A 5.2-m-diameter merry-go-round is initially turning with a 3.0s period. It slows down and stops in 28s.

Question

i found the v = 5.4 m/s, How many revolutions does the merry-go-round make as it stops?

Answer 1

Average rev/min = (1/2)*(20 rev/min)
= 10 rev/min

No. of revolutions = 10 rev/min*(28/60 min = 4.66

You do not need to know the diameter.