# Homework Help: Help please! I don't understand rotational physics!

1. Oct 19, 2009

### shannon12

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

A 75kg, 30 cm long rod hangs vertically on a frictionless, horizontal axle that passes through its centre. A 10kg ball of clay travelling horizontally at 2.5 m/s hits and sticks to the very bottom tip of the rod.
To what maximum angle (measured from the vertical) does the rod (with the clay ball attached) rotate?

2. Relevant equations

3. The attempt at a solution

Not sure where to start on this, apart from calculating the moment of inertia for the rod and for the ball and adding them together to get a total inertia?
From there, I have no clue.

2. Oct 19, 2009

### rl.bhat

Hi Shannon12, welcome to PF.
What is the momentum of of the clay ball? What is the moment of momentum on the rod?
What is the relation between moment of inertia, angular velocity and momentum of momentum?

3. Oct 19, 2009

### shannon12

Thank-you! :D
Umm ok so momentum = mv = 0.01 x 2.5 = 0.25 kg m^-1 s^-1 for the clay ball
And the momentum of the rod is I = 1/12 mL^2 = 1/12 (0.075)(0.3^2) = 5.625 E-4

For the second part, i'm not too certain. Í tried checking my book to see if there were any formulas but I couldn't find one..

4. Oct 19, 2009

### rl.bhat

Moment of the mud ball produces the angular momentum in the (rod+ mud ball) sustem.
So mv*L/2 = I*ω. Due to the angular velocity the system acquires kinetic energy. When the system comes to rest KE is converted to PE.
So 1/2*I*ω^2 = mgh = mgL/2(1 - cosθ)
Solve for θ.