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-   -   Is it dangerous to fire a gun upwards? (http://www.physicsforums.com/showthread.php?t=494000)

flyingpig Apr27-11 11:39 AM

Is it dangerous to fire a gun upwards?
 
1. The problem statement, all variables and given/known data

I am just wondering, if you fire something at some speed and acceleration, that something will come down even stronger right?

So you know in funerals, often there accompanied some men in some suits and they start firing a few times in the air. Why do they do that even if it is extremely dangerous?

thegreenlaser Apr27-11 12:00 PM

Re: Is it dangerous to fire a gun upwards?
 
Theoretically, it will come down with exactly the same speed it went up with. Assuming no air resistance, then the mechanical energy of the bullet is conserved. i.e. all the kinetic energy you give the bullet by firing it up will be converted to potential energy as it travels upwards, and then converted back into kinetic energy as it travels back downwards. Since energy is conserved, the bullet will have the exact same amount of kinetic energy when it hits the ground as it did when it was fired:

[tex]E_{kinetic} = \frac{1}{2} mv^2[/tex]

The mass of the bullet won't change, so the magnitude of the velocity will be the same.

Add in air resistance, which is a non-conservative force (unlike gravity), and you'll find that the bullet is actually slowed down along its path, losing energy to things like heat and sound. (so the final kinetic energy is less than the initial kinetic energy) There's also the factor of terminal velocity. At a certain speed the air resistance force pushing the bullet up is equal to the gravitational force pulling the bullet down, and so it will no longer accelerate downward. i.e. there is a limit to how fast the bullet can be traveling when it hits the ground.

Under very ideal circumstances, the best you'll get is a bullet hitting the ground at the exact same speed. In reality, it's likely to be a smaller speed.

SteamKing Apr27-11 07:48 PM

Re: Is it dangerous to fire a gun upwards?
 
If the Honor Guard at a funeral is well led, they only fire blanks in order to avoid killing anyone downrange. Generally, the guard doesn't fire straight up.

flyingpig Apr28-11 02:50 PM

Re: Is it dangerous to fire a gun upwards?
 
Quote:

Quote by thegreenlaser (Post 3270624)
Theoretically, it will come down with exactly the same speed it went up with. Assuming no air resistance, then the mechanical energy of the bullet is conserved. i.e. all the kinetic energy you give the bullet by firing it up will be converted to potential energy as it travels upwards, and then converted back into kinetic energy as it travels back downwards. Since energy is conserved, the bullet will have the exact same amount of kinetic energy when it hits the ground as it did when it was fired:

[tex]E_{kinetic} = \frac{1}{2} mv^2[/tex]

The mass of the bullet won't change, so the magnitude of the velocity will be the same.

Add in air resistance, which is a non-conservative force (unlike gravity), and you'll find that the bullet is actually slowed down along its path, losing energy to things like heat and sound. (so the final kinetic energy is less than the initial kinetic energy) There's also the factor of terminal velocity. At a certain speed the air resistance force pushing the bullet up is equal to the gravitational force pulling the bullet down, and so it will no longer accelerate downward. i.e. there is a limit to how fast the bullet can be traveling when it hits the ground.

Under very ideal circumstances, the best you'll get is a bullet hitting the ground at the exact same speed. In reality, it's likely to be a smaller speed.

Why would the final KE be less? Doesn't it come down stronger?

http://img402.imageshack.us/img402/7159/unledqn.png

Mattowander Apr28-11 03:01 PM

Re: Is it dangerous to fire a gun upwards?
 
When it is falling, the force due to drag would be pointing upward, not downward.

Pengwuino Apr28-11 03:02 PM

Re: Is it dangerous to fire a gun upwards?
 
The air resistance is always in the opposite direction of the direction of travel. A bullet falling down will experience an air resistance upward.

Also yah, as someone said, I bet they just fire blanks as bullets do kill people when fired like that. Over here, shooting off weapons during new years is banned because people have been killed by bullets hitting people upon falling down.

Also, think about the idea of something having more energy when it goes down to the same height. If it had more energy, you could send it back upwards using a U-shaped ramp and when it came down it would have even more energy than before and you've basically built a device that creates energy from nothing! Bad Physics!


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