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 the surface is frictionless, the only forces acting on the blocks are gravity and the force F pushing the large block. By Newton's second law, the acceleration of both blocks is a = F/(M+m), where M is the mass of the large block and m is the mass of the small block. In this case, a = 1.2/(1.0+2.0) = 0.4 m/s^2. Since the two blocks move with the same acceleration and do not slide relative to each other, the force of the large block on the small block must be equal in magnitude and opposite in direction to the horizontal component of the force of the small block on the large block. Therefore, the frictional force acting on the small block is f = m*a = 2.0*0.4 = 0.8 N to the left. The horizontal component of the force of the large block on the small block is then 0.8 N to the right, by Newton's third law (action-reaction). Therefore, the correct answer is (C) 0.40 N to the right.

Two Blocks Are Accelerated Across a Horizontal Frictionless Surface as Shown.Frictional