# Torque, rotational inertia and angular acceleartion

1. Oct 12, 2013

### urbano

1. The problem statement, all variables and given/known data

In order to link these into a logical sequence, how do they fit together

2. Relevant equations

I = rotational inertia a = angular acceleration (I know a different symbol is usually used)
τ= force x moment arm distance

3. The attempt at a solution

so....to get my head around how these interact

Torque is the amount of rotational force applied to a body.

rotational inertia is how much the body will resist this force

angular acceleration is how dpenedne ton these two.

Situation one.

If I applied 5Nm of torque to a body that had a rotational inertia greater than this then the body wouldn't move and its angular acceleration would be 0

Situation two.

If I applied 5Nm of torque to a body that had a rotational inertia less than this then the body would move BUTwhat if it moves at a constant velocity and doesn't accelerate ? It still has 0 acceleration but is moving ?

I unsure how torque can length of moment arm x force AND moment of inertia x angular acceleration

if there is is 0 angular acceleration then moment of inertia x 0 = 0 so ther eis 0 Torque

but if I'm applying 5Nm of toruqe how can there be 0 torque ?

I must have missed something fundamental somewhere

2. Oct 12, 2013

### SteamKing

Staff Emeritus
Yeah, it's called units. Look up the units of rotational inertia and angular acceleration.

BTW, applying a torque to a rotating body always acts to change the angular velocity. If the body is rotating at a constant angular velocity, what must the net torque be on the system?

Hint: Angular motion has formulas analogous to those of rectilinear motion.

3. Oct 12, 2013

### urbano

what happens though if the body isn't rotating and I want to start it rotating but can't apply enough force to do so ? I'm still applying torque though ....aren't I ?

4. Oct 13, 2013

### SteamKing

Staff Emeritus
Yes, you are. How quickly it starts to rotate depends on the amount of torque you apply and what the inertia of the body is.

5. Oct 13, 2013

### NihalSh

like SteamKing said, yes you are. But the net torque is zero, hence it doesn't rotate or simply some other torque resists the torque you are applying.