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Angular momentum to force

  1. Oct 14, 2010 #1
    Hello
    I am busy with a design project and need some help determining the force that can be exerted by a rotating body at its center of gravity.
    Picture a motorcycle performing a wheelie, the center of gravity rotates about the rear axle. Given that the mass and angular acceleration are known, how would I find the force necessary to stop the angular motion about the rear axle at the center of gravity?
     
  2. jcsd
  3. Oct 15, 2010 #2
    Torque=(force)(distance) or T=Ia
    I= moment of inertia and a is angular acceleration .
    How about take a bathroom scale and lift the bike up at the angle the front tire is when doing the wheelie and set it on there . And the will give you the force and then multiply it by the distance from the center of the back wheel and this will give you the torque about that point. These are just some ideas .
     
  4. Oct 16, 2010 #3
    Thanks cragar but that doesn't actually help me. I need the force resulting from the angular momentum. During a wheelie all the weight is on the back wheel anyway so putting it on a scale wouldn't tell me anything.
     
  5. Oct 16, 2010 #4

    Dale

    Staff: Mentor

    The dimensions of angular momentum are force * length * time. So there is no possible way to convert to a force without some additional information. You will need to specify the lever arm length and the stopping time. Once you have specified that then it is a simple conversion.
     
  6. Oct 16, 2010 #5
    Well if the wheelbase is 1.4m, take the CG to be in the middle so lever arm will be 0.7m. Stopping time is difficult to estimate but I would say about 0.3s. Is it a simple impulse equation?
     
  7. Oct 16, 2010 #6

    Dale

    Staff: Mentor

    I don't know what you mean by impulse equation. Just take your angular momentum, divide by the lever arm length and the stopping time, and that will give you the required force.
     
  8. Oct 17, 2010 #7
    Impulse is the force over time. How would I get the angular momentum? not sure of the formula
    thanks
     
  9. Oct 17, 2010 #8

    Dale

    Staff: Mentor

    I'm sorry, this is confusing. I thought you already had the angular momentum.

    Why don't you describe, as completely and clearly as possible, what information you have and what information you want.
     
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