Re: a little help? classical mechanics, torque and friction, rotation

On Jul 22, 12:53 pm, Fallingeagle <falling_eag...@xxxxxxxxxxx> wrote:
On 22 Jul, 18:10, Randy Poe <poespam-t...@xxxxxxxxx> wrote:



On Jul 22, 11:46 am, Fallingeagle <falling_eag...@xxxxxxxxxxx> wrote:

This is not a simple thing, a billiard ball, because of
multiple rotational and translational modes, rotation
combined with sliding and gyroscope effects, and
elastic impact. Good luck.

don't care about that! gyroscope effects and elastic stuff...all I
want to know is why DIRECTION of friction is to the RIGHT, when it is
rotating clockwise, and got pushed from left to right!!!! do YOU
understand why?

It's not, it's to the left.

and after that I want to understand how you go from here, to pure
rutation where the sum af all torqes is MINUS the torque of
friction!!!!!

Initially it is rolling with some angular rate w. If it were just
rolling,
no sliding, it would be moving forward at w*r. However, it is sliding,
meaning that the forward motion is faster than w*r.

The ball is rotating clockwise, and sliding to the right.

This causes sliding friction, a force which acts to the left. It
also causes a clockwise torque. Since these forces act
(a) against the sliding motion and (b) in the same direction
as the rotation, the effect is to reduce the sliding component
until there is no sliding and the forward velocity v' = w'*r.

- Randy

hmm..okei, I think you are on to something.. are there "two" friction
forces at work here?

No, just one. Friction acts where the ball contacts the table
(there is only one such point) and it acts to counter
the direction of motion (there is only one direction of
motion).

but look at the picture at this site:

http://webphysics.davidson.edu/faculty/dmb/PY430/Friction/rolling.html

This wheel is rolling in place. "Notice that the wheel does
not have a forward velocity as yet." Where the wheel is
contacting the ground, it is moving left. The friction force,
acting in the opposite direction, is pointing to the right.

In your problem, where the wheel is contacting the
ground it is sliding to the right. The frictional force
is to the left.

this is how my exercise is too. The friction is trying to stop the
rotation, and therefore accelerate the center of mass...

No, friction is never adding energy to a system. This is
not how your exercise is. Your exercise does not meet
the description that "the wheel does not have a forward
velocity". In your exercise, the wheel has a forward
velocity.

- Randy

.

Relevant Pages

  • Re: a little help? classical mechanics, torque and friction, rotation
    ... no sliding, it would be moving forward at w*r. ... as the rotation, the effect is to reduce the sliding component ... Friction acts where the ball contacts the table ...
    (sci.physics)
  • Re: a little help? classical mechanics, torque and friction, rotation
    ... no sliding, it would be moving forward at w*r. ... This causes sliding friction, a force which acts to the left. ... as the rotation, the effect is to reduce the sliding component ...
    (sci.physics)
  • Re: Spinning a wheel with friction
    ... |> | Sliding friction acts at the bearings, and the energy lost to sliding ... |> | friction will be roughly constant over each revolution. ... | period from one rotation to the next and T is the duration of the ... wheel a turn, timing it with the stop watch in his cell phone. ...
    (sci.physics)
  • Re: Spinning a wheel with friction
    ... |> | Sliding friction acts at the bearings, and the energy lost to sliding ... |> | friction will be roughly constant over each revolution. ... | period from one rotation to the next and T is the duration of the ... wheel a turn, timing it with the stop watch in his cell phone. ...
    (sci.physics)
  • Re: Anomalous movement resistance in a spinning gyroscope
    ... rotation times (the spindle's top is being rubbed away which causes ... friction to rise) and not lengthen them, ... anticlockwise and in the clockwise directions, ... It is just a micro scale, whereas the disc brake scenario is ...
    (sci.physics)