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

Since both blocks move with the same acceleration, we can assume that the net force acting on them is the same. Thus, we can write:
F - frictional force = Ma
where M is the mass of the small block. Rearranging, we get:
frictional force = F - Ma = 1.2 N - (1.0 kg)(1.2 m/s^2) = 0.4 N to the right (as friction is in the opposite direction of the applied force F).

Two Blocks Are Accelerated Across a Horizontal Frictionless Surface as Shown