Question

A block of mass rests on a block of mass 1 which is on a tabletop. A light string passes over a frictionless pulley and connects the blocks. The coefficient of kinetic friction between the blocks and between the block of mass 1 and the tabletop is the same. A force pulls the upper block to the left and the lower block to the right. The blocks are moving at a constant speed.

Which of the following statements is true?
A. The block of mass and the table both exert a leftward-pointing kinetic friction force on the block of mass 1 .
B. The force of kinetic friction between the blocks, and between the block of mass 1 and the tabletop, have the same magnitude because the coefficient of friction is the same.
C. There is no kinetic friction because the blocks are moving at constant speed.
D. The kinetic friction force that acts on the block of mass points to the left.

Answer 1

The correct explanation includes that kinetic friction is present and acts in the opposite direction of motion, even when blocks move at a constant speed. Kinetic friction forces act to the left for both interfaces, but their magnitudes might differ as they are dependent on the normal forces at each surface, not just the coefficient of kinetic friction.

Step-by-step explanation:

The correct statement addressing the scenario where a block of mass m rests on a block of mass 1, with both blocks being pulled in opposite directions and moving at a constant speed, should account for the direction and the magnitude of the kinetic friction acting on the blocks.

Since the blocks are moving at a constant speed, this implies that the net force acting on the system is zero. Despite this, kinetic friction is indeed present; it acts in the opposite direction to the motion of the objects. Therefore, the kinetic friction between the block of mass m and the block of mass 1 points to the left, while the kinetic friction between the block of mass 1 and the tabletop also points to the left, counteracting the pulling force on the block of mass 1.

However, the magnitude of the kinetic friction forces may differ, depending on the normal forces acting on each interface. Thus, we cannot claim that the kinetic friction forces have the same magnitude solely based on the coefficient of kinetic friction being the same. The magnitude also depends on the normal force acting on each surface, which would differ between the block of mass m and block 1, and block 1 and the tabletop.