Question

A box is put on a scale which is adjusted to read zero , when the box is empty. A stream of pebbles is then poured into the base from a height h above its bottom at a rate of n pebbles/s. Each pebble has a mass m. If the pebbles collide with the box such that they immediately come to rest after collision, then the scale reading at time t after the pebbles begin to fill the box is –

• A. mnt
• B. mn [$\sqrt{2gh}$+ gt]
• C. mngt
• D. Zero

Answer 1

Answer: (B)

mn $$$\sqrt{2gh}$+ gt$$

Answer 2

Scale reading = v $\frac{dx}{dt}$ + (mnt) g

= $\sqrt{2gh}$mn + mng t

= mn ($\sqrt{2gh}$+ gt)