Torque on a wheel and angular acceleration.

[SherlockOhms]

So, torque is given by I (moment of inertia) x angular acceleration. Let's say that the wheel is a hollow cylinder so the M.O.I is mr^2, I think. If a torque, T is applied to the wheel and there is a frictional force acting on the wheel, calculate the angular acceleration. Well, T - r(F) = I x a (r being the radius of the wheel). My question is, why does the frictional force cause a moment that opposes the motion of the wheel? I would've thought it was the other way around. If the frictional force is acting backwards then wouldn't it's moment with the centre of the wheel cause it to rotate in a clockwise direction? Is the frictional force actually acting backwards or do I have this mixed up? Apologies for not sticking to the headings but they don't show up when posting from a phone.

 

[milesyoung]

Friction is the force that opposes the relative motion of the wheel surface and whatever surface it contacts.

If you apply torque that would have the surface of the wheel move relative to the surface it contacts then the friction will act in a direction to oppose this motion. Torque acting on the wheel in a clockwise direction would have the torque from friction act in a counterclockwise direction and vice versa.

 

[SherlockOhms]

Ohhh, ok. I get it now. Thanks for that!