Springs at work
Stand up very slowly. You should feel the muscles in your legs working. You should feel your feet push against the ground. At the same time as you feel this you should feel your muscles pushing your head and body upwards until you are standing up straight. Now walk forwards slowly and feel your feet push on the ground as you raise your heel. The pushes that you are making are called forces. You make another kind of force too ? a pulling force. Just sit down again and pull the mouse across the mat towards you to realise how your body makes a pulling force.

When you push on the ground, the ground pushes back with an equal force. If it pushed back with a weaker force you would sink into it ? just like you do on a muddy playing field. Sometimes, when you push down on an object, material that you are pushing on also gives way but it stores up another force as well at the same time. This force is called a strain force. The strain force builds up inside the material while it is being squashed but the moment you stop squashing the material a change takes place. The strain force pushes outwards and makes the material go back to its original shape.

If you take a spring and squash it, a strain force builds up inside the coils and remains there all the time the spring is squashed. Remove the squashing force and the strain force pushes the spring quickly back to its original position. The power in the strain force may be used to make something work. Think of the coiled spring that is squashed into a jack-in-the-box then released to give someone a shock. A pinball machine also has a spring which you squash and release but the power of the expanding spring sends a metal ball shooting around the game to score points.

Springs, which you squash, have gaps between their coils. They are called open-coiled springs but there is another kind. Some springs have coils so close together that you cannot squash them. These springs are called close-coiled springs. Although you cannot push a close-coiled spring, you can pull it. When you do, the spring stretches. Inside the close-coiled spring, or any material that stretches, another kind of strain force builds up. This force is called tension. The tension force is a pulling force. When the spring is no longer being stretched by a pull, the tension force pulls it back to its original shape. Have you seen someone using springs to exercise their chest muscles? These springs make a device called a chest expander. When the person pulls on the springs, they stretch across the chest. When the person relaxes, the tension forces in the springs pull them back to their original length.

We use the stain and tension forces in springs in many different ways. When you lie on your bed your weight presses down on the mattress. Inside the mattress are open-coiled springs. They become squashed as your weight pushes down on them and a strain force builds up in them. In fact the strain force becomes so large that it stops the spring from being completely squashed. This adds to your comfort. If the springs were squashed flat you would feel them pressed against the hard base of the bed.

While springs that squash are used for comfort, springs that stretch are used for closing doors and for weighing machines. At the top of many doors, especially doors in the corridors of schools and hospitals, are door-closing devices. These doors are called fire doors and the door-closing device may contain a close-coiled spring. When you open the door, you have to pull against the tension force in the spring. Sometimes the door may be hard to open because the spring is so strong. Once you are through the door and let it go, the strain force takes over and makes the spring shorter again. As the spring contracts it closes the door.

Next time you are using a device which has a spring in it such as a spring balance, think about how the spring is being used ? is it being squashed or stretched and what will the strain or tension forces do when the spring is released?

Where do we get our power from to make pushes and pulls?
Our power comes from energy in food. The body uses oxygen in the air to release the energy from food. Our muscles use this energy to pull on the bones and move our limbs or even the whole of the body.

Are our muscles like springs?
No. A muscle can only get shorter. It cannot go back to its original length on its own. It needs another muscle to pull it back to its original length. For example, the muscle at the front of your upper arm shortens to raise your lower arm but has another muscle behind your upper arm, which helps it straighten up again. Springs can go back to their original shape on their own. They just use the strain or tension forces they have stored up.

The springs in mattresses are cone-shaped. Why is this?
A spring that is wide squashes easily but a spring which is narrow squashes much less easily. When you lie on a mattress, the wide part of the coneshaped spring squashes easily and gently for maximum comfort. Then as you sink into the spring, the narrow part of the cone-shaped spring builds up a strong strain force to balance your weight and stop you squashing the spring flat.

Have people always had springy mattresses?
No. Coiled springs only came into use about two hundred years ago. Before that time, the mattress may have been stuffed with feathers, wool, straw and even dried pea pods.

What was the first use of a coiled spring?
They were used in mousetraps over six hundred years ago.

Why are the springs so strong on fire doors?
A fire needs oxygen in the air. If a fire starts in a building it draws in oxygen from all the rooms and corridors around it until a hole is burnt in the roof or walls and air can enter from outside. The springs on the fire doors are strong so that they can hold the door closed. If a fire breaks out in a building with fire doors oxygen cannot get as quickly to the fire and there is a better chance of everyone leaving the building safely and the fire brigade arriving before the fire becomes widespread.

What shape is the spring in a clock?
It is neither a close-coil nor an open-coil spring. It is a strip of metal that is wound round on itself. When the clock spring is wound up it forms tighter and tighter coils. A strain force builds up in the spring. When the key is removed from the clock the strain force makes the coils gradually unwind. As they do so, they push on the cogwheels inside the clock and they turn the hands across the face of the clock.

If a spring is completely stretched will it spring back into shape?
No. A spring can only be stretched a certain amount. If it is stretched too much it will not go back to its original shape.

Why can springs be used in weighing machines?
Because they increase in length steadily as the weight on them is increased. This only works up to a point because if the weight stretches the spring too much the spring is destroyed. Weighing machines have devices on them to stop the spring being stretched too much.