# Torque Needed for Rotor

*Most recent answer: 04/30/2018*

Q:

I have a mass that weighs about 20 kg and has a moment of inertia of about 0.0281 kg-m^2. I am trying to find the torque required to rotate this mass about its horizontal axis at 1 rpm, so that I may buy a suitable motor for it. I am having trouble figuring it out. I know the formula says that Torque is = inertia * angular acceleration but I am really struggling to find the angular acceleration. Is there any other way to calculate the required torque.

- Tarik Amer (age 21)

Dallas, Texas, USA

- Tarik Amer (age 21)

Dallas, Texas, USA

A:

What you're missing here is how quickly the motor needs to get the mass rotating. The angular acceleration is just the final angular velocity divided by the time allowed for getting to that from an initial angular velocity of zero.

You will need a little extra torque because of friction in the bearings, but probably that's not too important if you have good bearings.

Mike W.

*(published on 04/30/2018)*