Figure 8 The output work of the oars results from the oars pushing against the water so that the water pushes back against the oars. Observing What evidence can you see in the photo that a force is acting on the water?
Work Output of a Machine
The force that is exerted by a machine is called the output force. The distance the output force is exerted through is the output distance. The work output of a machine is the output force multiplied by the output distance. Look back at Figure 7 one more time to see the output force, distance, and work for the oars. How are the input work and output work related? Although nearly equal, the output work is less than the input work because of friction. All machines use some amount of input work to overcome friction.
As the rowers pull on one end of each oar, the other ends push against the water as shown in Figure 8. The force of the oar on the water causes an equal and opposite reaction force to be exerted by the water on the oar. It is this reaction force that propels the boat through the water.
Can you increase the work output of the oar by positioning it differently? Unless the new position decreases friction, the answer is no. If there is no change in the work input, there cannot be an increase in the work output. The only way to increase the work output is to increase the amount of work you put into the machine. You cannot get more work out of a machine than you put into it!
Section 14.2 Assessment
Reviewing Concepts
How can using a machine make a task easier to perform?- How does the work done on a machine compare to the work done by a machine?
What changes can a machine make to a force?
A machine produces a larger force than you exert to operate the machine. How does the input distance of the machine compare to its output distance?
You do 200 J of work pulling the oars of a rowboat. What can you say about the amount of work the oars do to move the boat? Explain.
How can you increase the work output of a machine?
Critical Thinking
Inferring When you swing a baseball bat, how does the output distance the end of the bat moves compare with the distance you move your hands through? Why might this difference be useful?
Applying Concepts An advertisement for a new type of wrench claims it reduces the force needed to tighten a bolt. If the advertisement is correct, what do you know to be true about the input distance?
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