A machine helps you do work by changing the amount or direction of the force you need to apply. There are six basic types of simple machines.
An inclined plane is a flat, slanted surface. A ramp is an example of an inclined plane. The longer the incline (the less steep the incline), the less input force you need to push or pull an object. You can increase the efficiency of an inclined plane by decreasing this friction. There would be less friction, for example, if you put the crate on a dolly with wheels and rolled it up the inclined plane instead of sliding it.
A wedge is a device that is thick at one end and tapers to a thin edge at the other end. It might be helpful to think of a wedge as an inclined plane (or two inclined planes back to back) that can move. In a wedge, instead of an object moving along the inclined plane, the inclined plane itself moves. Examples of a wedge include an ax and a zipper.
A screw can be thought of as an inclined plane wrapped around a cylinder. This spiral inclined plane forms the threads of the screw. The closer together the threads are, the greater the mechanical advantage. Other examples include bolts, faucets, and jar lids.
A lever is a rigid bar that is free to pivot, or rotate, about a fixed point. The fixed point that a lever pivots around is called the fulcrum. To understand how levers work, think about opening a paint can with a spoon. The spoon acts as a lever. The spoon rests against the edge of the can, which acts as the fulcrum.
A wheel and axle is a simple machine made of two circular or cylindrical objects that are fastened together and that rotate about a common axis. The larger object is called the wheel and the smaller object is called the axle. In a screwdriver, the handle is the wheel and the shaft is the axle.
A pulley is a grooved wheel with a rope (or a chain, or even a steel cable) wrapped around it. You use a pulley by pulling on the rope. As a result, you can change the amount and direction of your input force. A pulley that you attach to a structure is called a fixed pulley. A movable pulley is a pulley attached to something you want to move.