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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 air resistance, a frictional force that 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

RAF parachutist

RAF parachutist

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).

Frictional forces such as air resistance, friction and drag act against the direction of motion, so tend to slow the object down. Objects with large surface areas, such as parachutes, fall more slowly because they experience more air resistance.

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 downhill skier puts wax on their 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 terminal speed. This is when the upwards force (air resistance) and downwards force (weight) equal each other. At this point the object travels at its fastest speed.

A skydiver can change their terminal speed by adjusting their shape and position as they fall. For example:

  • If they go head down with their arms by their sides, the air resistance is reduced. Their speed increases until a new, faster terminal speed is reached
  • If they are spread-eagled, the air resistance is increased. Their speed decreases until a new, slower terminal speed is reached.

Cars and other vehicles have a ‘top speed’ – the terminal speed when the driving force from the engine is at its maximum. As the driving force from the engine increases, the speed of the vehicle will increase but so will air resistance.

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