How Dangerous is a Penny Dropped From a Skyscraper?

Dropping a Penny from the Empire State Building

In this video, Derek and Adam Savage test whether dropping a penny from the Empire State Building can kill someone walking on the sidewalk below. They also explore what it takes to create a deadly projectile.

Dropping a Penny from the Empire State Building

• A penny dropped from the Empire State Building would accelerate to over 300 kilometers per hour by the time it hits the ground.

• The MythBusters made contraptions to shoot pennies at each other, but they never tried dropping pennies from the height of the Empire State Building onto someone below.

• A penny dropped from the Empire State Building is not fatal, although it stings when hit by one.

Air Resistance

• Pennies are not more dangerous because of air resistance.

• On Earth, objects speed up at different rates due to air resistance.

Terminal Velocity

In this section, we learn about terminal velocity and how it is reached when the force of gravity pulling an object down is equal to the force of air resistance pushing it up. We also learn that every object has its own terminal velocity, which is the maximum speed it will reach in free fall through still air.

Forces of Air Resistance

• The hammer experiences a greater force of air resistance than the feather because air resistance is proportional to speed squared and the hammer gets going much faster than the feather. However, due to its weight being so much greater, drag is negligible in comparison.

• Objects that have the same size and shape experience the same air resistance. But if one is heavier, then it has a higher terminal velocity so it doesn't float at the same wind speed as the lighter object.

• Conversely, some objects are very different in size and shape like a person and a lacrosse ball, but they also experience very different forces of air resistance. And the key thing is that the ratio of their weight to air resistance is

the same for both bodies so they have the same terminal velocity.

Experiencing Terminal Velocity

• Every object has its own terminal velocity, which means they will both float together in a wind tunnel.

• Felix Baumgartner jumped from a weather balloon 39 kilometers above sea level and after just 40 seconds of free fall he reached a terminal velocity over 1300 kilometers per hour making him the first person to break sound barrier outside of a vehicle.

Raindrops vs Hailstones

In this section we learn about raindrops and hailstones and how their shapes affect their terminal velocities.

Raindrops

• Raindrops aren't shaped like cartoon raindrops; they are closer to spherical but flatter on the bottom where they encounter oncoming air.

• Raindrops have a low terminal velocity of just 25 kilometers per hour.

Hailstones

• Hailstones can get much bigger than raindrops, up to 20 centimeters in diameter.

• Hailstones can reach terminal velocities of over 200 kilometers per hour, which is around 10 times the terminal velocity of rain.

• The main reason hailstones have a higher terminal velocity than raindrops is that drag is proportional to cross-sectional area, so it scales with radius squared, whereas weight scales with radius cubed. So the bigger the hailstone, the greater its weight relative to its surface area and thus the higher its terminal velocity.

Terminal Velocity of Pennies

This section discusses how pennies reach terminal velocity after falling only around 15 meters. It also explains that pennies don't actually have a single terminal velocity.

Pennies and Terminal Velocity

• Pennies reach terminal velocity after falling only around 15 meters.

• The average speed of the pennies in the top of the frame is the same as at the bottom of the frame.

• Pennies have reached terminal velocity when they aren't speeding up anymore.

• Pennies dropped from different heights would feel the same because they would be going at the same speed.

• A penny has two terminal velocities, one on its face and one on its edge.

Wind Tunnel Experiment

This section describes Adam's wind tunnel experiment to show how pennies oscillate between their two terminal velocities.

Wind Tunnel Experiment

• Adam built a custom wind tunnel to witness how pennies oscillate between their two terminal velocities.

• The wind tunnel has a gradient of wind speeds from around 100 kilometers per hour at the bottom up to 25 kilometers per hour at the top.

• The back pressure created by this gradient popsicle causes tongue depressors to spin, which makes penny spins too.

• In this experiment, if a penny really has two different terminal velocities, it should oscillate up and down in this wind tunnel as a result.

Ballpoint Pens vs. Pennies

This section compares ballpoint pens with pennies in terms of their danger level when dropped from high buildings.

Ballpoint Pens vs. Pennies

• Ballpoint pens weigh about twice as much as a penny and have smaller cross-sectional areas than them.

• Ballpoint pens falling from a skyscraper like the Empire State Building could be lethal.

• Pennies aren't dangerous because their terminal velocity is at most about 80 kilometers per hour.

• Something more aerodynamic than pennies would have a higher terminal velocity and could be more dangerous when dropped from high buildings.

Debunking the Myth of Falling Pens

In this section, Derek and Adam discuss the myth that falling pens can be dangerous and debunk it using scientific principles.

Falling Pens are Not Dangerous

• If the myth were true, we would expect to see incidents of people being injured by falling pens everywhere. However, there is no evidence to support this claim.

• Ballpoint pens with their caps off are not dangerous when dropped from tall objects because they have too much drag relative to their weight to achieve a high terminal velocity. Narrow metal pens might still be dangerous.

• Air resistance depends on both the cross-sectional area and overall shape of an object, which is captured in a dimensionless number known as the Drag Coefficient. The smoother air flows around an object without creating vortices, the lower its drag coefficient will be.

• The word "bullet" comes from the French word "boule," meaning ball. The earliest bullets were small balls or boulettes, but people modified their shape to reduce drag and eventually settled on a modern bullet shape with a drag coefficient between 0.1 and 0.3.

What Happens When You Drop a Bullet from a Skyscraper?

In this section, Derek explains what happens when you drop a bullet from a skyscraper.

A Bullet Tumbles When Dropped From A Skyscraper

• If you drop a bullet from a skyscraper, it will tumble and likely end up falling on its side instead of pointy side down as one might expect. Cylinders tend to fall on their sides if given enough chance because they find the highest resistance as most stable during freefall.

• A bullet fired straight up slows down as its kinetic energy is turned into gravitational energy. At its highest point, which could be up to three kilometers high, it stops and then falls back down. As it starts to fall, it will tumble and experience far more air resistance than on the way up. By the time it reaches the ground, it will be much slower than when shot.

• If a bullet isn't fired completely vertically, then it poses much greater danger because at the peak of its trajectory only the vertical component of velocity is zero. It still maintains its horizontal velocity combined with spin imparted by grooves inside gun barrel keep moving pointy and forwards. As the bullet comes back to the ground, it speeds up to a significant fraction of its launch speed.

Dropping Deadly Projectiles

In this section, Derek talks about dropping deadly projectiles on enemies.

Fléchettes: Little Arrows Dropped from Planes

• In World War I, little pieces of metal called fléchettes were dropped out of planes. They looked like nails with little feathers on their backs to make sure they fell straight and were called "little arrows" in French. Some were up to 15 centimeters long.

• The US created similar weapons called Lazy Dogs later used in Korean and Vietnam wars that were a bit heftier but inflicted indiscriminate damage and unpredictable injuries without leaving unexploded ordinances.

Projectiles and Lethality

In this section, we learn about the lethality of different projectiles.

Lethal vs Non-Lethal Projectiles

• The lower limit of energy required to fracture a human skull is around 68 Joules. Anything that has kinetic energy greater than that is very likely to kill you.

• A raindrop at terminal velocity with its tiny mass will only deliver 2000th of a Joule. A falling penny has about a fifth of a Joule.

• A baseball and the largest hailstone measured deliver more than 80 Joules, which is plenty to crack your skull.

• Objects that weigh more than a few hundred grams traveling at terminal velocity are likely to be deadly.

• Blunt force trauma can be fatal, but objects like Flechette may not have enough energy to crack your skull.

Learning with Brilliant

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