Flat springs
Can you twang a ruler? Probably everybody can. It is one of those things that people learn while the teacher isn't looking. What happens when you twang a ruler? First you hang about half of it over the edge of your table. Next you hold down firmly the part that is resting on your table. Then you bend down the end of the ruler that is hanging out from the table. Finally you let go of the end and the ruler moves up and down and makes a rattling sound. But what is really happening in the ruler to make it move in this way?
Rulers are made from long strips of wood or plastic. When these materials are cut into these shapes, they have elastic properties. That means that if they are bent out of shape they will spring back to the original shape as soon as they can. When you bend down the end of the ruler, you are bending it out of shape. You are applying a pushing force to the end of the ruler. Inside the ruler a strain force builds up. While your finger is pushing down on the end of the ruler the strain force pushes back on your finger. It is not stronger than your pushing force so cannot push your finger out of the way. It has to wait until your pushing force has gone. This happens when you remove your finger.

When you take away your finger, the strain force suddenly has no force working to stop it, so it pushes up the end of the ruler. In fact, there is so much power in the strain force that it pushes the ruler even further upwards and sets up a strain force in the opposite direction. This pushes the end of the ruler down but, again, this is so powerful that it pushes the ruler further downwards and a smaller strain force forms which pushes the ruler upwards again. This pushing up and down of the strain forces occurs very quickly and each force is weaker than the last. This means that the ruler bends up and down a great deal when you let go the end but eventually the amount the end of the ruler bends up and down gradually decreases until it stops. When this happens, the ruler is straight. It is in its original shape ? the shape it was in before you pushed on its end.

The strain force in a ruler can be used to move an object. Take a small piece of paper and screw it up into a ball. Put the ball of paper on the end of the ruler and bend the end down. When you let go of the end, the strain force will make the ruler bend up rapidly and push on the ball. There is a pulling force on everything on the Earth. It is called gravity and pulls everything down to the ground. The push of the strain force on the ball is so strong that it is greater than the pull of gravity and it sends the ball into the air. Once the ball is in the air the strain force can no longer push on it. Only gravity pulls down on the ball and so it slows and eventually changes direction and falls to the ground. (Pick it up before you are accused of making litter!)

At some swimming baths there is some equipment like a giant twangy ruler. It is a springy diving board. As you walk along it, you can feel its springiness beneath your feet. If you run along it and jump down on its end, you make the end bend down. The strain force in the diving board pushes up with such power that it raises the end of the diving board and gives you a push as you jump off into the water.

Rulers and diving boards are two examples of flat springs but there are more. When you call for your friend on your way to school you may use a flat spring. It does not let you dive into their home but it lets your friend know that you are there. The flat spring is in the doorbell. When you push on the doorbell, the spring is bent down to another piece of metal and electricity flows through them to make the bell ring. When you stop pressing the button in the doorbell, the flat spring bounces back and the flow of electricity is stopped. This makes the bell stop ringing.

If you ride to school on a bus, there are flat springs beneath you, which make your ride less bumpy. These flat springs are called leaf springs. They are connected to the wheel and to the body of the bus. When the bus goes over a bump in the road, the leaf springs bend and stop the body of the bus rising up. When the bus has passed over the bump the strain forces in the leaf springs push them back into shape ready for the next bump. If the bus did not have leaf springs, you would be shaken about inside the bus.

Next time you are tempted to twang a ruler, don't think about the noise it will make, think about the forces that are acting to bring the movement about. Your teacher may even be impressed with what you know.

Are leaf springs shaped like leaves?
They are long strips of metal arranged in a small stack. There may be four or five strips in the stack. The strip at the bottom is the shortest. It may be less than half a metre in length. The strip at the top is the longest. It may be over a metre in length. The other strips have lengths in between these two. They are arranged so that from the bottom upwards the strips become longer and longer. In fact they look like an archer's bow. Think that behind a bus's wheel is an archer's bow arranged horizontally and bent so it is pointing to the ground. The ends of the bow are attached to the body of the bus and the centre of the bow is attached to the wheel. When the wheel goes onto a bump it pushes up on the bow and straightens it. When the wheel comes off the bump, it no longer pushes on the bow which bends again.

Do other vehicles have leaf springs?
Yes, they do, but the vehicles tend to be large. For example, you may see leaf springs around the wheels of a railway carriage. Leaf springs may be found on the back wheels of larger cars too.

What other kinds of springs do cars have?
They have open-coiled springs. Each spring connects the axle behind each wheel to the car body.

If cars have springs why don't they bounce up and down when they go over bumps?
Connected to each spring is a shock absorber. It is a little like a bicycle pump filled with oil. This means that any pushing and pulling is slowed down by the movement of the piston in the oil. When a spring is squashed or bent, this movement is slowed down by the oil inside the shock absorber. This slowing of the springs stops the car body and the passengers inside from moving up and down too quickly.

Is an archer's bow a flat'spring?
Yes, it is. At one time bows were made from yew wood. This is very springy wood. Today a range of materials may be used to make a bow but it still works in the same way. When you pull back on the bowstring, a strain force forms in the bow. You can feel the power of this force pulling on the bowstring as you pull back the arrow. When you let go of the bowstring the strain force has nothing to stop it acting so it pulls back the bow to its original shape. As the bow straightens it pulls on the string, which in turn pushes on the arrow and sends it into the air.

Why does the arrow fall to the ground?
Once the arrow loses contact with the bowstring, it is no longer being pushed. There are two forces acting on it now. They are the push of the air and the pull of gravity. You can feel the push of the air if you take a piece of cardboard and try and run with it. The push of the air on the arrow is only small because the arrow only has a small surface moving forwards through the air. The pull of gravity acts on everything on the Earth and pulls it all to the ground. As the arrow makes its flight, gravity gradually pulls it down to the ground.