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Old 03-04-2005, 11:18 PM
HG Pennypacker HG Pennypacker is offline
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terminal velocity on the moon

I know falls from above 30 feet in normal gravity are generally fatal, but what would terminal velocity be on the moon? And how far could you fall without getting hurt?
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Old 03-04-2005, 11:25 PM
Triskadecamus Triskadecamus is offline
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Terminal velocity is a consequence of atmospheric friction. The Moon has no atmosphere, so there is no terminal velocity. The rate of acceleration is one sixth of that you are used to, so you would be moving much slower after falling from the same height on the Moon.

I don't know the real number answer, but remember, once you are moving, the pain at the end of the fall comes from inertia, not gravity, so it hurts just as bad hitting the moon at fifty feet per second, as hitting the earth.

Tris
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Old 03-04-2005, 11:31 PM
David Simmons David Simmons is offline
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Quote:
Originally Posted by HG Pennypacker
I know falls from above 30 feet in normal gravity are generally fatal, but what would terminal velocity be on the moon? And how far could you fall without getting hurt?
Objects on the moon don't have a terminal velocity but rather just keep accelerating toward the moon because there is no air resistance. The thing to remember is that the thing that injures in falls is the acceleration and since the mass of an object is the same on the moon as on the earth whatever velocity is reached on the earth before the abrupt stop causes injury would also cause injury on the moon.

I assume a fall of maybe 15 ft has the potential to cause some injury. In that case the velocity after 15 ft. would be 31 ft/sec which would be reached after a fall of 90 ft on the moon.
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Old 03-04-2005, 11:35 PM
Mirage Mirage is offline
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Terminal Velocity is the fastest that you can fall through an atmosphere before your air resistance is greater than your gravitational pull, which is not the same as the velocity and impact that would be “terminal” to a human. On Earth without a drag chute or other decent-arresting device you’d be falling at 180 miles per hour or so, no matter what the original height you fell from assuming it was far enough to pick up this speed. This impact would be as fatal as a fall out of a 4th story window as it doesn’t take a much to kill a human in this way, and Earth’s gravity can create the requisite impact speed from this height.

On the moon the 30 foot drop would result in much less acceleration (and therefore a lower impact velocity) due to the moon’s weaker gravitational pull, and you would certainly survive. However without an atmosphere the moon has a much higher Terminal Velocity so a fall from a great height on the moon would result in a much greater impact as there would be no 180 mph maximum to your speed of descent.
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Old 03-05-2005, 09:31 AM
Fuji Kitakyusho Fuji Kitakyusho is offline
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I would add to this that for any given velocity, a much greater risk exists when falling on the moon, since the victim is presumably wearing a pressure suit which could rupture or have its face shield cracked on impact.
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Old 03-05-2005, 10:00 AM
Squink Squink is offline
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Percival Lowell calculated the velocity of an object falling from infinity onto various solar system objects:
Quote:
Mercury 2.2 miles a second (probable value).
Venus 6.6 " " " " "
Earth 6.9 " " "
Moon 1.5 " " "
Mars 3.1 " " "
Jupiter 37. " " " (mean value)
Saturn 22. " " " " "
Uranus 13. " " " " "
Neptune 14. " " " " "
Sun 382. " " "
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Old 03-05-2005, 11:33 AM
Agent Foxtrot Agent Foxtrot is offline
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I asked this same question a while ago in GQ. Here's the link.

Adam
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Old 03-05-2005, 01:47 PM
RM Mentock RM Mentock is offline
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Quote:
Originally Posted by Squink
Percival Lowell calculated the velocity of an object falling from infinity onto various solar system objects:
That should be the escape velocity, essentially, right?
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Old 03-05-2005, 03:21 PM
Squink Squink is offline
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Quote:
Originally Posted by RM Mentock
That should be the escape velocity, essentially, right?
Should be, just looked at in terms of falling.
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Old 03-05-2005, 04:14 PM
Valgard Valgard is offline
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Quote:
Originally Posted by Mirage
Terminal Velocity is the fastest that you can fall through an atmosphere before your air resistance is greater than your gravitational pull, which is not the same as the velocity and impact that would be “terminal” to a human. On Earth without a drag chute or other decent-arresting device you’d be falling at 180 miles per hour or so, no matter what the original height you fell from assuming it was far enough to pick up this speed.
Minor nitpick - terminal velocity on the Earth is more like 110-120mph for a person in a relaxed spread-eagled posture (takes roughly 1000 feet of freefall to reach that speed). You can hit 180mph in a no-lift dive but it's not easy to get the right body position, takes some effort.
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