# Homework Help: Angular change in velocity (momentum)

1. Oct 28, 2011

### physgrl

1. The problem statement, all variables and given/known data

A merry-go-round has a radius of 3.0 m and a rotational inertia of 600 kg m2. The merry-go-round is initially at rest. A 20 kg child is running at 5.0 m/s along a line tangent to the rim. Find the angular velocity of the merry-go-round after the child jumps on.

2. Relevant equations

p(angular)=Iω
p=mv
p(angular)=p*r

3. The attempt at a solution

i tried to use conservation of momentum by saying:

r*mv+0=(I+mr)ω

2. Oct 28, 2011

### Staff: Mentor

Right idea, but you used the wrong formula for the rotational inertia of the child. What's the rotational inertia of a point mass (we can treat the child as a point mass) that is some distance from the axis?

(Always check units. What are the units of I? Does mr have those units?)

3. Oct 28, 2011

### physgrl

Ohh so it will be mr^2 right? :)
How is it derived anyways?

4. Oct 28, 2011

### dacruick

5. Oct 28, 2011