Elastic sheets and elastic bands
Have you played with an elastic band? Probably the first thing you did with it was to stretch it between your thumb and your first finger. As you moved your thumb and finger further apart, you felt the elastic band dig deeper into your skin. You may then have brought your thumb and finger closer together again and saw the band become shorter. At the same time you felt less pressure on your skin. The next thing you might have done was to stretch the elastic band and pluck it. When you did this, the elastic band appeared as a blur and a droning sound reached your ears. Perhaps the last thing you did was take a small piece of paper, make it into a ball and use your elastic band as a catapult.

When you used the elastic band as a catapult, you moved your thumb and first finger wide apart to stretch the elastic band. Next you pressed the paper into one side of the elastic band. This stretched the side of the elastic band even more. Finally you let go of the paper which flew away from you hard across the room. What had happened to make the paper move so fast?

An elastic band is a springy material. It can be stretched but once the stretching stops it goes back to its original shape. Movements and shape changes are due to forces and energy. When you move your finger and thumb apart to stretch an elastic band, you are using energy. It comes from your food and is released in your muscles to make them work. The stored energy in your food becomes movement energy in your muscles and makes a stretching force on the elastic band. As the elastic band is stretched it stores energy inside it and makes a force called a strain force. As you stretch the elastic band, you can feel the power of the strain force pushing the band into your skin.

Once you have stretched the elastic band you have given it a store of energy and built up a strain force in it. This is not enough to send a piece of paper flying so when you bring the paper to the elastic band, you stretch the elastic band again. This builds up even more energy and a stronger strain force. When you have pulled the paper into the side of the elastic band and let go, the elastic band releases its stored energy and the strain force pushes on the paper. The stored energy in the elastic band is changed to movement energy and travels with the paper as it leaves your catapult.

There is a piece of equipment that can catapult you into the air. It is called a trampoline. It contains a strong elastic sheet. This can be stretched like an elastic band. When you jump onto a trampoline you use energy from your food to make your leg muscles work. As you jump through the air your body has movement energy. When you land in the middle of the trampoline your feet meet the elastic sheet. You move down into the sheet and as you do so, you sink more slowly until you stop. The moving energy that your body had has been stored inside the stretched elastic sheet. A strain force inside the sheet matches the force of your weight. If it did not, the sheet would split. The stretched sheet then releases its stored energy. It does this through the strain force, which pushes on your body so hard that you are flung into the air.

You do not stay in the air long. There is a force called gravity, which pulls everything down to the ground. As you rise in the air it pulls on your body and slows it down until it stops. The moving energy that the body had is changed to stored energy at the height you stopped above the trampoline. As you fall, the stored energy changes back to moving energy. When you bounce into the trampoline again your moving energy is stored in the elastic sheet, then the trampoline uses it to shoot you back up into the air. If you jump as you hit the elastic sheet you can shoot even higher and keep going as long as you want.

If you jump into a trampoline and do not move your arms and legs you eventually stop. Why is this?
It is due to the loss of energy. When you jump you use the stored energy in your body to make your body move. As you stretch the elastic sheet you slow down and lose moving energy and the sheet stores energy. It then releases this energy to push you into the air and gives it back to you. When you fall back onto the trampoline you give energy back to the sheet, then it gives it back to you and so on. At each change some of the energy is lost. For example, your feet rub against the surface of the sheet and some energy is lost as heat through friction. Eventually the energy is used up and you simply remain standing on the trampoline.

How can people jump higher on the trampoline?
They do just that. They jump as they hit the elastic sheet. This releases more energy from their body, which is used to increase the pushing force on the sheet. The extra pushing force stores up more energy in the sheet. This extra energy can then be used to shoot the person higher into the air. As long as a person jumps as they hit the elastic sheet they will keep bouncing. Just notice what people do when they want to come off a trampoline. They just hold themselves still and bounce less and less until they can walk off the sheet.

Where do we get our energy from?
Our energy comes from our food. You could also say that we get our energy from the Sun for the following reason. The Sun releases light and heat energy. Plants use light energy to make food and most of our food comes from plants. Meat comes from animals, which have eaten plants, so meat contains energy from the Sun too.

Are there different kinds of energy?
Yes, there are. There is stored energy. This can be energy which is stored in food, or it can be energy which is stored because a body is in a particular place. For example, if you knock an object off a shelf it will fall to the floor and show that it has a large amount of movement energy. If you knock an object over on the floor it may roll around but will not move as much. It did not have as much stored energy because it was on the floor and the other object was on the shelf. Objects higher from the ground have more stored energy than those closer to it. Light, sound, heat and electricity are other forms of energy.

Can energy change from one kind to another?
Yes, it can. For example some of the stored energy in your food changed to movement when you jumped into the trampoline. When you stretched the trampoline sheet your movement energy then changed into stored energy in the sheet. When the sheet pushed you into the air, the stored energy changed to movement energy again. As you rose to the top of your jump, your movement energy gradually changed to stored energy because you were high above the ground. When you fell back to the trampoline, the stored energy changed back to movement energy. If you had missed the trampoline and landed on the floor, some of the movement energy would have changed into sound energy and other people would hear you land with a thump. It might hurt, too.

Can energy be stored in an elastic band by twisting the band?
Yes, it can. When the elastic band is twisted, energy is stored and a strain force is built up. Then the stored energy is used by the strain force to untwist the elastic band. A twisted elastic band may be used to turn a propeller on a toy aeroplane or to make cotton reel dragster push its way across the floor.