How Does Angular Acceleration Arise in a Rotating Cylinder with Multiple Forces?

[Ready2GoXtr]

Homework Statement

http://ready2goxtr.googlepages.com/problem1055.jpg

A cylinder having a mass of 2.0kg can rotate about its central axis through point O. Forces are applied as show: F1 = 6.0N, F2 = 4.0N, F3 = 2.0 N, and F4 = 5.0N. Also, r = 5.0cm and R = 12 cm. Find the (a) magnitude and (b) direction of the angular acceleration of the cylinder.

Homework Equations

Tnet = I \alpha
T = rFsin(theta)
I = 1/2MR^2

The Attempt at a Solution

So far I figured i should find the Net torque. So..

T1 = F1R = 6 * .12 = .72 N * m
T2 = F1R = 4 * .12 = .48 N * m
T3 = F1Rsin(theta) =??
T4 = 0

How would I find T3?

 

[rock.freak667]

Is T3 just not 2.0*0.05 since the angle is 90?

 

[Ready2GoXtr]

Its 90 with r not R

 

[rock.freak667]

Why would you need to find it with R?

Torque=Frsine(theta)...r is the distance of the force from the centre of rotation.

 

[Ready2GoXtr]

If that was the case. couldn't I say T4 has a 90 degree angle too?

 

[rock.freak667]

If that was the case. couldn't I say T4 has a 90 degree angle too?

T4 has an angle with the length of R.But since you are taking torques about the centre,O, F4 produces no torque since the distance between the force and centre of rotation is 0.

 

[Ready2GoXtr]

Okay
So then i think I can do the Tnet => -F1 + F2 + F3 = Tnet Tnet = -.14 N * m

Tnet = Ialpha

Tnet/I = Alpha?

 

[rock.freak667]

Okay
So then i think I can do the Tnet => -F1 + F2 + F3 = Tnet Tnet = -.14 N * m

Tnet = Ialpha

Tnet/I = Alpha?

Yes, that should be correct.

 

[Ready2GoXtr]

hey. =) I am pretty good with paint huh+ D=