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Why Is There Such A Thing As "terminal Velocity"?

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Posted by Mark on June 17, 2002 17:13:14 UTC

Here on Earth it makse sense... terminal velocity is the maximum velocity an object shall achieve in freefall. It's caused by the upward force of air friction (atmospherice drag) being equal to the downward tug of gravity. Hence the two forces cancel out and we cease to accelerate. At this point we achieve maximum velocity in freefall. I've heard this speed was somewhere around 120 to 150 mph for humans in freefall.

What I don't understand is why there is a terminal velocity for any gravitating body regardless of whether or not it has an atmosphere. It's a different type of terminal velocity than what was explained above and just so happens to be equal to the escape velocity of the object. On Earth this would be about 11.4 km/s. That means even without the atmosphere, no object shall fall toward the Earth at a rate greater than 11.4 km/s regardless of how long it remains in freefall. After about 20 mins, it would no longer accelerate but just settle at a constant speed.

Why should a rock falling toward the moon ever reach a maximum velocity other than short of the speed of light if it falls long enough?

If a projectile were shot toward the Earth at 12 km/s, would the Earth's gravitational field have no effect on the object's speed?

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