Physics

# A Guide to Simple and Compound Machines

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When you think of a machine, you probably think of the washing machine you use to do the laundry or the car that takes you from place to place. Those are examples of very complex machines. The least complicated types of devices are known as simple machines.

The definition of a simple machine is a device that makes work easier by altering the amount or direction of an applied force. There are six types of simple machines: an inclined plane, a wedge, a lever, a screw, a pulley, and a wheel and axle.

An inclined plane is a slope like a ramp. It allows you to move an object from one height to another with less force, but the trade-off is that you have to move it a longer distance. An example is rolling a grocery cart up a ramp. If you had to pick up the cart to move it to the higher elevation it would be much harder, but to get it up the ramp you have to roll it the entire length of the ramp. In physics terms, you have traded a greater force (lifting up) over a shorter distance (the height of the step) for a lesser force (pushing) over a longer distance (the length of the ramp).

A wedge is an inclined plane used to redirect force. A wedge converts forward motion into sideways motion at a right angle to the blade. Nearly all cutting tools - knives, chisels, axes - are wedges. When you cut into a melon with a knife, you are using lesser force to cut downward through the melon. The blade of the knife diverts that force into stronger force that moves a short distance sideways in both directions to split the fruit.

A screw is a central rod with a thread wrapped around it to form a helix. This thread acts like an inclined plane to change the direction of force. It allows you to move the screw in a rotary motion which is then converted to forward or backward motion, depending on which way you turn the screw.

A lever is a straight rod or board that pivots on a point known as a fulcrum. If the fulcrum is in the middle of the lever, the force changes direction equally from one side to the other, up on one side and down on the other, like a seesaw. If the fulcrum is not in the middle, then you can exert a lesser weight or force over a longer distance by pushing down on one side to move a greater weight over a shorter distance upward on the other side. A crowbar is an example of this type of lever.

A pulley is a wheel, usually with a groove around the outside edge. Used with a rope or cord, it changes the direction of force. A davit that lifts a boat from the water uses a pulley. A flagpole has a pulley at the top so you can hoist the flag by pulling down on the rope on one side and the flag rises on the other side of the pulley.

A wheel and axle is a wheel with a fixed rod through the middle. When the wheel moves, the axle moves with it. Because the wheel has a larger circumference, when force is applied to turn the wheel, the longer motion around the perimeter of the wheel is converted to a shorter and stronger motion at the axle. Conversely, when a strong, shorter motion is used to turn the axle, the edge of the wheel moves a longer distance. Door knobs and bicycle wheels are examples of this type of machine.

You need one more basic item to complete your set, and that is a gear. Gears are used to change the direction of force, and are a special type of lever. The direction of one gear will always be opposite to the direction of the gear next to it. Gears can be combined in various ways. Bevel gears are set at an angle to one another. A rack and pinion is a single gear turning on a toothed rack. There are many other possibilities.

Now that you have examples of all the simple machines, you can put them together into compound machines. A pair of scissors is two wedges (the cutting blades) and two levers (the handles). A wheelbarrow is a wheel and axle and two levers (the handles). A manual can opener includes a wedge (the blade that cuts the can lid), two levers (the handles) and a wheel and axle (the hand crank you turn). You can continue adding simple machines together until you create complex machines like automobiles.

Now when you look at a machine like a garden hoe or an eggbeater, you can recognize the simple machines that go into the construction of everyday tools that make our lives easier.

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