• Space/Science
  • GeekSpeak
  • Mysteries of
    the Multiverse
  • Science Fiction
  • The Comestible Zone
  • Off-Topic
  • Community
  • Flame
  • CurrentEvents

Recent posts

Birthright Citizenship RobVG June 29, 2025 3:34 pm (CurrentEvents)

To be blunt, NASA is now dead RL June 27, 2025 11:56 am (Space/Science)

Musk trashes his own AI after it chose a liberal worldview. RobVG June 23, 2025 9:56 am (CurrentEvents)

Psyche keeps its date with an asteroid BuckGalaxy June 22, 2025 5:21 pm (Space/Science)

Just for the record... ER June 22, 2025 8:59 am (CurrentEvents)

The Three Unknowns After the U.S. Strike on Iran BuckGalaxy June 22, 2025 12:58 am (CurrentEvents)

There are only nine meals between mankind and anarchy. BuckGalaxy June 22, 2025 12:29 am (Flame)

Not ready for prime time BuckGalaxy June 19, 2025 12:18 pm (Space/Science)

hypocrisy ER June 15, 2025 2:30 pm (Flame)

NSIDC offline? ER June 12, 2025 12:19 pm (Space/Science)

Wouldn't it be nice BuckGalaxy June 11, 2025 3:13 pm (Off-Topic)

Thank You Falettinme Be Mice Elf Agin BuckGalaxy June 9, 2025 1:35 pm (Off-Topic)

Home » Space/Science

A black hole's escape velocity. June 10, 2022 6:39 pm ER

You will recall that in the previous thread, I cribbed (from Wikipedia) the formula for the escape velocity from a body’s surface

v= sqrt (2GM/r) where
v = escape velocity
G = universal gravitational constant
M = mass of the body to be escaped from
r = distance from the center of the mass

So suppose you have say, a ten solar mass (M) star with a radius (r) of say, 10 million kilometers.

The formula above allows you to calculate the escape velocity at its surface, a distance r from its center. Spherical objects tend to act gravitationally as if ALL their mass were concentrated at the center.

Now let us assume this star collapses into a black hole. a tiny blob with a radius (r) of say, just 10 kilometers. The mass (M) of the black hole is still ten solar masses but its radius is a million times smaller. So why is the black hole’s gravity so much stronger than the original star’s? Actually, it isn’t!. The gravity field at the original distance of “10 million kilometers” is exactly the same. The black hole isn’t any heavier than the original star. So why do black holes have such a fearsome reputation as infinitely deep gravity wells? G and M have not changed, so why is v so huge?

The reason is that r is much smaller now, and when you divide by a smaller number, the quotient gets bigger. The gravitational attraction is the same, but now you can get much closer to it.

So black holes don’t really suck all that much, you can just get close enough to be sucked in a lot easier.

    Search

    The Control Panel

    • Log in
    • Register