# Homework Help: Mass Moment of Inertia

1. Mar 28, 2012

### exidez

This is not a question from any textbook or assignment. It is a concept question in order to get a task done. I need to size a motor but just need to confirm my suspicion.

I am sizing a motor needed to open/close a roller door that curls around a drum. Not one of those modern ones. I need to have an estimate of the mass moment of inertia of the door.

My question is, if i calculate the mass moment of inertia of the door (about the axis of the roling drum) when it is coiled around the drum, is this the same as the mass moment of inertia of the door when its is closed (ie. similar to a hanging mass)?

My suspicion is that it is not and the mass moment of inertia is larger when the door is close (like a hanging mass from the drum)

The door is physically attached to the drum and the drum is not used a pulley.

//Edit
I just realised i think i am completely misunderstanding the concept. Mass moment of inertia is greatest when it is coiled up. When it is extended (like a hanging mass), there is just a load torque being applied to the drum. Only the drum inertia needs to be factored in the inertia equation.

Last edited: Mar 28, 2012
2. Mar 28, 2012

### dipole

Yes, I don't know the answer, but try calculating the torque the door will exert when its fully extended, and then calculate the torque needed to spin the drum when the door is fully opened, and see which is greater, and go by that value.

You'll need some way to estimate the resistance of the door to being rolled up though.