According to NASA, terminal velocity of a falling object is a function of air resistance, also known as drag:
We can determine the value of the terminal velocity by doing a little algebra and using the drag equation. Drag (D) depends on a drag coefficient, (Cd) the air density, (r) the square of the air velocity (V) and some reference area (A) of the object.
In addition, physics professor Louis Bloomfield of the University of Virginia tells us that different objects can have different terminal velocities based partly on their aerodynamic characteristics (and by extension, an object in a vacuum has no terminal veolicty whatsoever):
This terminal velocity is determined partly by the object's density and size and partly by its aerodynamics. Large, dense, and aerodynamic objects tend to have very large terminal velocities while small, low-density, non-aerodynamic objects tend to have very small terminal velocities.
He also tells us that firing bullets into the air is foolhardy, dangerous, and probably fatal if they hit a person on the way back own:
A bullet's terminal velocity is the downward speed at which the upward force of air resistance acting on it balances its downward weight. Once the falling bullet reaches this speed, it coasts downward at a steady rate. Because air resistance depends largely on surface area while weight depends on volume, larger bullets will drop faster than smaller bullets (just as a piece of chalk drops faster than chalk dust). While I am not sure of the exact speed of a dropping bullet, I expect it to be several hundred miles per hour. As to whether or not it can kill someone, the answer is most definitely yes. In fact, a distant cousin of mine was killed several years ago during Mardi Gras when a falling spent bullet pierced her brain. Firing bullets into the air is an extraordinarily foolish and inconsiderate action. In cultures where it's common to fire guns during celebrations, innocent people are frequently killed by these descending "party favors." If you ever see people shooting guns into the air, you should immediately seek cover in a basement. Their bullets will return to earth in less than thirty seconds and will be just as deadly when they arrive as if they had been shot right at you.
Just a gentleman's bet, no money involved.Posted by wasylik at July 9, 2002 11:54 PM