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Simple friction wheel problem

  1. Dec 13, 2006 #1
    My physics teacher is saying that:

    The friction force acting on a wheel that is spinning is = to the STATIC friction force of the rubber wheel material on the cement.

    That doesn't make any sense though because the static friction force is greater than the kinetic friction force so based on what he is saying it would be easier to slide the wheel across the ground sideways than roll it.

    The whole point of a wheel is to reduce friction isn't it?!

    Am I missing something here?

    Im not argueing that there are no static friction forces involved but it would not be the same force(mu) as a block of rubber laying on a cement block.

    I know rubber on cement isn't very accurate because they are not completely smooth surfaces.

    Yes but even so when you are trying roll a tire you are not trying to start the rubber sliding across the cement(where static friction force would come into play) you are simply trying to get it to roll.

    Yes there will be some friction acting between the rubber and the cement but it won't be the same as if you tried to start sliding it across the cement.
     
  2. jcsd
  3. Dec 13, 2006 #2

    berkeman

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    I think he means that the torque on the wheel that is making it spin is due to the static friction force, as long as the tire is not slipping on the surface. If it is rolling without slipping, then the static mu is appropriate. In fact, that is one of the tricks of race cars and race bikes -- you get max braking force when the tire is just on the verge of slipping. The tires kind of howl when you're right on the edge.
     
  4. Dec 13, 2006 #3
    He might have but I specifically asked him: "so are you saying that if we layed a rubber block on cement and slid it we would come up with the same static friction force as if we put a tire on cement and rolled it?"

    He said yes.

    I know it IS a static friction force that makes it so the tire rolls instead of slides but its still alot less than a block of rubber(or tire for that matter) being SLID.
     
  5. Dec 13, 2006 #4

    berkeman

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    Yeah, it does sound like there is some misunderstanding going on between your prof and you. Sliding a block gives the kinetic friction coefficient. Proof -- put the block on an inclined plane and tilt it up until it breaks loose. Then lower the angle until it is just barely still sliding. The highest angle gives you the static mu (via the tangent function, right?), and the lowest angle gives you the kinetic mu. Maybe run that classic experiment past your prof and ask him again what he meant by his previous comment. Oh, and be polite -- he probably just misspoke or was misunderstood.
     
  6. Dec 13, 2006 #5

    Doc Al

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    It's certainly true that when a car accelerates the force acting on the tires--as long as they don't slip--is static friction, not kinetic. That's probably your teacher's point.

    But you are also correct that it is easier to roll a tire across a cement floor than to drag it. What you need to realize is that if the tire rolls at a constant speed without slipping, the static friction between floor and tire is zero. (Of course, in real life, other dissipative forces are at work so you would need to push the tire to keep it going.)
     
  7. Dec 13, 2006 #6
    Really?

    I was thinking either that was the case or the static force between the tire and the cement was extremely low(because it only got high enough to make the wheel spin and then stayed about that high to keep it spinning instead of slipping).

    We did a problem pushing a wheelchair up an inclined plane and figured mu=.05N

    I also had a problem with that because a static force that low doesn't make sense for the materials involved(aside from that fact that they were just made up numbers not from real world data).

    Thanks for the information.
     
  8. Dec 14, 2006 #7

    berkeman

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    Yes really.

    And mu=0.05N is not correct. mu is dimensionless, and N is a unit of force. The mu is defined by the maximum force that can be had from friction. When the wheel is spinning up, a little force is supplied by friction to provide the angular acceleration (even though nowhere near the maximum available friction force is needed for slowly spinning up the wheel). When the wheel is rolling at a constant velocity, there is basically no frictional force in the direction of motion. And whenever a wheel is rolling, there is probably a little lateral frictional force keeping the wheel rolling straight.
     
  9. Dec 14, 2006 #8
    I understand where the teacher is coming from with this kind of explanation. The force that the rubber block resists (before it starts sliding) is the same as the force that the person in the wheelchair can apply (before their wheels slip). We use wheels to eliminate kinetic friction (neglecting the axle bearing, which is designed to slide much easier than the ground). When you put a tire on cement and push it, it is static friction that causes the tyre to start rotating. Same "force" as if, er, we pushed on a rubber block (and it didn't slide). In principle you could quantitatively measure the maximum static friction coefficient by pushing (ie. accelerating) a trolly exactly hard enough so that the static friction can't transfer angular momentum to the wheels quickly enough (ie. it skids), and you would calculate the same number for the coefficient as if you had held the trolly's breaks on and pushed until it slid.
     
    Last edited: Dec 14, 2006
  10. Dec 15, 2006 #9

    disregardthat

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    This is interesting. What other dissipative forces did you mean Doc Al?
     
  11. Dec 15, 2006 #10

    berkeman

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    I think he means practical stuff like tire flexing and bearing friction. The things that go into "rolling resistance" of the tires.
     
  12. Dec 15, 2006 #11

    Doc Al

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    That's exactly what I meant. :smile:
     
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