# Is gravity really enough to pull a person to supersonic velocity?

Asked by gambitking (4194) February 7th, 2012

I’m wondering how a person skydiving from 23 miles up can break the sound barrier just from the momentum of gravity. Wouldn’t terminal velocity prevent the guy from going supersonic?

Oh by the way, this is why i’m asking, for point of refrence:
http://www.csmonitor.com/Science/2012/0207/Red-Bull-fueled-daredevil-plans-record-skydive-from-edge-of-space

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It has already been done. Here

My assumption is terminal velocity only works if there is resistance to slow you down.

WestRiverrat (19915)

In your typical situation, no. Terminal velocity for a human on earth is about 200 km/h, while the speed of sound is about 1225 km/h. But in thinner atmospheres or on more massive planets, terminal velocity will be much higher. The situation you’re referencing is a free fall starting from very high, where the atmosphere is thin or nonexistant and terminal velocity is therefore very high or nonexistant. The atmosphere will slow him down once he reaches altitudes where it’s thicker, but the decceleration is probably too low to slow him much. I can do some calculations tomorrow if you’d like.

Mariah (24638)

@Mariah Is on the right track. Terminal velocity is a function of the gravitational pull of the body you are falling into, and the atmospheric resistance to your fall. You don’t have to go far above the Earth (relatively speaking) to completely escape the planet’s atmosphere. If you were flown far beyond the outer reaches of the atmosphere, and released on a trajectory that would carry you back to Earth under its gravitational pull, you would eventually reach escape velocity of 25,200 MPH—well above the speed of sound at sea level and normal pressure. Do not try this at home. You would burn up like a meteor when you hit the denser layers of Earth;s atmosphere.

ETpro (34428)

Along with the well-noted fact there isn’t much atmosphere to slow you down when up that high, this is not quite the right notion “momentum of gravity”. Gravity is an accelerating force. With very little to slow you down you will go faster and faster until something does slow you down.

One of the things that bugs me about the write-up about this feat is the use of “speed of sound” which is very ambiguous in this context… do they mean speed of sound in air at sea level ?(most common usage) or, technically the speed of sound up at 120 km – which is very different due to less dense air.

Looks like a well prepared stunt, good luck to them!

dabbler (15873)

That’s pretty much what I’ve been thinking…its possible due to the thinner atmosphere.

And @WestRiverrat , it actually hansn’t been done yet. The event you reference in the video was a 102,000 foot jump. The one I’m talking about will be 120,000 feet, and he stands to break four records in doing so…. including being the first person ever to break the sound barrier outside an aircraft.

gambitking (4194)

CWOTUS (25491)

@dabbler is correct. The speed of sound is different at different altitudes. You cannot go faster than sound by this kind of jump. Keep in mind that, in space, the speed of sound is unbelievably fast.

filmfann (44522)

To be somewhat more precise, @filmfann, “the speed of anything” depends on the medium. Even the speed of light is different in water than it is in our “air” atmosphere and different from what it is in the “vacuum” of space. (The difference in the speed of light in air vs. water is visible to anyone who has ever looked at the way a straw or a stick seems to be crooked as it enters the liquid.)

As far as we know, there is no “speed of sound” in a vacuum… or what we call “outer space”. I’m not sure if the speed of sound will be faster or slower in the outer atmosphere. Since the speed of sound is faster in water than it is in air, for example, it seems to me that added density – to a certain extent – aids in the propagation of the sound wave.

CWOTUS (25491)

@filmfann The speed of sound is no barrier in the way the speed of light is. Some pieces of space rock falling into the Earth’s gravitational well have attained speeds in excess of 40,000 MPH. A human body could do that as well. But they would burn up when they did hit the upper atmosphere. Sound doesn’t propagate through the vacuum of space. Sound is a wave of compression and decompression between immediately adjacent molecules in a medium like air, water or the Earth. The more tightly the molecules are packed, the faster sound waves can propagate through that medium. Sound travels at roughly 768 MPH under normal barometric pressure at sea level. As altitude 9and thus pressure or density of the gasses making up air) increasde, the speed of sound decreases. In iron, sound travels about 15 times faster than in air at sea level.

ETpro (34428)

@ETpro This question got me interested and I did some reading; from what I saw, the basic relationship is

c = sqrt(P/d)

Where c is the speed of sound, P is a coefficient called the bulk modulus, and d is density of the medium.

This implies the opposite of what you said – that speed of sound is actually inversely related to density – but the bulk modulus is a measure of how “stiff” the medium is (how much it resists compression) and accounts for the faster sound speed in iron as opposed to air.

Mariah (24638)

@ETpro I would bet that the meteor was going that fast when it hit the atmosphere, and slowed down once it did.

filmfann (44522)

@Mariah Sloppy wording on my part. I knew sound traveled faster at sea level than high altitude, Sound travels faster stillin water, and faster still in iron. I just chose poor wording to express why.

@filmfann Indeed, meteors do slow when they hit the atmosphere. But not that much. Check these pictures of what they are still able to do when they impact the earth.

ETpro (34428)

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