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The Wheel and AxleA wheel and axle is really two machines in one because you can use each part in different ways. The first way is to roll something along. Wheels help you move an object across the ground because they cut down on the amount of friction between what you're trying to move and the surface you're pulling it against. (The axle is the object that attaches the wheel to the object it's moving.) Since only the very bottom of the wheel touches the ground, there is less surface area to rub — and less friction. Imagine pulling a little red wagon without any wheels! Generally speaking, the bigger the wheel, the easier it is to make something roll.
The second way of using a wheel is like a lever in the round. A door knob or a faucet on a sink are really round levers, and the "fulcrum" is in the middle where the axle turns. Imagine if a door knob was replaced with a little rod. It would be much harder to open the door! Once again, there's a trade-off: The larger the diameter of the wheel, the less effort you need to turn it, but you have to move the wheel a greater distance to get the same work done.
