How Dangerous is a Penny Dropped From a Skyscraper?

TL;DR

Terminal velocity limits impact speed; once reached, extra height adds little to how fast an object hits.

Briefing Cornell Notes

Briefing

A penny dropped from the height of the Empire State Building won’t be lethal—not because it’s “safe,” but because it tops out at a limited speed set by air resistance. In still air, a penny would accelerate to roughly 80 km/h (about 300 km/h in the simplified no-drag estimate), yet real falling behavior quickly reaches terminal velocity, meaning the penny stops speeding up long before it hits the ground. The practical result: it stings and can bruise, but it doesn’t deliver enough energy to crack a skull.

That conclusion is tested in a staged, escalating drop from a helicopter. Derek and Adam Savage coordinate a series of penny releases—first controlled hits, then a full “dump” while Derek lies beneath the rotor wash. The helicopter’s downdraft adds a complication: the air flow changes the pennies’ trajectories and makes aiming harder, with pennies arcing and fluttering as they fall. Even so, the impacts land as small, bullet-like stings rather than catastrophic injuries. Adam’s helmet and Derek’s shoulder take hits, and the overall takeaway is blunt: the pennies hurt, but they aren’t fatal.

Why pennies don’t become deadly projectiles comes down to terminal velocity and the physics of drag. Objects in free fall accelerate until gravity pulling them down is balanced by air resistance pushing them up. Air resistance grows with speed squared, so once an object reaches its terminal velocity, additional height doesn’t meaningfully increase impact speed. The transcript contrasts this with the classic hammer-and-feather thought experiment: on Earth, the feather reaches terminal velocity quickly and then coasts, while the heavier hammer keeps accelerating longer because its weight-to-drag ratio is higher.

The same framework explains why rain and hail differ. Raindrops reach low terminal velocities (around 25 km/h in the wind-tunnel setup) and tend to be relatively harmless, while hail can exceed 200 km/h because hailstones grow much larger. Drag scales with cross-sectional area (radius squared), but weight scales with volume (radius cubed), so bigger hailstones have a much higher terminal velocity and carry far more kinetic energy.

The episode also tackles why “more aerodynamic” objects might be worse. Ballpoint pens are tested as a popular myth: despite being heavier than pennies and having smaller cross-sectional area, plastic pens still don’t achieve the speeds needed for lethal impacts when dropped from tall heights. The discussion then broadens to bullets: a bullet’s shape reduces drag, but a dropped bullet would likely tumble and end up falling on its side, changing its effective resistance and speed. Real-world lethality is tied to kinetic energy thresholds—about 68 joules to fracture a human skull—while a falling penny delivers only around a fifth of a joule. Larger, heavier objects at terminal velocity (baseballs, large hailstones) can exceed 80 joules and become genuinely dangerous.

Net: falling objects are a real hazard, but a few grams of non-aerodynamic metal aren’t enough. Terminal velocity, drag, and kinetic energy determine which projectiles can kill—and which mostly sting.

Cornell Notes

A penny dropped from extreme heights isn’t lethal mainly because it reaches terminal velocity quickly. Terminal velocity occurs when gravity is balanced by air resistance, and after that point extra height adds little to impact speed. The helicopter drop test shows pennies sting and bruise but don’t cause fatal trauma. The physics generalizes: heavier objects can have higher terminal velocities, while larger objects like hailstones reach much higher speeds because drag grows more slowly than weight. Lethality ultimately tracks kinetic energy—pennies deliver far less than the ~68 joules associated with skull fracture, while hail and baseballs can exceed that threshold.

What sets the maximum speed of a falling penny, and why doesn’t extra height keep increasing its impact speed?

Falling speed is capped by terminal velocity. As a penny accelerates, air resistance increases (proportional to speed squared). Terminal velocity is reached when the downward gravitational force equals the upward drag force. After that, the penny’s average speed stays roughly constant, so dropping from 15 meters versus hundreds of meters doesn’t significantly change how fast it hits—only how long it falls and how much it can flutter or tumble.

How did the helicopter experiment account for real-world complications like rotor wash and aiming?

The helicopter’s rotor wash creates a strong downdraft between the helicopter and the drop zone. That airflow alters trajectories, producing sideways motion and arcs rather than a straight vertical fall. Pennies also flutter and tumble, meaning they don’t behave like a single rigid orientation with one terminal velocity. The experiment still found that impacts felt like stings rather than fatal blows, even with these added air currents.

Why do hammer-and-feather results on Earth differ from the Moon’s near-vacuum?

On the Moon, both objects accelerate at the same rate because there’s essentially no air resistance. On Earth, air resistance matters: the feather reaches terminal velocity early and then falls at a near-constant speed, while the hammer keeps accelerating longer. The hammer’s larger weight-to-drag ratio means drag is relatively less important, so it doesn’t hit terminal velocity as quickly.

Why are hailstones far more dangerous than raindrops?

Hailstones can be much larger than raindrops. Drag scales with cross-sectional area (radius squared), while weight scales with volume (radius cubed). As hail grows, weight increases faster than drag, so hail reaches much higher terminal velocities—over 200 km/h in the discussion—leading to much greater kinetic energy at impact.

How does kinetic energy connect to whether a falling object can fracture a skull?

The transcript cites an approximate lower limit for skull fracture of about 68 joules. It compares typical energies: a raindrop at terminal velocity delivers about 1/2000 of a joule, a falling penny about 0.2 joule, while a baseball and large hailstones can deliver more than 80 joules. Objects that exceed that energy threshold are much more likely to cause fatal injury.

Why might a ballpoint pen myth fail even though pens are heavier than pennies?

The test uses ballpoint pens (about twice the mass of a penny) with smaller cross-sectional area, which increases the weight-to-drag ratio. But the experiment still finds pens aren’t lethal when dropped from tall heights because their terminal velocity remains too low—plastic pens have enough drag relative to weight that they don’t reach the speeds needed for dangerous kinetic energy.

Review Questions

What physical condition defines terminal velocity, and how does it limit the effect of increasing drop height?
Compare why hail reaches much higher terminal velocities than raindrops using the scaling of drag versus weight.
Using the ~68 joule skull-fracture threshold, explain why a penny is unlikely to be lethal while a baseball can be.

Key Points

1
Terminal velocity limits impact speed; once reached, extra height adds little to how fast an object hits.
2
Air resistance grows with speed squared, so drag becomes dominant as objects accelerate.
3
A penny’s impacts are typically non-fatal because its terminal velocity is low and its kinetic energy is far below skull-fracture levels.
4
Rotor wash and airflow can bend trajectories and make falling objects harder to aim, but they don’t change the core terminal-velocity limit for pennies.
5
Hail is dangerous because larger size boosts terminal velocity: drag scales with area while weight scales with volume.
6
Lethality correlates with kinetic energy (about 68 joules for skull fracture), not just height or weight alone.
7
Aerodynamic shape matters, but tumbling can negate it; a dropped bullet would likely fall in an orientation that increases drag and reduces speed.

Highlights

A penny dropped from extreme heights reaches a speed ceiling set by terminal velocity, so it doesn’t keep accelerating all the way to the ground.

The helicopter drop showed pennies sting and bruise but don’t deliver fatal force, even under rotor-wash airflow.

Drag versus weight scaling explains the rain–hail gap: hail’s larger size drives much higher terminal velocities.

The kinetic-energy threshold for skull fracture (~68 joules) clarifies why pennies (≈0.2 joule) are unlikely to kill while hail and baseballs can exceed 80 joules.

Ballpoint pens were tested as a skyscraper myth and found not to be lethal because they still don’t achieve sufficiently high terminal velocity.

Topics

Terminal Velocity
Air Resistance
Falling Objects
Hail vs Rain
Kinetic Energy Thresholds

Mentioned

Derek
Adam Savage
Felix Baumgartner
David Scott
Stephen Colbert