An angular mom problem

1. Nov 13, 2006

redribbbon

A uniform rod of mass 3.15×10^−2 kg and length 0.380 m rotates in a horizontal plane about a fixed axis through its center and perpendicular to the rod. Two small rings, each with mass 0.250 kg, are mounted so that they can slide along the rod. They are initially held by catches at positions a distance 5.20×10^−2 m on each side from the center of the rod, and the system is rotating at an angular velocity 34.0 rev/min. Without otherwise changing the system, the catches are released, and the rings slide outward along the rod and fly off at the ends.

now
I think that the its just pure conservation of momentum

Iwinitial=Iwfinal

so i do

((1/12)(3.5*10^-2)(.38)^2+2*.25*(5.2x10^-2)^2))
divided by
((1/12)(3.5*10^-2)(.38)^2+2*.25*(.38^2))
and then multiply the result by 34
but i get 0.8109
what i am doing wrong?

Last edited by a moderator: Nov 13, 2006
2. Nov 13, 2006

OlderDan

What is the question you are trying to answer? It would be good if you would identify all those numbers by the symbols they replaced, and units would go a long way to helping in that identification.

3. Nov 13, 2006

redribbbon

What is the angular speed of the system at the instant when the rings reach the ends of the rod?

the numbers represent

Which is just the moment of inertia of the system before it starts
divided by the moment of inertia of the system after the rings get to the edge
the only thing that changes is the radius of the ring which goes from (5.2*10^-2)m to .38m

and then multipy by the initial angular velocity to
get the final angular velocity

i just happen to end up with .8109, which is not right

Last edited by a moderator: Nov 13, 2006
4. Nov 13, 2006

OlderDan

Each ring only goes out to half the length of the rod

5. Nov 13, 2006

redribbbon

wow

i am an idiot

thanks alot for pointing that out