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Consider the following information:
Bob pushes against a rock of mass 50 kg for 20 minutes exerting an average force of 100 N but the rock never budges. He really wants to move the rock so he asks his wife Barbara to come out and help. She pushes on the rock with another 100 N and eventually the rock starts to move. As the couple keeps pushing, it becomes easier and easier to move.
At the end of the driveway is a ledge and the couple push the rock off the ledge. The rock falls for 3 seconds and eventually crashes into the riverbed below.

According to a physicist, work is defined as the force applied to an object multiplied by the distance the object moves during the time the force is applied. Using this definition, how much work did Bob do before Barbara helped him?
 

Explanation

Work is defined as the force (F) multiplied by the distance moved (d). Since the rock didn’t move, the work done is zero. Choice (A) is correct.
 

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