You’ve probably heard the story – if you throw a penny from a skyscraper, it might bury itself in the concrete or even kill an unsuspecting bystander below. The open-air observation deck of the Empire State Building in New York City is 86 floors up; a staggering 1,050 feet (320 meters) in the air. That’s a lot of time to pick up speed during the coin’s free fall. So, what kind of damage might a falling penny cause if it hit someone? Should you avoid walking near tall buildings without a hard hat or is this merely an old and well-worn myth?
Since no one at the Altair office was brave enough to volunteer to physically test this theory, we decided to use engineering simulation software to virtually validate the experiment.
Gravity causes the penny to fall rapidly toward the ground, but we also need to factor in air resistance, or drag force, caused by the penny colliding with air molecules as it falls. The faster the penny falls, the more drag it encounters, which slows down the object’s acceleration. Factoring the push-and-pull between gravity and drag allows us to determine the terminal velocity or maximum speed the penny will reach before impact. In order to represent the real-world behavior of the penny in flight, we used Altair AcuSolveTM, a computational fluid dynamics (CFD) software.CFD enables us to simulate the drag the penny would encounter as it falls, as well as determine how these forces would cause the penny to rotate in its descent. Many factors can influence the penny’s rotation during flight, such as the force and direction the penny is dropped or thrown, any wind or updraft, and even the amount of humidity in the air, so this simulation just represents one of many potential flight paths.
Within the simulation, we modeled the penny in free fall and were able to extract the drag due to air resistance as well as its terminal velocity, reaching a maximum speed of 38 mph (61 kph).
Now that the terminal velocity has been determined, it’s time for the moment of truth.
The penny hurdles toward the head of our crash test dummy at a speed of 38 mph (61 kph), and we used Altair RadiossTM, a finite element analysis software, to record the forces the penny will inflict on impact.
The penny strikes and bounces off the dummy’s head within less than a millisecond, inflicting about 350 newtons of force. According to a NASA report, an average male can exert around 1,000 newtons of force with a punch, so while the impact of a penny at 350 newtons would probably be painful, it would not cause any lasting damage. According to a study from the Journal of Forensic Sciences, a pellet weighing the same amount as a penny would need to travel at a velocity of 167.8 mph (270 kph) to break the skin. Although it may leave a welt, it would be unlikely for a penny traveling at 38 mph (61 kph) to even cause a cut on the head of our brave dummy.
The impact would also cause the head to experience a brief 0.5 millisecond spike of about 7.5 units of g-force, which measures the acceleration of the head as a result of the impact. In automotive crash tests, we typically target head acceleration values of more than 80 g over a duration of three milliseconds, so the 7.5 g spike observed in this simulation is quite small and unlikely to cause significant damage.
Thanks to the drag forces we modeled in CFD showing a slowing of the penny’s acceleration, we definitively determined that an impact event would not produce fatal amounts of force. With the old penny-dropped-off-a-building myth officially debunked, you can walk confidently the next time you’re in The Big Apple. Just don’t forget about the number one aerial danger in New York City… watch out for pigeons overhead!