Figure shows two identical blocks each of mass `m` kept on a smooth floor. Block `A` is connected to front wall with a just taut straight string and block `B` is connected to rear wall with a relaxed spring. Assume that the floor of the train car is smooth and exerts no horizontal forces on the blocks. Mark the correct statement(s).

Options

A. When train moves with constant velocity there is no force either in string or spring.
B. Immediately after the train speeds up string gets taut and spring is compressed such that force exerted by each on respective blocks is same.
C. When train slows down block `A` moves towards front wall.
D. When train slows down resultant force on block `A` is non zero in ground frame.

Answer

A, C, D

Explanation

Immediately after the train speeds up, an impulsive tension develops in string. But for spring it will take some time to compress it. when the train sows down string slackens and `A` moves towards front wall and no force on `A`.