How Dangerous is a Penny Dropped From a Skyscraper?
VeritasiumKey Moments
Dropping a penny from a skyscraper won't kill you due to air resistance, but hail and flechettes can be deadly.
Key Insights
Air resistance limits the terminal velocity of light objects like pennies, preventing them from reaching lethal speeds.
Terminal velocity is the maximum speed an object reaches when the force of air resistance equals the force of gravity.
The ratio of an object's weight to its air resistance determines its terminal velocity; heavier, more aerodynamic objects have higher terminal velocities.
Hailstones can be deadly due to their larger size and consequently higher terminal velocities compared to raindrops.
Historical weapons like flechettes and modern concerns like falling construction tools highlight the real dangers of falling objects with sufficient mass and aerodynamics.
A bullet fired upwards and falling back down is more dangerous than a dropped bullet because it retains horizontal velocity and tumbles less.
THE PENNY MYTH DEBUNKED
The common belief that a penny dropped from a skyscraper can be lethal is a myth. While a penny dropped from the Empire State Building would accelerate to a significant speed without air resistance, the reality is that air resistance drastically limits its velocity. The video demonstrates with experiments, including dropping pennies from a helicopter onto a person, that the impact is noticeable but not fatal. This is primarily due to the penny's low mass and relatively large surface area, which lead to a low terminal velocity.
UNDERSTANDING TERMINAL VELOCITY
Terminal velocity is the maximum speed an object reaches when falling through a fluid like air. It occurs when the downward force of gravity is balanced by the upward force of air resistance. Objects like feathers reach terminal velocity quickly because their large surface area relative to their weight creates significant air resistance. Conversely, dense objects like hammers accelerate for longer, reaching a higher terminal velocity because their weight overcomes air resistance more effectively.
THE ROLE OF AIR RESISTANCE AND SHAPE
Air resistance, or drag, depends on an object's speed, shape, and the density of the air. Objects with a higher drag coefficient, meaning they disrupt airflow more, experience greater resistance. A penny's flat shape causes it to flutter and tumble, leading to two different terminal velocities depending on whether it falls on its face or edge. This tumbling effect increases air resistance and further limits its speed. More aerodynamic shapes, like bullets, are designed to minimize drag, but even bullets tumble when dropped, reducing their potential impact speed.
HAIL VERSUS RAINDROPS: A MATTER OF SIZE AND SPEED
While raindrops have a low terminal velocity, typically around 25 kilometers per hour, and are not dangerous, hailstones present a significant risk. Hailstones can grow much larger than raindrops, with drag increasing by the square of their radius while weight increases by the cube. This means larger hailstones have a much higher ratio of weight to drag, resulting in significantly higher terminal velocities, some exceeding 200 kilometers per hour. This high speed and mass give hailstones enough kinetic energy to cause serious injury or fatalities.
LETHAL PROJECTILES: FROM FLECHETTES TO CONSTRUCTION TOOLS
Certain objects are indeed lethal when falling from great heights. Historically, weapons like flechettes (small, dart-like projectiles) were designed to cause casualties without explosives, relying on their sharp points and sufficient velocity to penetrate. Similarly, modern dangers include falling construction tools, large icicles, and even measuring tapes falling from skyscrapers, which can possess enough kinetic energy to cause severe injury or death. The energy required to fracture a human skull provides a benchmark for lethality, with objects significantly exceeding this energy being highly dangerous.
BULLETS AND CELEBRATORY GUNFIRE: ADDED DANGERS
A bullet fired straight up poses a greater risk than a dropped bullet. While a bullet fired upwards slows down due to gravity, it retains some of its initial kinetic energy and horizontal velocity. As it falls, it tumbles, increasing air resistance, but it can still reach a significant fraction of its launch speed by the time it returns to the ground. This is why celebratory gunfire, though accidental, is responsible for numerous fatalities worldwide, as the falling bullets can be lethal.
Mentioned in This Episode
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Falling Object Safety: Dos and Don'ts
Practical takeaways from this episode
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Kinetic Energy of Falling Objects
Data extracted from this episode
| Object | Kinetic Energy (Joules) | Lethality |
|---|---|---|
| Raindrop (terminal velocity) | 0.002 | Not lethal |
| Penny (from Empire State Building) | 0.2 | Not lethal |
| Baseball | >80 | Likely lethal |
| Largest Hailstone | >80 | Likely lethal |
Common Questions
No, a penny dropped from the Empire State Building will not kill someone. While it reaches a significant speed due to gravity, air resistance quickly limits its speed to terminal velocity, making the impact painful but not fatal.
Topics
Mentioned in this video
Used as a comparison for a penny's weight and as an object to analyze for terminal velocity and dropping behavior from height.
The force opposing an object's motion through the air, crucial in determining terminal velocity and the danger of falling objects.
The act of firing weapons into the air to celebrate, which can lead to people being killed by falling bullets.
Made a record-breaking jump from a helium balloon in 2012, reaching supersonic speeds due to lack of air resistance at high altitude.
The central object of the video's mythbusting experiment, testing if it could be lethal when dropped from a skyscraper.
Tested as potentially lethal projectiles when dropped from heights, weighing more than a penny but with significant drag.
Used to simulate a human impact when testing the lethality of falling ballpoint pens.
Small, nail-like projectiles with feathered backs, dropped from planes in WWI for military use.
Generic term used when referring to literature in contrast to scientific principles.
The maximum constant speed an object reaches when falling through a fluid, determined by the balance between gravity and drag.
A dimensionless number that quantifies the resistance of an object in a fluid environment, related to the object's shape and smoothness.
The energy an object possesses due to its motion, calculated as 1/2 * mass * velocity^2, and a key factor in determining impact lethality.
A television show that tested myths, with Adam Savage, one of its hosts, participating in this video's experiment.
Historical period during which flechettes were dropped from aircraft as projectiles.
A building in New York City where streets were closed in 2014 due to the danger of falling icicles.
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