How Dangerous is a Penny Dropped From a Skyscraper?

Summary

In this MythBusters-inspired experiment, Derek Muller and Adam Savage investigate the myth of a penny dropped from the Empire State Building being lethal. They discover that air resistance limits the terminal velocity of a penny, making it harmless. The video explores the concept of terminal velocity with various objects, including hailstones and bullets, and demonstrates how objects' shapes and weights affect their falling speeds. The duo also addresses the danger of falling objects in everyday life, concluding that while a penny isn't deadly, heavier objects can be.

Takeaways

• 🏙️ Dropping a penny from the Empire State Building won't kill someone below due to its low terminal velocity.
• 🔢 A penny weighs around 2.5 grams, which is less than half the weight of a bullet, and thus doesn't reach a lethal speed.
• 💨 Air resistance plays a significant role in determining the terminal velocity of an object in free fall.
• 🎾 Terminal velocity is the constant speed an object reaches when the force of gravity is balanced by air resistance.
• 🌪️ The MythBusters tested pennies as projectiles but never dropped them from a significant height like the Empire State Building.
• 🚁 A helicopter's downdraft can significantly affect the trajectory and impact of falling objects like pennies.
• 💧 Raindrops have a low terminal velocity of 25 km/h, which is not enough to cause serious harm.
• ❄️ Hail can reach terminal velocities over 200 km/h, making it much more dangerous than raindrops.
• 🔩 The shape and weight of an object determine its terminal velocity; heavier objects with less air resistance reach higher speeds.
• 🗼 Dropping a bullet from a skyscraper doesn't make it lethal as it tumbles and loses energy due to higher air resistance on the way down.
• 📝 The concept of dropping projectiles from heights has been used in warfare, such as flechettes dropped from planes in WWI.

Q & A

What is the weight of a penny mentioned in the script?

A penny weighs around two and a half grams.

How fast would a penny dropped from the Empire State Building hit the ground if air resistance is ignored?

If you ignore air resistance, a penny dropped from the Empire State Building would accelerate to over 300 kilometers per hour by the time it hits the ground.

What is terminal velocity and how does it relate to the falling penny?

Terminal velocity is the constant speed an object reaches when the force of gravity pulling it down is equal to the force of air resistance pushing it up. The penny reaches terminal velocity after falling only around 15 meters, meaning it wouldn't matter if it was dropped from 15 meters or 3000 meters, it would be going the same speed.

Why aren't pennies more dangerous when dropped from a great height?

Pennies aren't more dangerous due to their terminal velocity, which is at most about 80 kilometers per hour, and their fluttering and tumbling behavior as they fall.

What is the terminal velocity of raindrops and how does it compare to that of hail?

Raindrops have a low terminal velocity of just 25 kilometers per hour. Hail, on the other hand, can reach terminal velocities of over 200 kilometers per hour, which is around 10 times the terminal velocity of rain.

Why is hail more dangerous than raindrops despite being less dense?

Hail is more dangerous than raindrops because it can get much bigger, and drag is proportional to cross-sectional area, which scales with radius squared, whereas weight scales with radius cubed. So the bigger the hailstone, the faster its terminal velocity.

What is the relationship between the shape of an object and its drag coefficient?

The drag coefficient is a dimensionless number that describes how smoothly air can flow around an object without creating vortices. It depends not only on the cross-sectional area of the object but also on its overall shape.

How does the shape of a bullet affect its terminal velocity when dropped from a height?

A bullet dropped from a height would tumble and likely end up falling on its side due to its shape, which increases air resistance. This means it wouldn't regain much of the energy it lost when it was shot upwards, resulting in a much slower speed upon hitting the ground.

What is the energy required to fracture a human skull and how does it relate to falling objects?

The lower limit of the energy required to fracture a human skull is around 68 Joules. Objects that weigh more than a few hundred grams and travel at terminal velocity are likely to deliver more than this amount of energy and could be deadly.

Why are fléchettes, used in warfare, effective despite not having explosive power?

Fléchettes are effective because they can pierce helmets, leading to enemy casualties and nasty injuries. They don't require explosives, are cheap to produce, and don't leave unexploded ordinances in the field.