Friction Direction on Rolling Ball: Am I Missing Something?

[Rmehtany]

My question is the following. A ball is initially skidding and eventually starts rolling on a flat plane with friction, and later comes to a halt. Which direction does friction act? (see diagram)

If friction acts to the right, then the translational speed will go up, and that's not right.

If friction acts to the left, then the angular speed goes up, and that's not right as well.

Am I missing something? It seems too simple and I feel like a fool to have to ask this.

 

[berkeman]

My question is the following. A ball is initially skidding and eventually starts rolling on a flat plane with friction, and later comes to a halt. Which direction does friction act? (see diagram)

If friction acts to the right, then the translational speed will go up, and that's not right.

If friction acts to the left, then the angular speed goes up, and that's not right as well.

Am I missing something? It seems too simple and I feel like a fool to have to ask this.

If the ball is thrown to the right, and skids and then slowly starts to spin up, it is because the friction is acting to the left on the bottom of the ball. That slows the translation to the right, and spins up the angular speed of the ball in the clockwise direction.

 

[Rmehtany]

But then if the angular speed is going up, how can the ball reach static equilibrium

 

[berkeman]

But then if the angular speed is going up, how can the ball reach static equilibrium

Just like at a bowling alley -- the equilibrium is the ball rolling down the alley floor with some angular velocity and with a translational velocity that is less than it had when first thrown. Losses due to friction is one reason the linear velocity is slower, but there is also the energy that is transferred into the angular momentum of the ball as it spins up. Are you familiar with the Moment of Inertia (MOI) concept yet?

 

[Rmehtany]

Yea, I am... Ah, I see, but I still have a problem, if the angular momentum is increasing, how does the ball stop? Does it stop decreasing, and when? I understand that angular momentum increases in the beginning

 

[berkeman]

It is mainly rolling resistance (and air resistance) that will slowly bring the ball to a stop. If it's rolling on carpet, then the rolling resistance is high, so it stops sooner. If the rolling resistance is low (like on hardwood flooring), then the ball is going to roll a *long* way...

 

[rcgldr]

Wiki article:

http://en.wikipedia.org/wiki/Rolling_resistance