* They have a mechanical advantage, which is the ratio of the output force to the input force. The mechanical advantage can be greater than 1, less than 1, or equal to 1.
* They are reversible, meaning that they can be used to perform work in both directions. For example, a lever can be used to lift an object or to lower it.
* They are efficient, meaning that they do not waste energy. The efficiency of a simple machine is the ratio of the output work to the input work. The efficiency of a simple machine is always less than 100% because some energy is always lost to friction.
The six types of simple machines are:
* Levers are rigid objects that pivot around a fixed point called a fulcrum. Levers can be used to lift objects, move them, or hold them in place.
* Wheels and axles consist of a wheel attached to a shaft. When the wheel is turned, the shaft rotates. Wheels and axles can be used to lift objects, move them, or change the direction of a force.
* Pulleys are wheels with a grooved rim around which a rope or cable can be passed. Pulleys can be used to lift objects, move them, or change the direction of a force.
* Inclined planes are sloping surfaces. Inclined planes can be used to lift objects, move them, or change the direction of a force.
* Wedges are triangular-shaped objects that can be used to split objects apart, lift them, or move them.
* Screws are spiral-shaped objects that can be used to hold objects together, lift them, or move them.
