Input force is the initial force used to get a machine to begin working. Machines are designed to increase the input force for a larger output force.
The quality of a machine is measured by mechanical advantage. The mechanical advantage is the ratio of the output force to the input force. A higher amount of mechanical advantage means that a machine simplifies work better by minimizing the amount of input force necessary for a high output force. For example, a lever is a simple machine. To use a lever to move a log, one end is placed under the log and the other is extended over a fulcrum. The input force is pressure used to force the extended end of the lever downward. The lever increases that force so that the force under the log is greater than the input force. This causes the log to move with minimal effort.