# Constrained motion question (using moments of inertia/energy)

1. Apr 23, 2006

### Mutant_Fruit

I've attached a screenshot of the question. This is what i've tried so far:

I've calculated the initial energy of the system (which is all potential energy):
Energy of disc:
0.5*I*w^2 + 0.5 *m*v^2 + m*g*h
= 0 + 0 + 0

Energy of arm:
0.5*m*v^2 + m*g*h
= 0 + 4*9.81*1.5Cos(45)
= 41.62 joules

Energy of collar:
0.5*m*v^2 + m*g*h
= 0 + 1*9.81*3Cos(45)
= 20.81 joules

Then i moved time on until the supporting arm BC was horizontal and calculated the energy at that stage (which would be all kinetic). When i do that, i end up calculating that the speed of the collar is 1.04 m/s. However, the answer is supposed to be 7.315 m/s.

I'm doing something wrong, but i can't figure out what it is. Any hints on how this is supposed to be solved would be great.

Thanks.

http://img127.imageshack.us/img127/3230/question8wo.th.jpg [Broken]

Last edited by a moderator: May 2, 2017
2. Apr 24, 2006

### arildno

In all probability, your flaw lies in determining the kinetic energy term of the arm. Specify what you did.