Eli5 If you shoot a bullet into the air how does it gain enough velocity on the way down to kill someone?
In: 5
According to the US Centers for Disease Control and Prevention, falling bullets can hit the ground at speeds greater than 61 metres per second (m/s). Bullets travelling between 46 and 61 m/s penetrate skin. Faster than this, and they can penetrate the skull.
Celebratory gunfire can cause injuries that require emergency room treatment and death. In Puerto Rico, 19 people were injured by celebratory gunfire on New Year’s Eve 2003, and one died.
Source:
https://www.newscientist.com/lastword/mg25233622-900-can-bullets-fired-upwards-cause-injuries-when-they-return-to-earth/
Remember falling objects increase speed at 9.8 meters a second until they reach terminal velocity
Gravity.
In the absence of air, the bullet would land with the same speed it started with. It would slow down on the way up as it gained altitude, then regain the same speed on its way back down.
In reality, air resistance slows bullets pretty substantially, but bullets are smooth and dense, so it doesn’t slow them down as much as it would need to to make them safe. A typical terminal velocity for a bullet is a few hundred miles per hour, which is slower than a bullet freshly fired from a gun, but still more than enough to do damage when it hits.
What goes up must come down. It loses its kinetic energy (the force of it travelling) to the air as it goes up but as gravity pulls down it starts to pick up that kinetic energy again. Try throwing a soccer ball up in the air and letting it land on you.
Most bullets aren’t shot “straight up” into the air so they travel in an arc and maintain much of their initial velocity
Energy is conserved; i.e. it can change form but the total remains.
Ride a roller-coaster down and you gain speed as you lose height — you gain kinetic energy as you lose potential energy. And then you go up the next hill, and lose speed as you gain height.
Likewise for a bullet that you shoot up in the air — it loses speed as it rises, and then gets it back as it falls.
The situation is complicated by air resistance. As the bullet goes through the air some energy is lost, heating the air. Not much if the bullet is going point-first; more if it’s tumbling.
If they are shot straight up, then they don’t. They can only fall back at terminal velocity, which isn’t fast enough to cause any major damage. This is because the bullet goes straight up until it loses momentum, stops momentarily before falling back to earth.
However, if they are shot at an angle, then they don’t lose enough of their forward momentum to fall back to earth safely, and can potentially injure or kill someone.
You assume the bullet on the way down is like someone dropping one out of a hot air balloon and just opening their hand.
Anything that’s not dead straight up** will arc and not tumble but keep some of its energy and return to earth.
**There’s probably a something degree range that firing “straight” up will result in a tumbling bullet, but that’s for you to find out.
It doesn’t. If you shoot a bullet perfectly straight up into the air, then it will reach its maximum height, then fall at terminal velocity, which would hurt like hell but not injure you.
The problem is that firing completely vertically is borderline impossible for an unassisted human. So instead of going straight up, stopping, then falling, it follows an arc and maintains a lot of horizontal speed.
So it isn’t that the bullet gains enough velocity to kill someone, it’s that if fired at even a slight angle, it retains enough of its initial velocity to still kill someone.
Bullets shot straight up start tumbling near the apex where they have to change direction. They are non-lethal.
Bullets shot *nearly* straight up never completely come to a stop (they maintain some speed perpendicular to gravity) and can keep their aerodynamic spin-stabilization. These bullets have the possibility of being lethal.
Well without atmosphere, it will come back down at the exact same speed you shot it up. It decelerates upwards at the same speed it accelerates downwards.
With atmosphere that is still true up to terminal velocity. The terminal velocity of a bullet, aerodynamic and potentially made out of dense material (pretty much any solid metal would do, maybe not including aluminium?, But certainly lead) would definitely be high enough to do serious damage. Especially considering that most “tallest building in XYZ place” tout the fact that if you were to drop a small coin off the top it would kill someone at the bottom.
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