The diagram shows a fist displacing water as it punches into it with force.
Forces can cause displacement of objects and this is called work done by the force on the object. When a book falls off a table, gravity acts on the book and causes it to be displaced from the table to the floor. This means work was done on the book.
When studying work it is important to examine whether it is the force that causes displacement. It is only when the displacement is caused by the force that we can say the specific force has done work on the object.
Work is calculated by the following formula:
work = force × displacement
When the unit of the force is Newton (N) and displacement is measured in metres (m), the unit of work is Newton metres; 1 Newton metre is equal to 1 Joule (J).
In ancient times people invented simple machines that helped them overcome resistive forces and were able to do work that seemed impossible, like building the pyramids in Egypt.
Simple machines work in the following ways:
- transfer a force from one place to another
- change the direction of a force
- increase the magnitude of a force
- increase the displacement the force causes
- change the speed of the force.
Examples of simple machines are the lever, the wheel and axle, the pulley, the inclined plane, the wedge and the screw.
A machine produces a mechanical advantage when the force applied by the person using it (input force) is smaller than the force applied to the task (output force).