What Goes Up Must Come Down.

An interesting question that I came across the other day.

“ We often see images of crowds of people firing hundreds of rounds of bullets into the air in celebration. What happens to those bullets? Surely they cannot keep heading into space forever? When gravity finally takes over, why aren’t they falling and killing the people that fired them?”

A great answer by Dr Karl S. Kruszelnicki.

“I’ve never been in such a crowd, but I would guess that most of these revellers think their bullets can’t hurt them. They’d be wrong.

The first thing to realise is that what goes up must come down – albeit, in the case of a bullet, more slowly. A bullet is fired with a typical muzzle velocity of about 3,000 kmh. Once gases stop pushing it, and it has well and truly left the barrel of the gun, it begins to slow. When it is fired upwards, two forces are acting to slow it – the resistance of the air and the downward suck of gravity. Typically a bullet takes about 30 seconds to climb to a height of around three kilometres. Eventually it will come to a dead halt, then begin its descent.

The suck of gravity is not nearly as powerful as the explosive gases that first fired the bullet from the barrel, so the plummeting projectile will only accelerate to a speed of somewhere between 330and 770 kmh – depending on the weight and the shape of the bullet.

A speed of 770kmh may be much slower than 3000kmh, but it is still more than enough for a bullet to penetrate a human skull (you need only 220kmh to do that). Most people who have been hit by bullets falling from the sky are struck on their upper back, the top of their head or their shoulders.

After the end of the Gulf War, Kuwaitis celebrated by firing weapons into the air and 20 died from falling bullets.

In Los Angeles between 1985 and 1992, doctors at the King/Drew Medical Centre treated 118 people for random falling-bullet injuries. Thirty eight died. Practically all of the injuries were caused by holiday-wekend party-goers discharging weapons.

Unfortunately, there have been instances in Australia where bullets have been fired into the air during celebrations. A few years back, a nine year old girl (in Belfield, Sydney) was watching New Year’s Eve fireworks with her parents in their driveway. At 12.05am, January 1st, 2002, a bullet fell out of the sky and lodged in her upper arm. If she had been standing a few centimetres to one side, she could have been killed.”