Myth: Dropping a penny from a tall building will kill someone

The Myth:

Some of you may have heard that a penny dropped from a really tall building, like the Empire State Building for example, could potentially kill a pedestrian walking the streets down below. The logic behind this is that the penny would continue to accelerate while falling and reach such a speed, that by the time it reaches the ground and a potential victim, it will be travelling fast enough to do a fair amount of damage, even to the point of killing someone.

So city-slickers, should you be worried?

The Truth:

The simple answer is no. You can feel free to walk around a city without the fear of getting struck down (although, you might feel a slight sting!) by a rogue penny.

Why?

Well, because the science behind this myth just doesn’t quite add up.

You see, the problem with this is that people tend to assume that a penny would continue to accelerate until it is stopped. However, while this may be true in a vacuum where air particles are not present, that isn’t a valid representation of what our atmosphere is actually like.

A penny falling from the top of a building will experience air resistance – meaning its fall will be slowed down as it comes into contact with air particles. This force opposes gravity and is known as “drag force”. This force increases as the penny’s velocity increases, until the drag force becomes equal and opposite to the pull of gravity. Once this point is reached, the penny (or any object for that matter) will stop accelerating, reaching what is known as “terminal velocity”.

Now for some facts and figures!

• Louis Bloomfield (cited in Wolchover 2012), a physicist the University of Virginia, has discovered that a penny will reach terminal velocity in only about 15 metres from point of origin – that’s about the height of three giraffes stacked on top of each other!
• Once reaching terminal velocity, the penny is estimated to fall at a rate of 40 kph. Considering there are some humans in the world that can run that fast, it doesn’t seem very fast at all!
• If the penny did continuously accelerate as if it were in a vacuum, by the time it traveled the length of the building, it would reach speeds of roughly 335 kph.

So in essence, the penny simply isn’t traveling fast enough to do that much damage.

Still don’t believe me? Then check out the video below and watch as Adam Savage and Jamie Hyneman from the popular TV show ‘MythBusters‘ put themselves in the firing line of a penny travelling at terminal velocity. You will find that they are not only still very much so alive, but also discover that the penny wasn’t even able to penetrate a layer of skin on Adam’s hand.

MythBusters Jamie and Adam testing the myth in Penny Drop (2011).

Why does a penny travel so slowly, you ask? Well, there are a number of factors that come into play. One being its flat shape. Due to this it is hardly aerodynamic, causing it to flutter about in the air instead of piercing straight through. Any wind or upward draft would slow the penny down greatly. Being so light-weight also turns the penny into a snail, as heavier objects have a higher terminal velocity than lighter ones.

Now, the question is, what about an object that IS more aerodynamic, like a pen? What would happen then? Could a pen kill someone? Well… lets just say that next time I’m a few hundred metres up in the air, I’ll make sure any pens I have on me are nice and secure.

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Sources

Hiskey, D 2010, ‘Dropping a penny from the top of the Empire State Building isn’t dangerous’, retrieved 7 April 2016, <http://www.todayifoundout.com/index.php/2010/10/dropping-a-penny-from-the-top-of-the-empire-state-building-isnt-dangerous/>

Penny Drop | MythBusters 20122, YouTube, Discovery, 11 February, retrieved 7 April 2016, <https://www.youtube.com/watch?v=PHxvMLoKRWg>

Wolchover, N 2012, ‘Could a Penny Dropped Off a Skyscraper Actually Kill You?’ retrieved 7 April 2016, <http://www.scientificamerican.com/article/could-a-penny-dropped-off/>.