Air resistance
Would you like to go up into space? Could you imagine looking down on the Earth and seeing its oceans, continents and cloud formations? If you looked further across the blackness of space you would see the steady shining lights of millions of stars. You may have thought that it was a dangerous journey to send your spacecraft out into space as its rockets made you shake and shudder as you rose into the sky. Unfortunately the journey back is also dangerous. Surprisingly, the danger is due to air.

Around the surface of the Earth is a mixture of gases that is called the air or atmosphere. It rises from the planet's surface to about a thousand kilometres. This is the place where space begins. We tend to think that air is nothing but you can show someone that this idea is wrong. Just take two similar balloons, blow one up, and hang them on opposite sides of a coat hanger. The inflated balloon will make the coat hanger dip down because it is heavier than the empty balloon. The air in the balloon must be something because it has weight.

Air is made from tiny particles that can only be seen by the most powerful microscopes. They whizz about and knock into each other and just keep moving. You can think of them looking like the balls that move about in a lottery show. If you hold up a large piece of card in front of you, and walk very slowly, you may hardly feel the air particles pushing on it. If you then start to run you will suddenly feel the push of the air particles. The faster you move the harder they push. This fact about the push of air particles is important for spacecraft returning to Earth.

Out in space there are few particles of gas so spacecraft can travel very fast. There is virtually nothing to push against them as they move. When a spacecraft comes to within a thousand kilometres of the Earth it meets particles of air. Fortunately, at this height above the ground, there are only a few but they increase in number as you move down towards the Earth. When the spacecraft moves through the outer part of the air, the particles rub along its side and begin to slow it down. As the spacecraft moves lower and lower it meets more and more air particles and the push on the spacecraft's sides increases greatly. This push is called air resistance. It is friction in the air. Friction between two solids such as two pieces of wood can create so much heat that they burst into flames. Friction between air and the solid surface of the spacecraft can also produce a great deal of heat. This heat is strong enough to make the spacecraft catch fire with disastrous results. Spacecraft are designed to avoid this fate by having a heat-resistant shield on their outer surface. This is made from materials which can stand up to the friction of the air and prevent the spacecraft burning up as it enters the Earth's atmosphere.

Once the spacecraft has slowed down, the danger of fire is removed. The spacecraft, however, faces another danger. It may still be travelling too fast to land safely. Now the power of air resistance is used to make a safe landing. Parachutes are released from the back of the spacecraft. They open out and present a huge surface to the air rushing by. The air pushes on this surface with tremendous force and slows down the spacecraft so much that it can touch down lightly on the runways and come gently to a halt. Your trip from space has been both hindered and helped by air resistance.

Don't stars twinkle in space?
No. The twinkling is due to the air around the Earth. Stars are huge distances away from the Earth. This means that by the time light from them reaches us it is very weak. There are currents in the air, which move it up and down. When the weak starlight moves through the air, the air currents make it follow a wavy path instead of a straight one. This wavy path makes the stars appear to twinkle. Out in space, where there are no air particles, the stars shine with a steady light.

Do other planets have atmospheres?
Yes, they do. When we send a spacecraft to land on a planet we equip it with parachutes. As the spacecraft sinks through the planet's atmosphere a radio message is sent from Earth to open the parachutes. The particles in the atmosphere of the planet push on the parachutes and slow down the spacecraft so much that it can make a gentle landing which does not damage its equipment on board.

What can help an object move quickly though the air?
A streamlined shape. The front of the object needs to be pointed or wedge-shaped to cut through the air. Its sides need to be smooth and gently curved so the air can flow over them easily. The back of the object also needs to be pointed or wedge-shaped so the air which has been separated at the front of the object can gently merge.

What happens if the air at the back of an object does not gently merge?
The air forms small whirlwinds called eddies. They pull on the back of the object and slow it down.

Do cars have streamlined shapes?
Most of them do. If you look at a car from the side you may see that it is wedge-shaped with the thin end of the wedge at the front of the car. Many cars have sides which are slightly curved to let the air move over them smoothly. Cars designed to travel very fast and break speed records have pointed fronts and backs to reduce air resistance as much as possible.

Do aeroplanes have streamlined shapes?
Yes, they do. If you look at a large passenger plane you can see that its front is rounded like a teardrop. If you look at a fighter plane, which travels much faster than a passenger plane, you will see that it has a pointed front and rear.

As you look back along the aeroplane you can see that the body is tapered almost to a point at the tail. This prevents large eddies in the air developing behind the plane and pulling backwards on it. The surfaces of the aeroplane are smooth and curved to allow the air to pass over it easily.

Does the air help the aeroplane stay in the air?
Yes, it does but this is not due to air resistance. It is due to air pressure. If you look at a wing from the end you can see that the upper surface is more curved than the lower surface. As air moves over the lower surface of the wing it pushes upward with a large amount of pressure. As the air moves over the upper surface of the wing it pushes down on the wing surface with a much smaller amount of pressure. This low pressure is due to the way the air moves over the highly curved surface. This difference in pressure on the two sides of the wing makes the air below the wing push the wings and the aircraft upwards so it can fly.

Is there a wing on the back of a racing car?
No. This is a spoiler. It is like an upside-down wing because the pressure it creates acts downwards. This force pushes on the back wheels of the car and helps it grip the road more firmly. It helps the driver travel around a racecourse at greater speeds.

Do birds have streamlined shapes to fly?
Yes, they do. A spindle is a streamlined shape. It has a pointed front and back and smooth sides which swell outwards in the middle. The body of a bird is spindle-shaped.