# Homework Help: Rotating rigid body

1. Jun 14, 2017

### Faefnir

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

Two masses of 2 kg are attached to the extremities of a 5 cm long, negligible mass and thin bar, free to rotate around a horizontal axis passing through its center. A drop of wax falls on one of the two masses at $v = 3 \frac{m}{s}$ and sticks to it. Retrieve the angular velocity of the system immediately after the impact

2. Relevant equations

Angular moment:
$$L = I \cdot \omega$$
$$L = rp = rmv$$
Inertia moment:
$$I = (M + M + m) \cdot r^2$$

3. The attempt at a solution

I don't know if the wax drop mass was deliberately omitted in the text, but if so, how can I get it? Without that I can not get $\omega$. I know I should do some attempt, but the only thing that comes to mind is the inertia moment for a thin bar

$$I_{MC} = \frac{1}{12} m \cdot l^{2}$$

Can you help me? Thanks in advance

2. Jun 14, 2017

### Staff: Mentor

You'll need the mass of the wax drop to solve this. Where did you get the problem?

3. Jun 14, 2017

### Faefnir

An italian homework book. Indeed the latest edition dates back to 2003

4. Jun 14, 2017

### Staff: Mentor

Oh well. Without the mass the best you can do is solve things in terms of 'm', but that's clearly not the intention since quantities are given for everything else.

5. Jun 14, 2017

### Faefnir

Thank you! You have confirmed that I understand the subject well :D