Physics Inertia : Merrygoround
posted by HELP ME!! .
A playground merrygoround of radius 1.96 m has a moment of inertia 335 kg m2 and is rotating at 12.3 rev/min. A child with mass 43.1 kg jumps on the edge of the merrygoround. What is the new moment of inertia of the merrygoround and child, together? Answer in units of kg m2.

add them: Inew=335 kg m^2+43.5*1.96^2 kg m^2

Thank you, but what equation do you use? I cant seem to get the correct answer.What happens to the rev/min? Is it: inertia+child mass*radius^2

The rev/min is not needed.
here is the equation:
momentofINertiatotal=momentinertiaMerryGoRound + momentinertiaKid.
Then you add them.
That is it. Now, if the problem goes on, you may need to find what the new angular speed is, but that is a different problem, you didn't ask that. 
if you want to find the final angular speed, wf, you take:
momentinertiaMerryGoRound(initial angular speed) = (momentinertiaMerryGoRound + momentinertiaKid)(wf)
then solve for wf 
Correct

What is the moment of inertia of the kid?