# Kinetics of Rigid bodies

## Homework Statement

The rotor of an electric motor of mass 200 kg has a radius of gyration of 150 mm. Calculate the torque required
to accelerate it from rest to 1,500 rev/min in 6 seconds. Friction resistance may be neglected.

## Homework Equations

ƩMO = Iα
I = 1/2 (mr2
I am really really stuck here, I don't know what equations I need to use, if this were a straight line question I would use v = u + at, but it isn't so that is wrong

## The Attempt at a Solution

All I have so far is this
I = 1/2 (mr2
I = 0.5 * 200 * 0.152
I = 2.25 kgm2

Really lost. I just need some pointers please.

Kind regards

Rob

## Answers and Replies

PhanthomJay
Homework Helper
Gold Member
In calculating I, the radius of gyration is not the radius of a disk. You can look up the formula for it, which is $r = \sqrt{I/m}$ (and note how it is derived).

You can find the angular acceleration α using the kinematic equations of rotational motion (watch units).

Brilliant, those are the words I didn't know to find the equation I couldn't find of variables I couldn't relate.

Found them all.

Thank you