Calculating Moment of Inertia and Torque for a Rotating Cylinder

[pb23me]

Homework Statement

A grinding wheel is a uniform cylinder of with a radius of 8.50 cm and a mass of 0.580 kg. Calculate a) its moment of inertia about its center, and b) the applied torque needed to accelerate it from rest to 1500 rpm in 5.0 s if it is known to slow down from 1500 rpm to rest in 55.0 s.

Homework Equations

I=1/2(mr²)=.002kg/m²
\tau=I(\Deltaw/\Deltat)=300MN

The Attempt at a Solution

It doesn't seem that'' it is known to slow down from 1500 rpm to rest in 55.0 s'' ? Do i really need to use that information?

 

[zorro]

if it is known to slow down from 1500 rpm to rest in 55.0 s.
May be there is a resistive force which causes it to slow down to rest by itself from 1500rpm in 55s. You might want to think about this deceleration.

 

[asz304]

Another for torque is torque = Inertia*angularAcceleration. I think you need to find angular acceleration first. acceleration = radius*alpha. Your rpm is rev/min convert this to rad/s to get omega(angular speed).

Some formulas:
v=rw
1 rev= 2pi

There are a couple of substitutions that you might need to do.

 

[pb23me]

ok so i calculated the torque required disregarding the resistive force and got \tau=.063M/N then i calculated the torque of the resistive force and got \tau=-.006M/N so i figured i just add .006M/N of torque to .063 and got .069M/N

 

[zorro]

Assuming that your values are correct, you are right acc. to me.