Rolling, Torque, and Angular Momentum (Physics Question)

[Mr. Hanky]

Hey, if you really want to be picky, you can say the physics of tire dynamics is far more sophisticated than simple static/dynamic friction properties (which really only relevant for objects that are infinitely smooth and have non-flexible surfaces). Arguably, the "frictional force" of a tire is always static, and when it is slipping (which is technically always occurring to varied degrees and varied locations within the contact patch), there is the additional behavior of material shear.

[Aaron]

Jesus, did any of you go to MIT?

[Mr. Hanky]

No, but I stayed at a Holiday Inn Express, once.

[Brandon B]

Originally Posted by Mr. Hanky

Hey, if you really want to be picky, you can say the physics of tire dynamics is far more sophisticated than simple static/dynamic friction properties (which really only relevant for objects that are infinitely smooth and have non-flexible surfaces). Arguably, the "frictional force" of a tire is always static, and when it is slipping (which is technically always occurring to varied degrees and varied locations within the contact patch), there is the additional behavior of material shear.

Yeah, tell that to the CHP with their little skid mark measurer and their little notebook of equations.

I did when they told me I was speeding once. They don't even use different coefficients for different road surfaces or different tire types.

[Brandon B]

Originally Posted by MatrixS2000

True, I should not have said no friction but a little friction...or the car would never slow down, which is again impossible while on the planet....

Well, you could build a really big vacuum chamber . . .

[turboedguy]

Originally Posted by vladittude0583

Hey guys, I am having a little hard time understanding a concept dealing with translational and rotational motion. For instance, if we have a solid sphere rigid body rolling down on an incline, we have both translation kinetic energy with regards to the center of mass and that of rotational kinetic energy associated with the particles of the sphere. Anyhow, my textbook mentions that it is static friction involved in ensuring this rigid body rolls "smoothly" down the incline without slipping. My question is, why is it static friction? I thought static friction needs to be overcome for a body to start moving? Can someone clarify this? Thanks.

Originally Posted by Mr. Hanky

The only thing that is implied when it comes to "static friction", is that 2 surfaces are NOT slipping. So if the rolling object has a diameter of d, then the angular velocity and translational velocity are simply governed by v=w*d/2 (v is velocity, w is omega or rotational velocity).

If the velocity is constant, then there will be a constant rotational velocity. If there is acceleration, then there will be a corresponding rotational acceleration. Under a no slip condition, the v=w*d/2 relation is in effect, at all times. When slip does occur, then this relation no longer holds true (and you then have to involve a dynamic friction coefficient and angular + translational acceleration/deceleration).

Uh,...What if C-A-T really spelled "dog"?...

[Anthony P]

did not read the other answers, so sorry if it has been answered. (and understood)

let's say / is the ramp and O is our object. What we end up is with O/ The part of the O that touches the ramp is on the right of its centre of gravity. That part (because of friction) wants to stick there and not move , but the centre of gravity being on the left pulls the O down so it rolls a bit, the next snapshot is the same as before (centre of gravity pulling the O to the left down the ramp and friction sticking that part of the O touching the ramp. You can see the opposite if you go with a none O object , for example if you have a triangular object going down the ramp V/ either gravity will be enough to overcome friction and it will slide down or not and it will stay there (you can test that by either changing the static friction (i.e. making it more slippery) or the angle (which then changes the forward motion and friction).

[Anthony P]

To address this comment directly, this is more in the context of 2 flat objects in contact with each other (though it is still technically true for a round + flat object)....
Now if the round object cannot freely rotate, then the case becomes similar to the 2 flat objects scenario. Nothing is going to move until the static friction is overcome, which will result in the round object sliding (but not necessarily rolling) across the flat object.

yup, well don't forget what you learnt from calculous that every curve (and so any round object) can be thought as an being drawn by an ever increacing number of flat lines (i.e. the tangent line at any point), just that like a slinky in a normal round object the centre of gravity moves to the next flat surface(of the object)