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Can a coin dropped from a great height be deadly? – ABC Radio National

Hi, Doctor Karl here.

Have you ever prepared for the Sydney Harbor Bridge Climb? You have to get everything out of your pockets, especially metal coins. How is that possible? Could you really kill someone by dropping a coin from a great height?

It turns out that people all over the world have done different versions of this experiment and the answer is almost certainly a resounding “no” – at least with regular coins.

In the early 2000s, University of Virginia physicist Lou Bloomfield tried to mimic dropping a coin from a tall building. He cleverly combined helium balloons with a radio-controlled mechanism to release the coin once the balloon was high enough. Then he tried to catch the falling coins. He caught, and was even hit, by some of the falling coins. He said it wasn’t painful and it felt like a finger being slapped on the forehead – “…but not even very hard”.

According to the popular science magazine The Scientific American, he said: “The pennies didn’t hurt. They bounced off me and it felt like I was being hit by insects, large raindrops, or small hailstones. No bruises, no injuries. I was out the whole time laughing.”

Now, since a coin is made of thick metal, how come the falling coins didn’t even hurt it? The short answer is that they didn’t fall fast enough.

On the one hand, you have the ever-powerful pull of gravity that pulls the coin toward the ground faster and faster with each passing second — but to get this continued acceleration, the coin has to fall into a vacuum. However, our common currency is falling down in our atmosphere. So gravity is opposed by the atmosphere. In addition, our coin does not fall vertically straight down, but flutters on the ground like a leaf – sliding to the left and then to the right while aligned roughly horizontally.

Now I’m talking about your average coin, which weighs a gram or two, and about an inch or two in diameter. I’m certainly not talking about the South African gold coin called the Krugerrand, which weighs about 33 grams and is about 33 millimeters in diameter. If one of these fell down first, it could build up significant speed and cause some damage – thanks to its weight and vertical alignment.

But with your average falling coin, after about 50 feet of drop, the pull of gravity is offset by the resistance of the wind — and the coin reaches the speed horribly known as its Terminal Velocity. In our atmosphere, however, this speed is relatively low – about 45-50 km/h. If there were no atmosphere, a coin could accelerate to about 233 mph, which is fast enough to do some damage if it hits someone.

According to studies by the American National Rifle Association, 110 km/h is enough to penetrate the skin, while 220 km/h breaks bone.

Atmosphere is one thing, form another. For example, a falling heavy metal ballpoint pen can cause massive damage if dropped from a great height. If it fell vertically like an arrow, the combination of its terminal velocity of about 300 km/h, combined with its small, hard pointed end, could penetrate a human skull.

Sure, vertically fired bullets certainly cause damage on the way back to Earth. Depending on the weight and shape of the falling bullet, they can reach speeds between 330 and 770 km/h on the way down. In Kuwait, after the end of the Gulf War, the Kuwaitis celebrated by firing guns into the air – and 22 people died from falling bullets.

In Los Angeles, King-Drew Medical Center treated 118 people for random injuries from falling bullets between 1985 and 1992. 38 of them died, while most of the survivors had severe long-term disabilities, such as chronic pain, seizures, spinal cord injury and quadriplegia.

In Sydney, a nine-year-old girl from Sydney sat with her parents from their driveway watching fireworks to celebrate the New Year 2002. Five minutes after midnight, a bullet fell from the sky and lodged in her upper arm. A few inches closer to her midline, and she could have been dead.

The Mythbusters, Adam Savage and Jamie Hyneman, tried a variation on this experiment. They made a mini cannon that could fire a small coin at about 100 km/h – about twice the speed of a falling coin. They aimed it at one of their favorite props, a ballistic dummy, which took only minor damage. Then, made brave by this knowledge, they fired coins at each other. Yes, they did sting, but no, they caused no damage. When a coin fell on Adam’s hand, he said it barely hurt. And that coin moved faster than a coin that fell from the Empire State Building or the Sydney Harbor Bridge.

But if it rained cats and dogs, well, that would be a whole different box of frogs.

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