The Wedge

The wedge is a kind of inclined plane. The difference is that an inclined plane stays in one place, but a wedge moves. One common use of a wedge is for splitting logs to make firewood. Wedges are sometimes used under doors to prevent them from blowing shut. The Egyptians may have used wedges to split the rocks that were used in building the pyramids. By hammering several wedges into a crack between two layers of rock, the rock could be weakened. Eventually the rock would split. 

The Screw

The screw is similar to the inclined plane and the wedge. It is an inclined plane that is rolled up. You can see this if you cut a right-angle triangle of paper, darken the long side, and roll it around a pencil as shown in the drawing. The pattern that results is much like the thread on a screw. If the inclined plane has a gentle slope, the thread will be fine. If the slope is steep, the thread will be coarse. You can follow the edge of the paper around the pencil with your finger and see how it makes a spiral. Screws are used to join pieces of wood or metal. When you use a wood screw, you apply a turning force to the screwdriver with your hand and wrist. As the screw turns, the threads pull the screw into the wood. In other words, a screw changes a turning force into a pulling force. Threads have many uses. The lids of jars and pop bottles are often threaded so that they can be closed tightly. Bolts and nuts depend on threads for doing their task of holding things together.

The Pulley

The pulley is a wheel that turns around an axle. A groove in the edge of the wheel allows a rope to pass over it or under it without slipping off. There are two kinds of pulleys fixed pulleys and movable pulleys. In a fixed pulley, the wheel turns but the arm holding the wheel is fixed in one place. A movable pulley moves up or down while the wheel turns. 

Sometimes the two kinds of pulleys are combined. The drawing at the left on the next page shows one fixed pulley and one movable pulley, used together to lift a load. The load is easier to lift this way than with a single fixed pulley. In fact, you can overcome a resistance of 50 N in this way with an effort of only 25 N. A block and tackle uses two fixed pulleys and two movable pulleys. With this combination, you can overcome a resistance of 100 N with an effort of 25 N.

The Wheel and Axle

 A wheel and axle machine is just that a wheel fixed to an axle, like the steering wheel of a car. When a steering wheel is turned, the axle, or shaft, turns with it. It would be impossible to turn the steering axle by grasping it in your hands. The wheel that is attached to the top of the axle makes steering possible. By grasping the rim of the wheel, the driver can easily apply enough force to turn the axle. The doorknob is another common example of the wheel and axle machine. As with the pulley and the lever, the advantage of greater force is matched by a disadvantage of increased distance.

The Lever

The lever may have been the first machine man learned to use. Cavemen almost certainly knew how to move a heavy rock with the help of a strong pole. All they had to do was to force one end of the pole under the rock and put a smaller rock on the ground under the pole near the same end. By pushing down on the other end of the pole, they could raise the rock. 

About 4000 years ago, the Great Pyramid was built in Egypt. It took about 30 years to build. There is no record of how it was built, but the lever may have been the principal machine used. Great teams of workmen may have levered the massive blocks of limestone out of quarries (workweek), dragged them to the site of construction, and then used inclined planes built of sand to raise them up onto the pyramid. Each block has a mass of about. Today, levers are more popular than ever. They are used in many household tools, such as scissors, pliers, the crowbar, and the claw hammer. 

Simple Machines

There are just six kinds of simple machines. Each kind has many uses. Big machines are merely many simple machines put together. The six kinds of simple machines are the lever, the pulley, the wheel and axle, the inclined plane, the wedge, and the screw.

The Meaning of Work

In everyday conversation, we use the word work in various ways. "Going to work", "doing homework", "a writer's work" these are some common expressions using the word. The meaning of "work" varies. But, to the scientist, "work" has the same meaning every time it is used. In science, we say that "work" has been done only if a force of some kind has moved an object from one place to another place. "Work is done when a force moves an object through a distance" is how scientists describe it. 

You are doing work, in the scientific sense, if you lift a bag of potatoes onto your shoulders. You use a force to raise the bag through the distance from the floor to your shoulders. Once the bag is on your shoulders, no more work is being done because there is no motion. The amount of work you did depends on the amount of force you had to use and the distance through which you moved the potatoes. For example, you do work whenever you stretch a spring or an elastic band. The more you stretch it, the more work you do.

Machines 

The modern world is filled with machines. Some of them are very simple, such as the can opener, the shovel, and the saw Others are more complicated, such as the bicycle, the lawn-mower, and the crane. All machines are designed to make work easier. There are three ways in which they can do this by increasing the force, by increasing the speed, and by changing the direction of the force. Here is an example of each of the three ways.