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Why do wheels work?

  1. May 6, 2009 #1
    I know that moving things is a lot easier with wheels, and I suspect it has something to do with the friction on the wheels versus the friction on just a regular sliding object.

    But how do you prove, with the laws and equations of physics, that it takes less force to move something when there are wheels involved?
     
  2. jcsd
  3. May 6, 2009 #2

    russ_watters

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    As you say, it isn't about the wheels per se, it is about what the wheels do to the friction. A rolling wheel supported by ball bearings has no sliding friction anywhere. If you have a wheel on a shaft with no bearings (ie, like a hot wheels or pinewood derby car), you will have sliding friction, but with a limited contact area and typically with a lubricant. I guess you could compare that with having a chunk of wood or metal sliding down a wood or metal ramp that is covered in oil.
     
  4. May 6, 2009 #3

    chroot

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    Look closely at a wheel rolling across the ground. You'll notice that at the point of contact, where a wheel meets the ground, there is no relative movement. That means there is no kinetic (sliding) friction at all.

    If wheels were perfect, rolling them down the road would cost no energy at all! Once you get them moving, they'd roll forever. Of course, in the real world, there are a number of ways energy is lost. For example, in an automobile, some energy is lost as the tires continually deform as they rotate. This loss is called "rolling resistance."

    - Warren
     
  5. May 7, 2009 #4

    rcgldr

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    Also the radius of the axle is much smaller than the outer radius of the wheel, reducing the linear force at the perimeter of the wheel versus the friction torque at the axle.

    An alternative to a wheel would be a series of cylinders to allow the object to move along over the rolling cylinders. In some warehouses, the floor is covered with foam beads to make moving boxes very easy. These deform under the weight of someone walking on them (which eliminates the walking hazard), but don't significantly deform under the low pressure from the boxes and they act as ball bearings.
     
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