A block is pulled along a horizontal surface with a constant horizontal force of magnitude . The surface exerts a frictional force of constant magnitude
on the block. The graph of speed
as a function of time
for the block is shown above.
If, instead, the block slides without friction but is pulled with the same horizontal force of magnitude , which of the following would be a possible new graph of speed
as a function of time
? (The dashed line represents the old graph.)
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The correct answer is (A). Without friction, there is still a constant force on the block. This means a constant acceleration, so the speed curve will still be a straight line. Removing friction will not cause the block to be in motion before it is pulled, so the initial speed remains zero. Finally, since friction would act against the pulling force, the net force on the block is greater without friction, so the acceleration and thus the slope of the speed graph would be greater