# Some students want to calculate the work done by friction as an object with unknown mass moves along a straight line on a rough horizontal surface. The students have a force probe, a meterstick, and a stopwatch. Which of the following will allow the students to take the measurements needed to calculate the work done by friction?

A. Pulling the block at an unknown constant acceleration with the force probe for a measured time.
B. Pulling the block at an unknown constant speed with the force probe for a measured time.
C. Pulling the block at an unknown constant acceleration with the force probe for a measured distance.
D. Pulling the block at an unknown constant speed with the force probe for a measured distance.

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1. If you pull the block with a constant speed, this means that there is no net acceleration, and hence the pulling force (force displayed on the force probe) equals the magnitude of the frictional force.
Hence, a constant speed would allow you to identify the magnitude of the frictional force.

Further, Work = Force * Distance along direction of Force

Hence, the distance must be measured.

The answer is D.

(Note that the work done would be negative since in this case the direction of displacement is exactly opposite to that of the frictional force)

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