Two blocks are accelerated across a horizontal frictionless surface as shown. Frictional forces keep the two blocks from sliding relative to each other, and the two move with the same acceleration. If F = 1.2 N and M = 1.0 kg, what is the horizontal component (frictional force) of the force of the large block on the small block?
A)0.40 N to the left
B)0.80 N to the right
C)0.40 N to the right
D)0.80 N to the left
E)1.20 N to the left

Question

Two blocks are accelerated across a horizontal frictionless surface as shown. Frictional forces keep the two blocks from sliding relative to each other, and the two move with the same acceleration. If F = 1.2 N and M = 1.0 kg, what is the horizontal component (frictional force) of the force of the large block on the small block? 
A)0.40 N to the left 
B)0.80 N to the right 
C)0.40 N to the right 
D)0.80 N to the left 
E)1.20 N to the left

Quizplus · Accepted Answer

The force applied on the small block is transmitted to the larger block via the string. Therefore, the force of the larger block on the small block is also 1.2 N to the right. The frictional force on the small block due to the larger block is equal in magnitude and opposite in direction to the force of the larger block on the small block, which is 1.2 N to the left. Since the two blocks move with the same acceleration, the net force in the horizontal direction on the larger block must be equal to the net force on the smaller block. Therefore, the frictional force on the larger block due to the smaller block must also be 0.4 N to the right. Thus, the horizontal component (frictional force) of the force of the large block on the small block is 0.4 N to the right.

Two Blocks Are Accelerated Across a Horizontal Frictionless Surface as Shown