Calculating Angular Acceleration with Friction on an Inclined Plane

[ripper9100]

Homework Statement

A solid uniform cylinder with mass 5 kg is being pulled with a constant force of 45 Newtons up a 30 degree incline. The force is acting on the cylinders center and is parallel to the incline. What is its angular acceleration?

I have a good idea on how to do the problem by using Newtons second law for a rigid body. The only part I'm confused about is on what the moment of inertia should be. I think Its either (.5mr^2) or (.5mr^2 +mr^2)?

please help

 

[e(ho0n3]

Perhaps this link will help: http://en.wikipedia.org/wiki/List_of_moments_of_inertia

 

[ripper9100]

im also iffy on the net Torque. I am thinking that friction is the only force causing torque since the 45 N force is acting on the cylinders center.
am i right?

 

[e(ho0n3]

Yes. I agree.

 

[ripper9100]

thanks for the help

 

[Doc Al]

I have a good idea on how to do the problem by using Newtons second law for a rigid body. The only part I'm confused about is on what the moment of inertia should be. I think Its either (.5mr^2) or (.5mr^2 +mr^2)?

The moment of inertia depends on what you are using as the rotational axis; either expression will work, if you're careful.

im also iffy on the net Torque. I am thinking that friction is the only force causing torque since the 45 N force is acting on the cylinders center.

Again, it depends on the axis of rotation. If you are using the center of mass as the axis, then you are correct.

Even though you have a choice, I recommend using the center of mass of the cylinder as your axis--I think it gives the best understanding of what's going on.

 

[ripper9100]

what about the friction force is it up the incline or down the incline? I am confused about because usually the direction of friction force is opposite the direction of motion.

 

[Doc Al]

Remember that friction acts to oppose slipping between surfaces. If there were no friction, which way would the surfaces (cylinder bottom and incline) slip with respect to each other? Use that to figure which way friction must act.