between the moving parts inside a car engine. The oil holds the surfaces apart and can flow between them. The reduced friction means there is less wear on the car’s moving parts and less heat produced.
Ice causes very little friction, which is why it is easy to slip over on an icy day.
However, this is a good thing for ice skating and sledging.