Angular Momentum with a Circular Platform

[destrukt]

The Question:

A 45kg child runs at 3.0m/s and jumps tangentially onto a stationary merry-go-round. The moment of inertia of the merry-go-round is 360 kgm^2 and the radius is 2.0m. What is the angular velocity of the child and the merry-go-round?

My attempts:

I'm not sure how to incorporate the speed of the running child into this question. I've made some attempts at getting the answer but none included the speed of the running child, and I know it has to include it.

First I tried solving this with respect to conservation of angular momentum...but I got nowhere with that. I'm pretty sure it has something to do with conservation of angular momentum but I can't figure out where to start with the question. Anyone's help would be appreciated.

 

[Doc Al]

Yes, it's a conservation of angular momentum problem. Hint: What's the angular momentum of the running child before she jumps on the merry-go-round?

 

[Borxter]

should have thought about it more, sorry about the wrong answer.

 

[Doc Al]

I think i got the solution:

Im assuming no friction. Conservation of KE:

You cannot assume conservation of energy--the child and merry-go-round undergo an inelastic collision.

Also... please provide help, but do not provide complete solutions--let others do their own work. (Please do provide help, though! )

 

[destrukt]

Hey thanks for the help doc, I got it...feel so stupid for asking as the solution was like 3 lines long. And yea there is no conservation of energy as the next question was to find out the mechanical energy lost to friction !