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Work

Simple Machines

A machine is a device that helps people do work against some resistive force. Some machines are powered by engines, motors and even animals, while many others simply use human power. The basis of all complex machines comes from their simple components. When these components are by themselves, they are called simple machines. The most common simple machines are the lever, rollers, the ramp, and the pulley.

Questions you may have are:

  • What is the background of machines?

  • How does a lever, roller, and pulley work?

  • What about machines that run forever?

Background of inventions

Before engines and motors were invented, people had to do things like lifting heavy loads by hand. Using an animal could help, but what they really needed is some clever ways to either make work easier or faster. Ancient people invented, simple machines that would help them overcome resistive forces and allow them to do the desired work against those forces.

Ancient Egyptians

The ancient Egyptians, for example, used such inventions to help them build the pyramids. They used levers to pick up large blocks of stone. They put those blocks on rollers to move from one area to another. Then they used ramps to move the blocks up to the top of the pyramid they were building.

Ancient Romans

The ancient Romans used catapults to throw stones at their enemies. The catapult was a large lever. They used a pulley to pull down the arm of the catapult. The device was set on wheels--an advanced version of rollers--to move it from place to place.

Today

We still use those simple machines today, by themselves and as part of more complex machines.

The lever

In order to lift up a heavy object, such as a huge rock, the lever was invented. We still use it today to lift things. It also can be used as a teeter-totter and a catapult.

Push on E to lift up the weight R with this lever

Suppose you wanted to lift up a box that weighed 200 pounds. If you used a lever with the distance from the pivot point or fulcrum F to the weight RA = 1 foot and the other distance EA = 10 feet, then you would only have to push down at E with 20 pounds of force.

This is because of the rule that E * EA = R * RA. In other words,
20 pounds times 10 feet = 200 pounds times 1 foot.

Although you have what they call a mechanical advantage in being able to lift this heavy weight, you are actually doing the same amount of work, because you have to push the 20 pound force 10 times as far as the 200 pound box moves. For example, to life the box 3 inches, you have to push the 20 pound force for 30 inches.

Since Work is Force times Distance, then you can see that:

20 pounds x 30 inches = 200 pounds x 3 inches.

Teeter-totter

A teeter-totter is a lever that children use as a play thing. Since the children are usually approximately the same weight, the fulcrum is placed in the center of the board.

A child sits on each end of the board, and they take turns lifting each other off the ground.

Catapult

A catapult is a lever in reverse. A heavy weight is dropped on the short side of the level, catapulting a lighter weight from the longer side of the lever. This works because not only does the weight on the long arm of the lever (EA in the picture above) travel a greater distance, but it also goes faster.

If EA = 10 feet, RA = 1 foot, and you drop R = 200 pounds at 32 feet per second, weight E = 20 pounds will fly in the air at 320 feet per second!

Different configurations

There are different configurations for levers, where the fulcrum is placed on one end or the other. The principles still hold.

Rollers

The resistive force of friction is one of the first thing ancient people wanted to overcome. In order to construct buildings they had to drag large blocks from one place to another. Friction made the job difficult.

One way to reduce friction is to lubricate the sliding surface with oil. But this was not practical in most situations. Then someone came up with the idea of placing logs or rollers under the object, so that the friction was greatly reduced.

They used a lever to lift the object onto the rollers, and then used the rollers to reduce friction and easily move the heavy object from one place to another.

After hundreds of years, rollers were replaced by the invention of the wheel on an axle. A wheel is simply a refinement of the roller.

Movers use a rack with wheels called a dolly to move heavy furniture. Where else have you seen wheels being used to reduce the force of friction?

Ramp

Although rollers reduce friction, a ramp may be used to gain a mechanical advantage or to reduce the amount of force required. But remember that the amount of work required is the same (not counting the losses due to friction).

A ramp is also called an inclined plane. By rolling an object up a hill or a ramp, you require less strength than required to pick the object up the same height, but you compensate by traveling a greater distance. This ability to move an object to another height works the same as with a lever.

"Hey, stick-man. Roll that barrel up the ramp!"

If you lifted a barrel that weighed 200 Newtons up 6 meters in height, the work against gravity would be 200 x 6 = 1200 Nm. If you rolled that barrel up a ramp 12 meters long, you would only have to push with a force of 100 N. That is because 200 x 6 = 100 x 12.

(Note that by rolling instead of sliding, you are drastically reducing the extra resistance from friction.)

Variations on the ramp or inclined plane include the door-jam or wedge and the threads of a screw or bolt. Threads are simply a ramp that are wrapped around a cylinder like a bolt.

The pulley

A pulley is a way to use your own weight to lift an object to another height. The same force must be used, but it simply changes direction. You pull down and the weight goes up.

Pull on the rope to lift the heavy weight

Perpetual motion machines

Many people have tried to invent a machine that would run forever, once you got it started. They call such a machine a perpetual motion machine.

Of course, the big problem with a machine running forever concerns the losses due to friction. To overcome those losses, many of these machines have been designed to use gravity for power. Unfortunately, that just doesn't work. Like the saying goes, "What goes up must come down."

There are other machines that improve their efficiency by using other means of power. An inventor recently came up with an automobile with a large flywheel. When the car went downhill, the flywheel would gather energy to be used when the car went uphill. But it still needed an engine to make up the difference of what was lost due to the resistive forces.

In conclusion

Simple machines usually exchange using a smaller force over a greater distance to move a heavy object over a short distance. The work required is the same, but the force required is less. The are also simple machines that help to reduce the resistance of friction or such.





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