Two blocks, A and B, are being pulled to the right along a horizontal surface by a horizontal 100-N pull, as shown in the figure. Both of them are moving together at a constant velocity of 2.0 m/s to the right, and both weigh the same. Which of the figures below shows a correct free-body diagram of the horizontal forces acting on the lower block, B?
E) None of these diagrams is correct.

Question

Two blocks, A and B, are being pulled to the right along a horizontal surface by a horizontal 100-N pull, as shown in the figure. Both of them are moving together at a constant velocity of 2.0 m/s to the right, and both weigh the same.  Which of the figures below shows a correct free-body diagram of the horizontal forces acting on the lower block, B?
E) None of these diagrams is correct.

Quizplus · Accepted Answer

The correct free-body diagram of the horizontal forces acting on the lower block, B, should show only the force of friction acting to the left (opposing the direction of motion) and the horizontal pull force acting to the right (in the direction of motion), as no other horizontal forces are present. Choice B shows this correctly. In choice A, an incorrect downward force is included, in choice C an incorrect upward force is included, and in choice D, an additional force acting to the left is included, which is not present in the situation described. Therefore, the best choice is B.

Two Blocks, a and B, Are Being Pulled to the Right