Simple machines make our lives easier by allowing us to do tasks with less force. The mechanical advantage is the factor of which a machine multiplies the force put into it. For example, if a simple machine has a mechanical advantage of 3 it means that it made the work you did 3 times easier or you only had to put in 1/3 the force you would have if you didn’t have the simple machine.
A screw is an example of a simple machine. Think of it as an inclined plane wrapped around itself. It is used to hold things together.
To calculate the mechanical advantage of a screw you must divide the length around the thread by the length of the screw. Therefore the closer the threads of the screw are to each other, the higher the mechanical advantage is and the less force you need to input!
MA= length around thread/length of screw
Source: Prentice Hall Science Explorer. Motion and Forces. Copywright 2007 Pearson Education Inc.