As Britain unveils a new pound coin, we put to rest once and for all the question of whether falling change can, in fact, be lethal?
It’s one of Itchy and Scratchy’s finest moments. From atop the Empire State Building, the mouse drops a penny. With a fury only the furies could conjure, the metal disc hurtles down with enough speed to incinerate the cat. Classic.
But is The Simpsons the perfectly accurate guide to reality we all assume it to be? Is the urban myth true: can a coin dropped from the top of a skyscraper actually kill someone?
The short answer, says physicist Jon Butterworth of University College London, is no. It couldn’t even burn flesh – except in certain circumstances.
All dropped objects experience constant acceleration – in other words, they go faster and faster, their speed constantly increasing, at a rate of just under 10 metres per second. But air resistance also increases the further the object has fallen. When air resistance matches gravity, the object will achieve “terminal velocity” – maximum speed, after which it no longer accelerates. At a weight of 7.12g, a 2p coin would in theory reach terminal velocity at just over 19km an hour, hardly fast enough to kill you. (The actual speed it would reach depends on a variety of factors, from humidity to air pressure.)
The new British pound coin proposed this week, if it weighs the same 9.5g as the current one, won’t be able to do much more damage either – although its bevelled edges might sting a little more.
So is it ever possible for a coin dropped from a skyscraper to be lethal? Only if chucked down a very long container sucked free of all gas – because the coin could continue accelerating for ever.
“If you dropped a coin from high enough in a vacuum chamber, it could possibly kill you,” Butterworth confirms. True vacuums, however, only exist in the lab: even space, once thought devoid of all matter, is sprinkled with gas and dust (though no skyscrapers as of yet).
A breadboard, on the other hand, is a different story. Dropped from the Empire State Building, which is 381 metres high, a 220g wooden breadboard on its side would take just under nine seconds to hit the ground and would hit you at a speed of 311km/hr.
So where does the penny myth come from? Butterworth suggests it might have something to do with the psychological impact of skyscrapers. “Maybe standing on tall buildings gives people an exaggerated sense of their own power.”