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.

Outlines

00:00

🌆 MythBusting Pennies from the Empire State Building

Derek and Adam Savage explore the myth that a penny dropped from the Empire State Building could be lethal. They discuss the weight and potential velocity of a penny, comparing it to a bullet. Despite the penny reaching speeds of over 300 km/h when dropped from such a height, it does not cause serious harm due to air resistance. The duo conducts an experiment where Adam drops pennies on Derek from a helicopter, simulating the scenario. The pennies, while stinging upon impact, do not inflict serious injury, proving the myth false.

05:00

🌀 Understanding Terminal Velocity and Air Resistance

The concept of terminal velocity is explained, which is the constant speed an object reaches when the force of gravity is balanced by air resistance. The video uses the example of a hammer and feather to illustrate how air resistance affects different objects. It is shown that heavier objects like the hammer experience greater air resistance but continue to accelerate due to their weight, while lighter objects like feathers reach terminal velocity quickly. Derek experiences this firsthand with indoor skydiving, demonstrating how objects with the same weight-to-air resistance ratio have the same terminal velocity, regardless of their shape or size.

10:04

🎾 Testing the Danger of Falling Objects

The video investigates the danger of various falling objects by examining their terminal velocities and the energy required to fracture a human skull. It is revealed that while a penny or a ballpoint pen dropped from a height is unlikely to be lethal due to their low terminal velocities, objects like baseballs or large hailstones can be dangerous. The team conducts an experiment with a ballistics gel dummy to test the impact of falling pens, finding them not to be lethal. The segment also touches on the historical use of kinetic projectiles like flechettes in warfare and the risks of falling objects in everyday life.

15:05

🔍 Deep Dive into the Physics of Falling Objects

This segment delves deeper into the physics of falling objects, discussing how the drag coefficient affects the terminal velocity. It explains how bullets, despite their weight, tumble and fall on their sides due to their shape causing them to experience more air resistance. The video also addresses the kinetic energy and potential lethality of falling objects, comparing the energy delivered by a falling penny to that of a baseball or a hailstone. It concludes by emphasizing that while small, non-aerodynamic objects are generally not fatal when falling, larger objects traveling at terminal velocity can be deadly.

20:05

🎓 Educational Resources with Brilliant Sponsorship

The video concludes with a sponsorship message for Brilliant, an educational platform that offers interactive learning in STEM fields. Derek encourages viewers to explore Brilliant for advanced topics and fundamental concepts, emphasizing the importance of hands-on learning and problem-solving. He shares his experience with video-based education and how platforms like Brilliant complement video content by allowing users to experiment with simulations and receive immediate feedback. A special offer for Veritasium viewers is provided, thanking both Brilliant and the viewers for their support.

Keywords

💡Terminal Velocity

Terminal velocity is the constant speed that an object reaches when the resistance of the medium through which it is falling (here, air) balances out the force of gravity. In the video, it's discussed in the context of how different objects like pennies, feathers, and hailstones reach different terminal velocities due to their weight and shape. The video demonstrates that a penny dropped from the Empire State Building would not be lethal because it quickly reaches a terminal velocity that is not high enough to cause significant harm, contrasting with heavier and more aerodynamic objects.

💡Air Resistance

Air resistance, also known as drag, is the force that opposes the motion of an object through the air. It is a key factor in determining an object's terminal velocity. The video explains that air resistance is proportional to the square of the object's speed and its cross-sectional area. It's illustrated through the example of a penny dropped from a height; air resistance prevents it from accelerating indefinitely, thus it doesn't reach lethal speeds.

💡Cross-Sectional Area

The cross-sectional area of an object is the area of a surface that is perpendicular to the direction of motion. It plays a significant role in determining the drag force experienced by an object in a fluid medium like air. The video mentions that objects with smaller cross-sectional areas experience less air resistance, which can lead to higher terminal velocities, as seen in the discussion about ballpoint pens versus pennies.

💡Drag Coefficient

The drag coefficient is a dimensionless number that describes the resistance of an object moving through a fluid like air. It depends on the shape of the object and how smoothly the air can flow around it. The video explains how the drag coefficient is crucial in determining the terminal velocity of objects like bullets, which have been shaped to minimize air resistance and thus have a lower drag coefficient compared to spheres.

💡Kinetic Energy

Kinetic energy is the energy an object possesses due to its motion. It is calculated as one half times the mass of the object times the square of its velocity. In the video, kinetic energy is discussed in relation to the potential danger of falling objects. The video states that objects with enough kinetic energy to fracture a human skull (around 68 Joules) are considered lethal, which is used to compare the potential harm of different falling objects like pennies, hailstones, and icicles.

💡Fléchettes

Fléchettes are small, dart-like projectiles, mentioned in the video in the context of historical military use. They were designed to fall straight due to their shape and small feathers, which increased their terminal velocity and made them potentially lethal. The video contrasts flechettes with pennies, highlighting the difference in their aerodynamic properties and the resulting differences in terminal velocity and danger.

💡Hailstones

Hailstones are balls of ice that form in thunderstorms. The video discusses how hailstones can reach high terminal velocities due to their larger size and mass, making them potentially dangerous. It contrasts hail with raindrops, which have a much lower terminal velocity and are not as dangerous. The video also mentions that hail can cause fatalities, emphasizing the role of size and density in determining the potential harm of falling objects.

💡Ballistics Gel

Ballistics gel is a material that mimics the properties of human flesh and is used to test the impact of projectiles. In the video, a ballistics gel dummy is used to test the impact of a ballpoint pen dropped from a height, simulating what would happen if it hit a person. The video concludes that the pen is not lethal, as it doesn't penetrate deeply into the gel, illustrating the importance of testing to understand the effects of falling objects.

💡Empire State Building

The Empire State Building is used in the video as a reference point for the height from which objects are dropped to test their potential to cause harm. The video investigates the myth that a penny dropped from the top could kill someone below, using it as a basis for exploring concepts like terminal velocity and air resistance. The Empire State Building serves as a dramatic and relatable example to engage the viewer with the physics concepts being discussed.

💡MythBusters

MythBusters is a popular science television show that aimed to test the validity of various myths and urban legends. The video references the show and its methods, particularly when discussing the testing of myths related to the potential harm of objects dropped from significant heights. The mention of MythBusters adds a layer of familiarity and credibility to the experiments and discussions in the video.

Highlights

Dropping a penny from the Empire State Building is a popular myth about its potential to cause harm.

The Empire State Building's ledges are filled with change, indicating the myth's widespread curiosity.

A penny dropped from a height has the potential to reach speeds comparable to a bullet due to gravity.

MythBusters tested the impact of pennies but never replicated the Empire State Building drop.

The experiment involved a helicopter and Adam Savage to simulate the penny drop.

The helicopter's downdraft affected the pennies' trajectory, introducing an uncontrolled variable.

Despite the fall, the pennies did not cause severe harm, confirming the myth as busted.

Air resistance plays a crucial role in determining an object's terminal velocity, which is key to the pennies' safety.

Terminal velocity is the constant speed an object reaches when air resistance equals gravitational force.

Experiments with hammers and feathers demonstrate the impact of air resistance on different objects.

The ratio of weight to air resistance dictates the terminal velocity, explaining why heavier objects fall faster.

Indoor skydiving illustrates how objects of different weights can have the same terminal velocity.

Felix Baumgartner's jump showed how reduced air resistance at high altitudes increases terminal velocity.

Raindrops have a low terminal velocity due to their shape and size, making them harmless.

Hail can be deadly due to its potential to reach high terminal velocities and carry significant kinetic energy.

Penny tumbling behavior indicates it has two terminal velocities, which was demonstrated in a custom wind tunnel.

The myth that ballpoint pens dropped from heights could be lethal was tested using a ballistics gel dummy.

Drag coefficient, a dimensionless number, determines how air resistance affects an object's fall.

Bullets tumble when dropped, making them less dangerous than one might expect from their shape.

Historically, fléchettes were used as kinetic projectiles in warfare due to their ability to pierce helmets.

The energy required to fracture a human skull is around 68 Joules, providing a benchmark for lethal falling objects.

Transcripts

- [Derek] What would happen if you dropped a penny

off the Empire State Building?

Could it kill someone walking on the sidewalk below?

What does it take to create a deadly projectile?

Well, I'm gonna put this to the test

with original MythBuster Adam Savage.

He's going up in a helicopter to throw pennies at me.

No one hasn't heard that story.

- When you talk about it,

people are like, "Oh yeah,

the penny from the Empire State Building."

And when we went to the Empire State Building,

every ledge below the observation deck

is filled with change.

I just love the idea of all these people.

They're not murderers, but they're like,

it's probably not true.

They're like throwing pennies over the side

thinking it's probably not true.

- [Derek] A penny weighs around two and a half grams,

which is about half to a quarter the weight of a bullet.

If you ignore air resistance,

a penny dropped from the Empire State Building,

which is 443 meters to the very top,

would accelerate to over 300 kilometers per hour

by the time it hits the ground.

That's around half as fast as a typical bullet.

The MythBusters made contraptions

to shoot pennies at each other.

- Stephen Colbert shot me in the ass with it a few times.

- How did it feel?

It's a baseball pitcher throwing a penny hard at you.

- Yeah. - It'll sting.

- [Derek] But they never tried the ultimate test.

Dropping pennies

from the height of the Empire State Building

onto someone below.

And that's what we're gonna do here with Adam.

- That'd be great watching it bounce off your body.

- [Derek] Yeah, yeah.

- I say this thinking it's always been my body until now.

First one, I'll drop the pennies

to see where it's gonna land.

You're gonna walk there.

I'll throw a second one,

that's good for you. - Yeah.

- And then you're ready for the full dump.

- Yep.

(tense music) (helicopter engine revving)

- I'm going underneath helicopter

where Adam Savage is gonna drop a whole bunch of pennies

on me.

What are we even doing?

I know I agreed to do this

and I didn't think I'd get hurt,

but as I'm walking out under the helicopter,

I started to think, no one has actually done this.

We planned for pennies falling through still air,

but the helicopter creates a huge downdraft

to support its weight.

- In 3, 2, 1.

- Oh boy, I hear them landing around me.

I start to imagine pennies gouging into my shoulders.

You can see how tense my body looks.

- Ah, one hit my helmet

in 3, 2, 1.

- Aaaaaa-hahaha-owwww, that hit my shoulder.

AAAAaAAh.

(slow-motion screaming)

- That's really good.

In 3, 2, 1.

- They feel like tiny little bullets.

I feel like I'm gonna be bruised after this.

Oh boy.

Okay, I'm laying down, I'm going for it.

- Here we go, doing the whole thing.

In 3, 2, 1.

(Guitar riff)

Unbelievable.

There you have it.

A penny dropped from the Empire State Building

is not gonna hurt.

I mean it hurts a little, but not a lot.

You'll be alright.

- That was great.

I saw little dust clouds all around you.

- Amazing.

- Tell me what it was like down here.

- I was terrified.

I got out there and the rotor wash is so heavy.

I'm like, maybe we haven't calculated for rotor wash.

It stung to get hit by pennies falling that far,

but it certainly wasn't fatal.

(upbeat music)

So why aren't pennies more dangerous?

Well, the reason is air resistance.

- [David Scott] And I'll drop the two of 'em here

and hopefully they'll hit the ground at the same time.

- Take the classic experiment of a hammer and feather

dropped simultaneously on the moon.

In the near vacuum of the moon's surface,

both objects speed up at the same rate

due to the moon's gravity,

and they're both still accelerating

when they hit the ground at the same time.

- [David Scott] How 'bout that?

- Repeat the experiment on earth.

And of course, the hammer lands way before the feather.

If you watch the feather closely,

you'll notice that it doesn't speed up as it falls.

For most of its journey,

it's moving at a constant speed

known as its terminal velocity.

Terminal velocity is reached when the force of gravity

pulling an object down

is equal to the force of air resistance pushing it up.

In this case,

which object, the hammer or the feather,

experiences a greater force of air resistance.

Well, I bet most people would say the feather

because its motion is clearly affected by drag.

But the answer is actually the hammer.

Air resistance is proportional to speed squared

and the hammer gets going much faster than the feather

so it experiences the larger force of air resistance,

but its weight is so much greater

that drag is negligible in comparison.

And that's why the hammer keeps speeding up

while the feather reaches terminal velocity early

and just stays at that speed.

It all comes down to the ratio of weight to air resistance.

Every object has its own terminal velocity,

the maximum speed it will reach

in free fall through still air.

And I went indoor skydiving to experience this first hand.

That was so amazing, that was totally wild.

Objects that have the same size and shape

experience the same air resistance.

But if one is heavier, here I've got two identical balls,

but this one has water added to it, so it's 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,

and obviously they have very different weights,

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,

which means they will both float together in the tunnel.

If you transported the sky diver and lacrosse ball

to the stratosphere,

they would continue to fall together,

but their joint terminal velocity would be much faster.

In 2012, Felix Baumgartner jumped from a weather balloon

39 kilometers above sea level.

After just 40 seconds of free fall,

he reached a terminal velocity

over 1300 kilometers per hour.

It was 25% faster than the speed of sound

making him the first person to break the sound barrier

outside of a vehicle.

What allowed him to do this was the lack of air

at that altitude.

Air resistance is directly proportional

to the density of air you're moving through.

And at that altitude,

the air is 60 times less dense than at sea level.

As he continued falling into thicker atmosphere,

the increasing air density reduced his terminal velocity

and by two and a half kilometers above sea level,

he had slowed to 200 kilometers per hour,

at which point he opened his parachute.

Now rain also falls kilometers,

but through the thicker air of the troposphere.

One of the coolest things in the wind tunnel

was to see water floating.

Poured from a jug,

it quickly breaks up into droplets

the same size as raindrops

from around 0.5 to four millimeters in diameter.

And standing there,

you can experience what it would be like

to fall with raindrops.

(soft music)

They have a low terminal velocity

of just 25 kilometers per hour

and that's what the wind speed was set to

for this demonstration.

And what you can see

is that raindrops aren't shaped like cartoon raindrops.

They are closer to spherical,

but a bit flatter on the bottom

where they encounter oncoming air.

If a raindrop gets too big,

it flattens out, caves in in the middle

and briefly resembles a little parachute

before breaking up into smaller droplets.

So raindrops aren't damaging,

but it's a different story for hail.

(hail hitting the ground)

- [Man] I just completely lost my windshield right here.

- [Derek] Every year in the US,

hail injures around 20 people

and since 2000 it's caused four fatalities.

That's because hail can reach terminal velocities

of over 200 kilometers per hour.

That's around 10 times the terminal velocity of rain.

But why is its terminal velocity so much higher

even though ice itself is slightly less dense

than liquid water?

The main thing is that hail can get much bigger

than a raindrop.

Hailstones have been measured up to 20 centimeters

in diameter.

Now 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 faster its terminal velocity.

It also has more mass,

so it carries even more kinetic energy

and packs a bigger punch when it hits something.

Pennies reach terminal velocity after falling

only around 15 meters.

You can see in this shot the average speed of the pennies

in the top of the frame

is the same as at the bottom of the frame.

They aren't speeding up.

They've reached terminal velocity.

So it wouldn't matter if pennies were dropped on you

from 15 meters or 300 meters or 3000 meters,

it would feel the same

because they would be going the same speed.

In fact, we didn't take the helicopter

all the way up to Empire State Building height

because that wouldn't have increased

the speed of the pennies at all

and it would've just made aiming much harder.

- By the end, I'm throwing to account

for this secondary air current

that's moving between you and the helicopter.

And so the pennies are making this like 12 foot arc

all the way over and then coming back.

- [Derek] One of the reasons pennies are so hard to aim

is because they flutter and tumble as they fall.

This tumbling behavior means pennies don't actually have

a single terminal velocity.

- A penny actually has two terminal velocities

and it oscillates between them.

So it's got one on its face and one on its edge.

I've got a wind tunnel that can show you

exactly how that works.

It's really beautiful.

- [Derek] I gotta see that.

- Yeah, it's really neat.

- Throw to the... - Yeah.

- [Derek] Adam built a custom wind tunnel

to witness this for himself.

So I went to San Francisco to his cave to check it out.

- This is literally like my MythBusters origin story,

this device.

I'm so delighted to fire this up again.

- It's like looking at a piece of piece of history.

How old is this?

- 19 years old now.

- It's old enough to drive and vote, but not drink.

- There's people watching this video,

you know? - Yeah.

- Who weren't alive when you were making this.

- [Derek] Because of the holes which allow air to escape,

this wind tunnel has a gradient of wind speeds

from around a hundred kilometers per hour at the bottom

up to 25 kilometers per hour at the top.

- This creates a little bit of back pressure

that popsicle, the tongue depressors up here,

and that back pressure is relieved by these holes enough

so that the penny spins.

- [Derek] If a penny really has

two different terminal velocities,

it should oscillate up and down in this wind tunnel

as a result.

- There you go.

(sound of air rushing by)

- That's amazing to see it oscillate, right?

- I know.

The fact that it goes up and down

and then comes back up again.

- Yeah. - Oh yeah.

- It was just hanging out like that.

- When in 2003 I dropped the penny on the top

and it went up and down, like I'm still,

every single time I tell that story, I get goosebumps

because I remember that feeling of like, Oh wow.

- [Derek] We made a separate video on Adam's channel

that discusses the wind tunnel in more detail.

So check it out after this.

So the reason pennies aren't dangerous

is because their terminal velocity is at most

about 80 kilometers per hour.

Yeah, it's not gonna hurt you.

- That's busted. - Right.

(both laughing)

- Think I'm allowed to say like.

- Like old habit.

(both laughing)

But something more aerodynamic

would have a higher terminal velocity.

And this has led some to suggest ballpoint pens

falling from a skyscraper like the Empire State Building

could be lethal.

- But supposedly.

- Yeah, a pen, a ballpoint pen.

- Is as dangerous as a penny is mythically dangerous?

- Yeah, yeah, like that's.

Is meant to actually be lethal.

- It's worth really trying.

- [Derek] These pens weigh about twice as much as a penny

and they have a smaller cross-sectional area.

- All right, I'm gonna start the first drop.

- [Derek] So this will increase the ratio of weight to drag,

but will it be enough?

Now, because I'm not sure what will happen,

I'm not putting my body on the line for this one.

Instead we'll use a ballistics gel dummy.

- Here we go.

In 3, 2, 1.

Oh, that's very close.

Back up again (murmuring)

3, 2, 1.

Oh oh, very close, very close.

Here we go, 3, 2, 1.

Oh, oh, almost.

Okay, 3, 2, 1.

Oh, almost.

3, 2, 1.

Okay, last one.

3, 2, 1.

Ah, over.

- That might be it

- Yeah, we're outta pens

(helicopter revving)

The second to last drop,

we had almost no crosswinds at all.

So they dropped perfectly down

and kind of hit right below where we were

and they were all cattywampus.

If the myth was true, I would expect to see this everywhere.

- [Derek] Right. - [Adam] Right?

That's what I would expect to see.

And I don't see a single one

and I didn't after we dropped 'em all.

- Are you gonna call it I?

- (laughs) I will, sure.

I'll come back outta retirement to say busted.

Pens are not dangerous falling from tall objects.

Ball point pens with their caps off.

- [Derek] Narrow metal pens might still be dangerous,

but these plastic ones still seem to have too much drag

relative to their weight for them to achieve

a high terminal velocity.

One of the curious things about air resistance

is that it depends not only on the cross sectional area

of the object, but also on its overall shape.

This dependence is captured in a dimensionless number

known as the Drag Coefficient.

The Drag Coefficient is all about

how smoothly air can flow around an object

and without creating vortices.

The word bullet comes from the French boule meaning ball.

So a boulette is a small ball,

exactly what the earliest bullets were.

But the drag Coefficient of a sphere is 0.5

so people modified the shape to reduce drag

and eventually, they settled on a modern bullet shape,

which has a drag coefficient between 0.1 and 0.3.

So drag coefficient is the reason a bullet

is no longer a boulette.

So what would happen if you dropped a bullet

from a skyscraper?

Not what you'd expect.

Instead of falling pointy side down,

a bullet would tumble and likely end up falling on its side.

- The thing is that cylinders tend to fall on their sides

if given enough chance. - Really?

- But the object falls

in relation to the highest resistance.

It ends up finding the highest resistance

as the most stable.

- Why doesn't it fall in lowest resistance?

- I know. - That seems intuitive.

- Bullets, if you let them,

they'll fall and make bullet shape holes--

- On their side.

A bullet fired straight up slows down as its kinetic energy

is turned into gravitational energy.

And at its highest point,

which could be up to three kilometers high,

it stops and then falls back down.

At that moment, it's just like dropping a bullet

from a really tall building.

As it starts to fall,

it will tumble and so it experiences far more air resistance

than on the way up.

And so it's not gonna get back a lot of that energy

that went into its height,

which means that by the time it reaches the ground,

it will be much slower than it was shot.

Now if the bullet isn't fired completely vertical,

then it poses much greater danger.

At the peak of its trajectory,

only the vertical component of velocity is zero.

It still maintains its horizontal velocity,

and that combined with the spin imparted to the bullet

by the grooves inside the gun barrel,

keep it moving pointy and forwards.

And so as the bullet comes back to the ground,

it speeds up to a significant fraction of its launch speed.

There are hundreds of cases of people being struck

and killed by celebratory gunfire from all over the world.

Now this is accidental,

but the concept of dropping deadly projectiles on enemies

is almost as old as aircraft.

In World War I,

these little pieces of metal were dropped out of planes

and they look like nails with little feathers on the back

to make sure they fall straight.

They're called fléchettes,

which is French for little arrow,

but some were up to 15 centimeters long.

- That is great.

I totally wanna make a thing that shoots these.

(Derek laughing)

- And how big were these things?

- About the size of a dart,

a little heavier than a standard pub dart.

And there were like endless different shapes.

- [Derek] From a military perspective,

the advantages were they didn't require any explosives

and they were cheap to produce and deploy at scale.

They could pierce helmets leading to enemy casualties

and some nasty injuries.

- They found darts that had gone through a rider

and his horse.

- That's insane.

- But I also just love the idea of a guy in an open cockpit,

cloth and wood plane just hurling handfuls of darts out.

That is like a 10 year old's idea of warfare, right?

Then I'm gonna hit him with darts.

- Later, the US created similar weapons called Lazy Dogs,

which were a bit heftier used in the Korean

and Vietnam wars.

The damage they inflicted was indiscriminate

and unpredictable,

but at least they didn't leave unexploded ordinances

in the field.

And militaries continue to use

kinetic projectiles to this day,

For example, to make precision strikes on terrorist leaders.

Falling objects are also dangerous in civilian life.

Nearly 700 Americans die each year

by being struck by a falling object.

These range from loose tiles and bricks

to falling construction tools,

falling rocks and tree branches,

and even icicles.

Death by icicle is rare

but they were a serious enough concern

that in the winter of 2014,

streets around New York's One World Trade Center

were closed due to the danger caused

by icicles hanging on the building.

So which projectiles are lethal and which aren't?

Honestly, a lot of them are.

The lower limit of the energy required

to fracture a human skull is around 68 Joules.

So 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.

But a baseball and the largest hailstone measured

deliver more than 80 Joules.

That is plenty to crack your skull.

In 2014, a man was killed

when he was hit by a falling measuring tape

that had fallen 50 stories.

And this is just calculating for blunt force trauma.

The energy stored in a falling Flechette

is not enough to crack your skull,

but it can apply a large force to a very small area.

So yeah,

a penny falling from the Empire State Building

won't kill you.

A pen likely won't either.

Anything that weighs a few grams and isn't aerodynamic

isn't going to be fatal.

But objects that weigh more than a few hundred grams

traveling at terminal velocity are likely to be deadly.

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