# Direction of Friction for Rolling Motion

1. Aug 6, 2013

### better361

A ball is attached with a string on the side. When the string is pulled up, static friction accelerates the ball forward while the tension rotates the ball.
When a ball is placed on a incline with friction, is kinetic friction opposing the motion and giving the ball torque while gravity accelerates the ball down? Is there also static friction is this case?

Also, does the ball lose energy in either case due to friction?

2. Aug 7, 2013

### tiny-tim

hi better361!
in rolling (without slipping), there is only static friction (plus "rolling resistance" due to deformation)

does static friction ever do work? (hint: what is the definition of work done?)
what is the question?

3. Aug 7, 2013

### better361

So would the direction of static friction be pointing to up on the incline then if there is no kinetic friction?

I would say static friction doesn't do any work because it only applies to stationary objects, and the ball can be treated as one at a particular instant. Am I right?

Ohh, I was supposed to ask the similar questions on that problem as the incline one. I thought that the only way for the ball to accelerate on the level plane was from static friction, because the other two forces, the tension in the string and gravity, are added with the normal force so that there is no net force in the upwards direction.
So, would the tension need to overcome the torque from the static friction to start rolling or would the ball roll as soon as I apply a bit of tension to the string?

Also, if a uniform electric field were to act on a ball with some charge, and the ball is on a surface with friction, what direction is the friction when I let the ball go?

4. Aug 10, 2013

### tiny-tim

hi better361!

(i'm sorry for taking so long to reply )
yes, static friction from a stationary surface doesn't do work

work done, usually written as force "dot" displacement, is technically ∫ force "dot" velocity dt

the velocity is of the point of application of the force, so if (as here) that is stationary, the work done is zero

of course, if the surface the ball is on is moving, then the point of application is moving, and so the static friction does do work!

(eg a box on an accelerating trailer has increasing kinetic energy, supplied entirely by the static friction )​
static friction is not fixed at µN, it is less than µN, and is proportional to the tension

so yes, it starts moving (rolling) even with a tiny amount of tension
the total electric force will act through the centre of the ball, so the result is the same as for any mechanical force (or torque) applied at the centre

5. Aug 10, 2013

### better361

What makes the ball start to roll if the force is applied through the center? If static friction points in the direction of motion, what is supplying a torque in the opposite direction to make the ball roll properly?

How exactly is the static friction proportional to friction? If the equation is r(T-F)=Iα, where F is the force from friction, wouldn't T, tension, need to be greater than friction for rolling to occur?

Also, why does the direction of friction depend on the radius at which I apply the torque?(new scenario)

Last edited: Aug 10, 2013
6. Aug 16, 2013

### better361

bump on questions from my last post.

7. Aug 16, 2013

### rcgldr

If there is force applied to the ball through the center, then the static friction exerted onto the surface of the ball is in the opposite direction of acceleration, resulting in angular acceleration or angular decleration of the ball.

8. Aug 17, 2013