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Creating a self propelling wheel

  1. Oct 5, 2007 #1
    well, not exactly.

    I'm working on a little project for school (art school, ironically) and I think I'm a bit in over my rudimentary physics education.

    Basically, my idea is to create a wide, hollow wheel with a motor hidden inside of it, so that to the casual observer the wheel appears to moving of its own volition.

    My execution is to have the motor and its battery attached solely at the axis, which thus turns the wheel. My understanding is that the force exerted by the motor will travel down the radius of the wheel, gain mechanical advantage, and hit the ground. Due to Newton's 3rd law, that amplified force will bounce back up the wheel and exert force against the motor. So to counteract that, I'll need to have a ballast of some sort connected to the motor with enough mass to negate the acceleration caused by the original force, plus the mechanical advantage. This should prevent the motor from turning uselessly on itself, and the wheel should move forward.

    Am I right in believing this?

    Further questions:

    I'm looking at a twelve watt motor for this, am I correct in assuming for my calculations that it exerts, at any given instant, twelve newtons?

    newtons per meter per second:
    watt = mass*m^2/s^3 = mass*m/s^2 = newton

    will the motor need to exert enough force to accelerate the mass of the entire rig, or simply the mass of the wheel relative to the motor?

    Is friction the only limitation on top speed?

    Is this project even feasible?
  2. jcsd
  3. Oct 5, 2007 #2


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    Gold Member

    Welcome to PF, Roast Beast.
    I'm just on my way to bed, and am somewhat inebriated, so I can't get into this in any detail. One thing that I will mention, though, is that any self-propelled wheel that I've seen has been driven from the inside of the rim rather than having an axle. A definite advantage of that is that you can keep your motor on the bottom to lower your centre of gravity.
  4. Oct 5, 2007 #3
    No. Watts are a unit for power, Newtons are not, so that part is just wrong.

    This kind of project *is* feasible, but I don't think it will be productive for you to try to analyse it theoretically first, partly because the cheap motors and so forth in the real world do not behave in a manner that is quite as simple to describe as for the ideal motors you might consider in a first-year university physics course. Just get some cheap hobby motors, mock up a wheel out of cardboard or similar, and experiment a bit yourself.
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