Multiple-object systems without friction: A 6.00-kg block is in contact with a 4.00-kg block on a horizontal frictionless surface as shown in the figure. The 6.00-kg block is being pushed by a horizontal 20.0-N force as shown. What is the magnitude of the force that the 6.00-kg block exerts on the 4.00-kg block?
A) 6.00 N
B) 20.0 N
C) 8.00 N
D) 4.00 N
E) 10.0 N

Question

Multiple-object systems without friction: A 6.00-kg block is in contact with a 4.00-kg block on a horizontal frictionless surface as shown in the figure. The 6.00-kg block is being pushed by a horizontal 20.0-N force as shown. What is the magnitude of the force that the 6.00-kg block exerts on the 4.00-kg block? 
A) 6.00 N 
B) 20.0 N 
C) 8.00 N 
D) 4.00 N 
E) 10.0 N

Quizplus · Accepted Answer

According to Newton's third law, the force that the 6.00-kg block exerts on the 4.00-kg block is equal in magnitude but opposite in direction to the force that the 4.00-kg block exerts on the 6.00-kg block. Since the 6.00-kg block is being pushed with a 20.0-N force, the force that it exerts on the 4.00-kg block is also 20.0 N. However, since the 4.00-kg block is not moving, we know that the net force acting on it is zero. Therefore, the force of the 6.00-kg block on the 4.00-kg block must be equal and opposite to the force that the 4.00-kg block exerts on the 6.00-kg block, which is 8.00 N (calculated using F = ma where m is the mass of the 6.00-kg block and a is the acceleration of the system, which is equal to the force divided by the total mass of the system). Therefore, the magnitude of the force that the 6.00-kg block exerts on the 4.00-kg block is 8.00 N.

Multiple-Object Systems Without Friction: a 6