Final answer:
The most appropriate word to fill in the blank is 'input'. When you apply a force to a machine, you are providing the machine with an input. This input force can be used by the machine to produce an output.
Step-by-step explanation:
The most appropriate word to fill in the blank is input. In the context of the question, when you apply a force to a machine, you are providing the machine with an input. This input force can be used by the machine to produce an output.
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