Match the rocks with the material they are made of.

N = 50 rev

Explanation:

Initially tub is at rest and then accelerated uniformly to its final speed

so we have

time taken by it to reach the final speed

t = 8 second

now the total number of revolutions is given as

now we have

Now when its lid is opened its safety switch is off due to which it is uniformly decelerated and comes to rest in 12 s

so we have

So here we have total number of revolutions given as