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Science

Falling safely

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Falling objects increase their speed as they fall, because their weight (the force of gravity) pulls them to Earth. They also experience an upward force called air resistance (drag), which slows them down.

Objects fall faster until they reach their terminal speed, which is reached when the upward (air resistance) and downward (weight) forces are equal.

Falling objects

When an object is dropped it gets faster and faster as it falls. This happens because their weight (the force of gravity) pulls them down towards the centre of the Earth.

As they fall through the air, they also experience an upward force called air resistance (drag).

 

RAF parachutist

RAF parachutist

Objects with large surface areas, such as parachutes or shuttlecocks fall more slowly because they experience more air resistance.

Frictional forces such as air resistance, friction and drag act against the direction of motion, so tend to slow the object down.

This fact is put to good use in the design of the parachute and shuttlecock.

The size of frictional forces can be reduced by streamlining the object or lubricating any moving parts.

Reducing frictional force

Examples of reducing frictional force by streamlining include:

  • A sports car is wedge-shaped to reduce air resistance and so increase top speed.
  • Lorries and caravans have deflectors to reduce both air resistance and fuel consumption.
  • A car with a roof box has increased air resistance and fuel consumption and a lower top speed.
  • A downhill skier puts wax on the skis to reduce friction and so increase the top speed.

Terminal speed

When objects fall through the Earth's atmosphere they get faster and faster until they reach a speed where the upwards force (air resistance) and downwards force (weight) equal each other. At this point the object travels at its fastest speed called terminal speed.

What happens when you drop a coin and small feather at the same time?

They both have a similar surface area but the feather weighs less so has a smaller force of gravity pulling it down.

As the feather falls its upwards air resistance increases and soon equals its downwards weight, so it then travels at terminal speed. The coin is heavier and has to be travelling a lot faster before its air resistance is large enough to equal its weight.

Terminal speed on the moon

Watch

Watch an astronaut dropping a feather and a hammer on the moon.

The astronaut, David Scott carried out a famous experiment on the Moon. He dropped a hammer and a feather at the same time and found they landed together. As there is no air on the Moon there is no air resistance. The only force on both the hammer and feather was the Moon’s gravity which made them both fall with the same acceleration.

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