1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How do wheels decrease the amount of energy needed to move an object?

  1. Jul 4, 2009 #1
    I was thinking about it, and the only thing I could come up with is that it reduces friction with the surface the object is sitting on and perhaps it distributes weight evenly?
     
  2. jcsd
  3. Jul 4, 2009 #2

    negitron

    User Avatar
    Science Advisor

    That's it; rolling friction is much less than sliding friction. It's the same reason a ball will roll down an inclined plane, rather than slide down.
     
  4. Jul 4, 2009 #3
    Why is rolling friction less? It seems like common sense that it is, but why is it?
     
  5. Jul 4, 2009 #4

    Pengwuino

    User Avatar
    Gold Member

    Edit: Nevermind, didn't understand the question.
     
    Last edited: Jul 4, 2009
  6. Jul 4, 2009 #5

    atyy

    User Avatar
    Science Advisor

    But does rolling friction come into play in ideal rolling without slipping - in such a case the point of contact is always stationary?
     
  7. Jul 4, 2009 #6
    I wouldnt think so, no. If its completely perfect, then no rubbing means no friction.
     
  8. Jul 4, 2009 #7
    In physics, mechanical work is the amount of energy transferred by a force acting over a distance. As noted already, with a rotating wheel you can use a lot less force to turn a wheel with a load on it relative to pushing the weight over the ground or carrying it yourself.
    It's a type of mechanical advantage relative to brute force.
     
  9. Jul 4, 2009 #8

    negitron

    User Avatar
    Science Advisor

    Rolling friction occurs even when the wheel doesn't slip. Friction, as a general case, occurs when electrostatic forces act between the surface atoms of one body moving over the surface atoms of another. Sliding is not the only case in which this occurs. For a wheel, there is always some portion of it in contact with the surface over which it rolls, and as it turns, atoms on the wheel's leading surface are continually being mashed down onto the surface of travel where electrostatic forces weakly bind them then are pulled apart again at the trailing surface--the energy required to pull those weak bonds apart is what we call friction.
     
  10. Jul 4, 2009 #9

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    The question was why rolling friction is less than sliding friction. The area of contact of a rolling wheel is the same, as if the wheel slides. It is not the size of the contact area, but it's movement relative to the ground: In the rolling case the area of contact has no horizontal speed.
     
  11. Jul 4, 2009 #10

    negitron

    User Avatar
    Science Advisor

    You can immediately falsify this statement by asking yourself why brakes work, since the area of contact with the ground doesn't change, the wheels just stop turning.
     
  12. Jul 4, 2009 #11

    rcgldr

    User Avatar
    Homework Helper

    In the case of a wheel, the friction at the axle creates an opposing torque to movment of the wheel. The force required to overcome that force equals that torque divided by the radius of the wheel, and since the radius of the wheel is much larger than the radius of the axis, the amount of force at the rim of the wheel required to overcome the resistance of a plain bearing axis, is relatively small.

    An alternative to a wheel is to use a set of moving rollers, which eliminates the axis issue completely, but then you'd need a constant supply of rollers to place under a moving platform.

    Another way to take advantage of this is to place moving rollers (roller bearings) or spheres (ball bearings) between the axis and the inner hole of a wheel, to reduce the opposing torque compared to a simple bearing axis.
     
  13. Jul 4, 2009 #12

    Pengwuino

    User Avatar
    Gold Member

    .... what are you talking about?
     
  14. Jul 4, 2009 #13

    negitron

    User Avatar
    Science Advisor

    Your post. It was wrong, or at best, highly unclear.
     
  15. Jul 4, 2009 #14
    I know that most macroscopic friction for non-deformable substances is caused by electrostatic forces, but I can't quite envision your description. Wouldn't the energy gained (in the form of the rotational KE of the wheel, I guess) due to the weak binding of atoms at the the leading surface equals the work required to pull the atoms apart at the trailing surface?

    ...I think I'm wrong, because I could say the same thing about sliding friction: the energy gained from an atom on surface A binding with a new atom on surface B equals the work required to pull the atom on surface A apart with an old atom on surface B - therefore sliding friction is nonexistent... But, that's incorrect.
     
  16. Jul 4, 2009 #15

    Pengwuino

    User Avatar
    Gold Member

    Ah nevermind, I see what's wrong, I didn't understand the original question. I was thinking a rolling wheel vs. sliding something like a brick as opposed to rolling a wheel vs. sliding that same wheel. Well that makes it quite a different situation :rofl:
     
    Last edited: Jul 4, 2009
  17. Jul 4, 2009 #16
    I see what negitron is trying to say. If the wheels aren't turning, the friction increases regardless of the small contact area of the wheel touching the ground. However, could changing the atoms which are reacting with the ground decrease friction? Or am I just wrong?
     
  18. Jul 4, 2009 #17

    negitron

    User Avatar
    Science Advisor

    Exactly. The contact area does not change, but the friction rises dramatically. Ergo, the reason rolling friction is less than sliding friction has nothing to do with contact area.

    I'm not sure what you mean here. You mean using a different material?
     
  19. Jul 4, 2009 #18
    No, I mean when the wheel turns, new atoms are making contact with the ground. Could electrostatic forces build up from the same atoms keeping contact with the ground?
     
  20. Jul 4, 2009 #19

    negitron

    User Avatar
    Science Advisor

    Perhaps I'm dense today; I still don't quite understand what you're asking. Sorry.
     
  21. Jul 4, 2009 #20
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook