# simple math (but hard 4 me)

Discussion in 'Homework Help' started by isakiwanuka2013, May 13, 2013.

1. ### isakiwanuka2013 Thread Starter New Member

May 12, 2013
4
0
I have a thin disk that

In weight is 30 Kg in total

It is 0.5 meters in diameter

So moment of inertia is = I = 0.25xMxR^2

So I = 0.25x30x0.25^2 = 0.46875 kg-m^2

i want to suggest a stepper motor to be brought with 4.5Nm

my angular velocity is 0.6282 rad/sec

so now i want to work out torque but have no idea???

Torque of a Body in Angular Motion
T = J α (2)

where

J = polar moment of inertia (kg m2, lbm ft2)

α = angular acceleration (rad/s2)

but i have angular velocity

i tink if i integrate velocity i get accelartion therefor rad/s ^2

2. ### WBahn Moderator

Mar 31, 2012
23,379
7,092
What is the 4.5Nm for? Those are units of torque.

The motor has to deliver power for two reasons: (1) to change the speed of the disk, hence you need to decide what the fastest acceleration you need is, and (2) to overcome the friction (and any other) losses, which you haven't even touched on.

3. ### WBahn Moderator

Mar 31, 2012
23,379
7,092
Oh, and you integrate acceleration to get velocity. To get acceleration from velocity, you differentiate.