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Q & A: If you shoot a bullet straight up... (reprise)

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Most recent answer: 09/26/2009
Q:
In additiion to your question about theoretical speed of a falling bullet when shot straight up. When you shoot a gun straight up into the air at lets say 2200 ft per second, around how fast would it be going when it hits the ground in real life (air resistance calculated)? Thanks for your time.
- Daniel (age 10)
Sherwood Park, Alberta, Canada
A:
Daniel -

Actually, the speed at which the bullet will fall back to the ground won’t really depend on how fast it left the gun. This is because the force of air resistance depends on velocity. As the bullet falls, it speeds up (due to gravity). The faster it goes, the more air resistance it feels. Eventually it reaches a speed at which the force of gravity pushing down on it is equal to the force of air resistance pushing up. When all the forces balance out like this, the bullet stops speeding up - this is called the .

In order to calculate the exact terminal velocity of your bullet, there are a lot of things that you would have to know - such as the weight and shape of the bullet, the wind speed, the altitude, etc. One estimate that I found online is about 300 ft/sec (about 200 mph), which sounds like a reasonable guess.

-Tamara

(published on 10/22/2007)

Follow-Up #1: falling in air

Q:
"Eventually it reaches a speed at which the force of gravity pushing down on it is equal to the force of air resistance pushing up...." You mean the force of gravity PULLING down on it? and; The weight of the bullet (or anything for that matter) is not a consideration. Two objects of the same weight and same shape (aerodynamics) will fall at the same rate and hit the ground at the same time. The concept is "free-fall" and is not affected in anyway if trajectory is straight unlike with velocity if the bullet was fired downwards at the ground from say an aeroplane!
- Chris Wilson
UNITED KINGDOM
A:
There are a number of problems with your claims.
1. It is meaningless to argue about whether gravity 'pushes' or 'pulls'. The words don't mean anything different in this context.

2. Different objects do indeed fall at different rates, depending on the overall weight, size, shape, etc. If there were no air friction, the objects would fall in exactly the same way regardless of their properties, but that's not the case we are discussing.

3. I'm not sure I understand your last paragraph. Often, the phrase 'free fall' refers to situations in which something is falling without much effect from air friction. Then, in the frame of the falling object, there's no perceptible gravity. People in that situation usually find it disconcerting, or even nauseating.

Mike W.

(published on 09/03/2008)

Follow-Up #2: Terminal velocity of a bullet

Q:
If, as Tamara suggest, a bullet shot straight up descend at approx 200 mph, how come nobody gets shot dead in cowboy movies when dudes shoot off their guns in celebration or as warnings of one thing or another? Just dumb luck? Is the terminal velocity of the bullet REALLY that fast? TC
- tom clements (age 61)
moraga, ca, usa
A:
It must be dumb luck.   I sure wouldn't like to be hit by a 2 or 3 hundred mph slug.  Here is another web site that pretty much agrees with Tamara's guess.  http://www.goaskgrandpa.com/sample13.htm

LeeH

(published on 09/26/2009)

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