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Science

Weight and friction

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# Falling objects

You should be able to describe the forces affecting a falling object at different stages of its fall. Usually, you need to think about two forces:

1. The weight of the object. This is a force acting downwards, caused by the object’s mass the Earth’s gravitational field.
2. Air resistance. This is a frictional force acting in the opposite direction to the movement of the object.

## Three stages of falling

When an object is dropped, we can identify three stages before it hits the ground:

1. At the start, the object accelerates downwards because of its weight. There is no air resistance. There is a resultant force acting downwards.
2. As it gains speed, the object’s weight stays the same, but the air resistance on it increases. There is a resultant force acting downwards.
3. Eventually, the object’s weight is balanced by the air resistance. There is no resultant force and the object reaches a steady speed, called the terminal velocity.

Check your understanding of this by studying the parachutist animation.

## Terminal velocity

What happens if you drop a feather and a coin together? The feather and the coin have roughly the same surface area, so when they begin to fall they have about the same air resistance.

As the feather falls, its air resistance increases until it soon balances the weight of the feather. The feather now falls at its terminal velocity. But the coin is much heavier, so it has to travel quite fast before air resistance is large enough to balance its weight. In fact, it probably hits the ground before it reaches its terminal velocity.

## On the Moon

An astronaut on the Moon carried out a famous experiment. He dropped a hammer and a feather at the same time and found that they landed together. The Moon's gravity is too weak for it to hold onto an atmosphere, so there is no air resistance. When the hammer and feather were dropped, they fell together with the same acceleration.

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