# Weight of a black hole

1. Aug 19, 2009

### sleventh

Hello all,
I am wondering how would you calculate the weight of a black hole?

2. Aug 19, 2009

### 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.

3. Aug 19, 2009

### marcus

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.

4. Aug 20, 2009

### sleventh

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
sleventh

5. Aug 20, 2009

### LithiumHelios

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.