Work Input and Work Output
To row the boat in Figure 7, the rower pulls back on each oar handle and the other end of each oar pushes against the water. Work is done on the oars (the machine) by pulling on them, and the oars do work on the water to move the boat. Recall from Chapter 12 that friction acts against the motion of any object. 
Because of friction, the work done by a machine is always less than the work done on the machine.
Work Input to a Machine
The force you exert on a machine is called the input force. The distance the input force acts through is known as the input distance. The work done by the input force acting through the input distance is called the work input. The work input equals the input force multiplied by the input distance.
Each oar in Figure 7 is a machine. For the oar, the input force is the force exerted on the handle and the input distance is the distance the handle moves. The work input is the work you do to move the handle. You can increase the work input by increasing the input distance, increasing the input force, or increasing both at once.
Figure 7 The oars of the boat act as machines that increase the distance over which the force acts. Predicting If the oar is pushed farther away from the boat, how will the force needed to pull the oar through the water change?
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