Forces_Concept_8

Subject Facts

Machines are made up from mechanEsms, which are ways of turning a force into a big force or a small movement into a big movement (but not both at the same time). By using levers, pulleys or gears, you can make a small motor move a big load or move a small load more quickly (again not both at the same time).

Levers

A lever is a simple mechanism. It can be used to apply a small force over a long distance (with the same effect as applying a large force over a short distance). A door is a lever. Try placing your finger on a point near the handle and pushing it open at a steady pace, Now try placing your finger near the hinge and opening the door, You will notice that the force required was much smaller the first time, but that you had to push for much longer.

Here's another example. Think of a see-saw shared by yourself and a small child. How do you get a balance? Since you will be providing the bigger force (weight), you will need to position yourself a shorter distance from the pivot point, or fulcrum, than the child. If you are twice as heavy as the child then the child will need to be twice as far from the pivot point as you in order to balance you.

Levers are literally all around us, from tools like scissors and hammers to switches like twist knobs and car indicator stalks. All of them work in the same way: the further you get from the pivot point, the less force and the more movement you need to do the same work. Archimedes said that if he had a big enough lever; he could move the world!

Pulleys

A pulley is another way of transmitting and magnifying force by increasing the distance over which a force is applied, using one pulley (see Figure 12), you need to pull with a force slightly greater than the weight of the load to get it moving, and you need to pull over the same distance that you want to raise the load. However, it is more convenient and easier to pull downwards (working with gravity) than to pull up.

With two pulleys, you only need half of the force; but you will need to pull twice as far to raise the load by the same amount, With three pulleys, it will be one third of the force but three times the distance. The more pulleys you use in practice, the more friction will build up, so you will need to pull with a greater force than you might have expected.

Gears

These work to the same principle as levers and pulleys: scaling force down and distance up, or vice versa. In Figure 13, for every revolution of the big gear wheel (28 teeth), the small gear wheel (seven teeth) will go round four times. If these gears were on a bicycle, linked by a chain, which gear would you attach to the pedals and which to the wheel? f you attached the pedals to the big gear and the wheel to the small gear then the wheel would go round four times for every time that you turned the pedals. That would be fast; but your feet would have to push with quite a force, As long as you were going downhill it would be okay. What about the reverse arrangement: you have to turn the pedals four times for the rear wheel to go around once? This will be slow, but it will take very little force to press on the pedals, and you will be able to remain in balance at very slow speeds. You will thus be able to pedal up quite steep slopes without losing control or getting too tired.

Teaching ideas

Looking for levers (observing)

The children can explore the classroom or schooe to identify examples of levers. As a class, they can share their discoveries.

A long lever (testing)

The children can investigate leverage using a a metre rule, a book, string and some weights (see Figure 14). How far along the ruler does the weight need to go in order to lift the book? What effect does it have if they: