Question

A horizontal plank has a rectangular block placed on it. The plank starts oscillating vertically and simple harmonically with an amplitude of $40cm$. The block just loses contact with the plank when the later is momentarily at rest. Then

• A. the period of oscillation is $2\pi /5s$
• B. the block weighs double its weight when the plank is at one of the positions of momentary at rest
• C. the block weighs $1.5$ times its weight on the plank half way down
• D. the block weighs its true weight on the plank, when the latter moves fastest

Answer 1

Answer: (B)

At one of the extreme position, weight of block $=$ restoring force. At the other extreme position, weight of block and restoring force both act downward direction. So the force on block at is doubled than its weight.