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Homework Help: Grav. Force - Mass of an object

  1. Feb 17, 2007 #1
    1. The problem statement, all variables and given/known data


    A proton moving at 0.999 of the speed of light orbits a black hole 4362km from the center of the attractor. What is the mass of the black hole?

    2. Relevant equations

    F=(Gm1*m2)/d^2

    3. The attempt at a solution

    Assuming the above is correct, a proton I think has constant mass of 1.6726*[(10)^(-27)], I know the distance, and G is constant. Since what I need is mass of black hole I'm assuming there's a way to find the force using the proton's speed.
     
  2. jcsd
  3. Feb 17, 2007 #2

    hage567

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    The gravity of the black hole is acting as the central force. So what is the definition of a central force? Think Newton's second law for circular motion.
     
  4. Feb 17, 2007 #3
    super, i figured put the problem, the force would be equal to (m*v^2)/r

    now the only thing im missing is v, which is .999 of the speed of light, but what does that mean?
     
  5. Feb 17, 2007 #4

    hage567

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    Do you know what the speed of light is in a vacuum? It's a constant. The 0.999 is how much of the speed of light the proton is moving at.
     
  6. Feb 17, 2007 #5
    ok, so if the speed of light is 299,792,458 m/s, then the proton's speed would be that times .999 right?
     
  7. Feb 17, 2007 #6

    hage567

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    That's right.
     
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