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Weight of a black hole

  1. Aug 19, 2009 #1
    Hello all,
    I am wondering how would you calculate the weight of a black hole?
  2. jcsd
  3. Aug 19, 2009 #2


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    Staff: Mentor

    Do you mean mass? The weight of an object is the force of gravity between it and another object. That implies your answer, though: you calculate the mass of objects in space by measuring interactions between objects.
  4. Aug 19, 2009 #3


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    Science Advisor
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    Dearly Missed

    A good way would be to use the google calculator, which knows all the basic constants and units so you don't have to look anything up in a textbook.

    A formula for the kilograms of a black hole is:

    M = c2R/(2G)

    G is Newton's constant, google will automatically put the value in for you if you just type G.
    R is the black hole's radius. Let's try an example. Say the radius is one micron. Like a fine grain of dust.

    Put this into the google search window:
    "c^2*(1 micron)/(2G)"

    I just tried it in the google window (omitting the quote marks of course) and it said 6.7e20 kilograms
    That means 6.7 x 1020 kilograms.

    You can figure out how much that weighs in pounds, if you like that unit, or tons if you prefer. I like thinking in terms of a metric tonne (one thousand kilograms).
    This micronsize blackie weighs like 2/3 of a billion billion tonnes. Whew.
  5. Aug 20, 2009 #4
    excellent thank you very much, the reason i had said weight to begin with was because i was wondering how to calculate the interaction between a black hole and some other object taking into account the gravitational effects of the black hole. Thank you very much marcus, i also was wondering about mass and what equation would be used.
    thank you both
  6. Aug 20, 2009 #5
    It's also possible to use the velocity of objects in orbit around a black hole to calculate the mass. If, for example, we are looking at the velocity of the orbits of stars around the center of a galaxy (with a black hole at the center - use the orbit velocities to infer it's presence) then as long as the motion of the stars appears consistent with an orbit then a measurement of their velocity will allow us to calculate the amount of mass interior to the orbit. This is just one of the ways of measuring the interactions between a black hole and other objects as mentioned by russ_watters.
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