Two Blocks Are at Rest on a Horizontal Frictionless Surface

Multiple Choice

Two blocks are at rest on a horizontal frictionless surface with a compressed spring between them.The 10-kg block accelerates when the string holding the system in compression is cut and the 20-kg box accelerates in the opposite direction.What is the magnitude of the force from the expanding spring on the 20-kg box when the 10-kg box is undergoing an acceleration of 8.0 ?

A) 40 N
B) 160 N
C) (160/3) N
D) 80 N