### KS3 Physics

Energy
Motion and Forces
Waves
Electricity and Electromagnetism
Matter
Space Physics

# What is Gravity?

Gravity is an invisible force that attracts objects towards each other. This becomes very noticeable when dealing with large bodies of mass, such as planets. This is because the greater the object’s mass is, the greater the gravitational force. Gravitational forces pull inward, towards the object’s centre. This is why on Earth, we are pulled towards the ground.

All planets, including Earth, in our solar system orbit the Sun, while the Moon orbits Earth. This happens due to the force of gravity.

## Gravitational Pull

All objects have mass, which changes only when you add or remove mass. However, weight does change because it’s a force that depends on gravity.

As seen in the example below on the left, when on Earth, the elevator falls due to gravity. This happens because it is within Earth’s gravitational pull. In space, a person becomes weightless and floats because they are outside Earth’s gravitational pull. Therefore, he is no longer drawn towards Earth’s centre.

## Air Resistance

When an object falls towards Earth, it gets faster due to the force of gravity. The rate at which an object falls to the ground depends on how much air resistance there is.

In this case, the opposing force is air resistance, an upward force of friction that aims to slow the object down. Air resistance opposes gravity. Air resistance increases when the surface area of the object increases.

However, objects fall differently depending on how gravity and air resistance interact. For example, if you dropped an apple and a feather at the same time:

Feathers float and take longer to reach the ground, while an apple falls straight down. This is because there is much greater air resistance when the feather is falling compared to the apple.

The apple can fall through the air at a faster rate. This is based on Galileo’s discovery that objects with greater density (or more mass) will fall faster than less dense objects (or those with less mass) due to air resistance.

When a feather and an apple are dropped at the same time, air resistance causes the feather to drop fall more slowly than the apple. However, in a vacuum (such as space), where there is no air resistance, the apple and the feather would fall at the same rate and reach the ground at the same time.

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