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 would have a force arrow pointing to the left, representing the frictional force opposing the motion, and an arrow pointing to the right representing the tension force of 100 N applied to both blocks. Option B correctly shows these two forces acting on block B. Option A shows a force arrow pointing down (the weight of the block) and doesn't include the frictional force. Option C shows a normal force arrow pointing up, which is correct, but no force arrow representing the frictional force. Option D shows force arrows pointing in opposite directions, which is not possible when the blocks are moving together at a constant velocity. Option E is not a valid choice since one of the diagrams must be correct.

Two Blocks,A and B,are Being Pulled to the Right Along