Posted by HELP ME!! on .
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.

Physics Inertia : Merrygoround 
bobpursley,
add them: Inew=335 kg m^2+43.5*1.96^2 kg m^2

Physics Inertia : Merrygoround 
HELP ME!!,
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

Physics Inertia : Merrygoround 
bobpursley,
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. 
Physics Inertia : Merrygoround 
nora,
if you want to find the final angular speed, wf, you take:
momentinertiaMerryGoRound(initial angular speed) = (momentinertiaMerryGoRound + momentinertiaKid)(wf)
then solve for wf 
Physics Inertia : Merrygoround 
John,
Correct

Physics Inertia : Merrygoround 
Ted,
What is the moment of inertia of the kid?