07x17 - Flying, Flipping and Driving

[bunniefuu]

[Dallas]
This is the Science of Stupid.

[klaxon blaring]

Yes, this is the show
where clear eyed science

meets pie-eyed stupidity,
with predictable results.

You'll see the amateur analysts

attempting unwise experiments
for unexplainable reasons.

[man, off-screen]
Whoa.

[Dallas] Then we'll describe
what went wrong and why,

by examining
the key scientific principles,

such as inelastic collisions,

muscle malfunction,

-and angular momentum.
-[thuds]

Fight with physics,

and you will fail.

[lively fiddle music]

Watch out, it's the Science of Stupid.

[electricity crackling]

In this episode, we'll be looking at...

the hidden perils of the paramotor,

the subtle pleasures of the stair flip...

[thuds]

...and the trouble with tensile strength.

[snaps]

But first, this.

[electricity crackling]

[shattering]

It's easy to understand
why mountain biking is so popular.

You get to enjoy the great outdoors,

it's an exercise that feels fun,

and you can do it almost anywhere.

But it's hard to beat the exhilaration

of bombing between the trees
along a forest trail.

[crashes]

As long as you look out for logs.

One solution to the
"log prob" is to hop over them,

which is easier said than done,

and of course,
best left to mountain bike experts,

particularly those with
an ear for the science.

Before he reaches the log,

the mountain biker pushes down
onto his pedals and handlebars

to pre-load the suspension.

This compresses it, so the force
from him pushing downwards

can produce an equal
and opposite reaction force

from the ground, propelling him upwards.

The suspension rebounding helps
to give additional upwards momentum,

ensuring he has enough time in the air
to adjust his body position

so that his center of mass

is towards the back
of his base of support for landing.

Another thing to watch out for
is soft ground,

as your wheel can dig in and
increase rolling resistance,

which in turn increases your chance
of going over the handlebars.

[rock music]

[boy, off-screen]
Full pelt.

[crashing]

[Dallas] Although a soft surface
does have some advantages.

[laughing]

This teenage biker's center of mass
is too far forward,

so when his front tire lands,

the rolling resistance
from the soft ground

rotates the bike around the wheel,

and he discovers why his mum
always tells him to wear a helmet.

[laughing]

[man, off-screen] I saw that
down there and you flying.

[Dallas] Let's see if this
fully grown man does any better.

-[thuds]
-[man grunts]

It seems age is no barrier
to scientific ignorance.

[man 1, off-screen]
It's like there was a rock?

[man 2]
No rock.

[Dallas]
He lands his jump off the first log

with his center of mass too far forwards,

and with all that weight
over his front wheel,

it's stopped dead
by that sneaky second log...

[thuds]

But he isn't.

Okay, let's see if these guys have got it.

Preloading?

Check.

Center of mass towards
back of base of support?

Good.

Controlled landing?

-[grunts]
-[crashing]

Yeah. There's always one, isn't there?

[man speaking in native language]

The third guy uses preloading

to gain upwards momentum,
but on landing,

his feet fall off the peddles,
he loses control...

-[grunts]
-[crashing]

...and gains a healthy respect
for physics.

[electricity crackling]

[metal clanging]

In 1980, Englishman Mike Byrne

strapped a large fan to a metal frame
and put it on his back.

It might sound like
he was having a breakdown,

but he'd actually invented
a new sport: paramotoring.

[rock music]

It's basically paragliding,

but with a very big advantage
of being able to control your thrust.

That's a two stroke motor,
just like you'd find in a lawn mower,

but while they aren't terribly powerful,

they give a tremendous feeling of freedom,

unless you get caught
on a literal symbol of freedom...

Like that one.

Yep, paramotors
aren't totally stress free.

Since you take off
on foot in most countries,

you don't even need a license to fly one,

but is that really a good thing?

I mean, whilst these are
fairly straight forward machines,

motor propeller, inflatable wing,
and a few strings,

they actually have to deal with
some complicated physics.

As our man runs to take off,

air is forced into vents,

inflating the crescent shaped wing,

forming it into an air foil
that generates lift.

The propeller attached to him
pushes the air backwards,

The air provides an equal
and opposite force,

and pushes the paramotor and our man

forwards at speeds of around


Now all he needs to do is watch out
for obstacles on the ground,

as paramotors can have difficulty
gaining height quickly.

[thuds]

That all sounds
reasonably straightforward on paper,

but let's see what it's like
trying them in the real world.

It seems sensible
to start at the beginning.

And for the paramotor,
that means take-off.

Now remember,
run until the vents fill your airfoil,

and apply power to the engine
to help generate lift.

Yeah, a bit more than that.
You only just made it over that hedge.

[crashing]

And he didn't quite make it over that one.

The issue here is that
he didn't climb fast enough,

and when you clip a bush,
or anything really,

that slows you dramatically.

[crashing]

Which is exactly what you don't want
when you're trying to take-off.

It's not a good start.

Let's see if this guy can do any better,

and let's face it,
it would be hard to do any worse.

Right, he's airborne. That's good.

But he's heading towards that tree...

And that is bad.

Remember, you need to keep your distance
from obstacles on the ground,

as you might not be able
to gain height quickly.

Okay, this guy has achieved
a good altitude,

so he's already doing better
than everyone else so far,

and when you're up this high,
you don't need to worry about obstacles,

so you can sit back, relax,
enjoy the view,

and listen to the gentle hum
of the engine.

[engine rumbling]

That hum doesn't sound very gentle.

The engine's cut out.

While it's running,
its extra velocity provides lift,

and more than makes up for its weight.

But stopped, it's just dead weight.

I should probably mention that this is
generally considered to be a safe hobby,

but that might not be so comforting

when you're hurtling towards
a crowded playing field.

And that was probably the most
valuable slide in baseball.

Now, I know what you're thinking,

because we're all thinking the same thing.

"This seems too dangerous for me,

"but I've already bought my paramotor.

Can I use it for anything
other than flying?"

Well, the good news is yes.

The bad news: I'm not convinced
that this is any safer.

[electricity crackling]

[metal clanking]

[crumbling]

[lively fiddle music]

This rather eager-looking chap
is about to demonstrate

a scientific principle,
but can you guess which one?

[shattering]

[electricity crackling]

[Dallas] We asked you what science
this vacationer was about to show us.

[man]
Whoa.

Ohhhh!

[splashing]

Yes, it was parabolic trajectory.

[screaming]

And a fair amount of impact force.

[splashing]

The dingy pulling away from
the yacht gives him both vertical

and horizontal velocity,
and when he lets go,

he follows a parabolic trajectory

that almost intersects with that dingy,

turning paradise into pain.

[splashing]

[electricity crackling]

[exploding]

We parkour experts, or traceur,

like to use the urban environment

as a platform for our incredible acrobatic

and hair raising stunts.

It's pretty heroic stuff,
but as we'll see,

that doesn't mean it's a good idea.

Apparently it's almost impossible

for traceurs to use stairs normally.

Instead of walking,
they usually flip up them,

which is really tricky.

And flip down them, which is easier...

[laughing]

But a lovely opportunity
for a quick cuddle.

[laughing]

As we've just seen, a controlled landing

is what you're hoping for
when flipping down stairs,

but you'll need science to work out
how to achieve one.

He strides forwards to create
enough horizontal momentum

to clear the stairs.

As he pushes off, he must also generate
angular momentum for the flip.

Tucking in reduces his moment of inertia,

an object's resistance
to rotational acceleration,

so he can flip fast enough to get
all the way around before landing.

So, you need the right amount
of horizontal and angular momentum,

and the correct moment of inertia
in order to land safely.

Get any of it wrong,

and the only place you'll be
landing yourself is in hospital,

so this one is best left
to the professionals.

Ooh, close.

[man, off-screen] All right, I got that.
Try one more time.

[Dallas]
Have another go.

And try and stay on your feet.

-[man yelling]
-[thuds]

For which you have to land on them.

This punk's run up generates
enough horizontal momentum,

and his jump enough angular momentum,

but he forgets to tuck in
to reduce his moment of inertia.

And that's a science no-no.

Will this kid remember to tuck in?

-[thuds]
-[observers exclaiming]

Well, yeah, but not nearly enough.

[man]
Aah!

[Dallas] Not enough tuck
means a large moment of inertia,

which leads to...

[man]
Oh!

[Dallas]
...a whole lot of pain.

This landing is a tricky business.

[man, off-screen]
Oh, he's going to do it now.

[Dallas] So, within seconds,
this guy will either be

the main attraction, or in traction.

-[man yells]
-[thuds]

I call that a win,
even if you didn't stay on your feet.

He produces too much angular
momentum on take-off,

and so his muscles aren't able
to absorb it on landing.

He was lucky, but you might not be,

so don't try this yourself.

[clattering]

[bubbling and sizzling]

All right, it's time to
pocket those smart phones,

put on your thinking caps and
get ready for today's science lesson,

the part of the show where we analyze
one particular scientific principle.

So, what do the following have in common?

Tree swinging...

[laughter]

Slacklining...

-[thuds]
-[man] Ooh.

[Dallas]
...and rowing. Very good for your core.

[snaps]

Bad for your neck.

If you said "ultimate tensile strength,"

then pick up 100 points and go straight
to the top of the class.

Ultimate tensile strength is a
measure of the force required

to pull a material to its breaking point.

Any material's ultimate tensile strength

is only as strong as its weakest point.

Here's the low down.

When a weight is applied
to this thin rope,

tensile stress causes it to stretch

until its ultimate tensile strength
is reached.

When the same weight is
applied to this thicker rope,

the larger cross-sectional area
dilutes the force on each section,

so its ultimate tensile strength
is not reached.

When the weight is dropped,
the same rope must counteract momentum,

which adds extra downward force,

so it's subjected to
additional tensile stress,

and the ultimate tensile strength
is exceeded.

A little side note,
a more elastic material than rope

would have an advantage
when the weight is dropped,

because it's able to stretch more,

allowing the downwards momentum
to be lost over a longer time,

thereby reducing the tensile stress
it experiences.

Okay, right, now time for a little quiz.

First question:
What is ultimate tensile strength?

Can these family tuggers
provide the answer?

[woman, in Spanish]
Uno, dos, tres.

-[snaps]
-[screaming]

-[laughing]
-[Dallas] They certainly can.

When they pull on the rope,

the two families apply
a lot of tensile stress,

which causes the rope to break
at its weakest point.

-[snaps]
-[screaming]

So ultimate tensile strength

is a measure of the force needed
to pull apart a material.

At least there won't be any arguments
over which team won.

Okay, question number two:

How does momentum affect
the tensile stress

a material is subjected to?

[man, off-screen]
All right, do it.

[Dallas]
You wouldn't catch me doing this.

[laughter]

And there's no one
to catch him doing that.

When he swings, his momentum
adds extra downwards force to the rope

than the force of his weight alone,

which increases the tensile stress.

-[rope snaps]
-[thuds]

Maybe he'll try something
more age appropriate next,

like basket weaving?

Question number three:

How does elasticity

affect the tensile stress
experienced by a material?

This bungee run could provide the answer.

[snaps]

And a couple of bruises
to remind you of it the next day.

When they run,
they apply force to the cord,

but as its elastic, it stretches slowly,

reducing their momentum,

and thereby reducing the tensile stress
experienced by the cord.

Just a shame it wasn't attached properly.

Class dismissed.

[boy]
Oh!

[glass shattering]

[electricity crackling]

[shattering]

[Dallas] I've sprained my neck,
as I've spent all morning

trying to get the perfect
social media post.

You know the one.

-Ooh.
-[camera shutter clicks]

The hair flip. It's best done on a beach.

But anywhere will do, really.

-[crashes]
-[woman] Ah! [laughs]

[Dallas]
As long as there's space.

[woman]
Ow!

[Dallas]
To perform a successful hair flip,

you need three things.

Hair. Check.

Rhythm, and an understanding
of the science.

Check.

As she swings her head,

centripetal force keeps her hair
going in a circle,

following her movements.

Due to its inertia, her hair
experiences a centrifugal force,

which causes it to straighten out
as it moves.

The faster she moves her head,

the greater the centrifugal force,

enabling her hair to defy gravity.

So, it's all about centripetal force
and centrifugal force,

but of course it's also about
avoiding impact force.

I'm getting a headache
just thinking about it.

The world's longest hair
is over 18 feet long,

about the height of a
particularly lofty giraffe.

This guy's isn't that long,
but it does look inconvenient.

-[man screaming]
-[clattering]

Yup, this fashionista has very long hair,

and the greater the weight of the hair,

the more centrifugal force is needed
to counteract gravity,

and when you're
shaking your head that much...

[thuds]

...it's hard to keep an eye on
your ridiculous shoes.

[upbeat music]

Here we go.
This looks like my type of party.

And it seems like this dancer
is working up to a hair flip.

It's a small price "toupee"
for looking that good.

As this party girl spins her head,

her hair piece spins in a circle, too,

following her movements
due to centripetal force,

but she's so enthusiastic

that the centrifugal force
becomes too much.

I'd just carry on, though.
I don't think anyone noticed.

But I could be wrong.

It's found its rightful owner.

[shattering]

Of course, the most famous
hair flippers in music

are the hard rocking head bangers,
like this guy,

who knows that practice makes perfect

when it comes to mastering centripetal
and centrifugal force.

So that's why my new house
is looking a bit...

-...wobbly.
-[crashing]

[electricity crackling]

[metal clanging]

Every once in a while,
we like to take a break from physics,

and enjoy some engineering,

and today we're looking at one
of the unsung wonders

of that world: the hull.

You may not know this,
but evidence suggests

that boats predate modern humans,

and were first built by our ancestor,
h*m* erectus.

Of course, hull technology
has moved on a bit since then,

allowing watercraft to master
all sorts of conditions...

-[water spraying]
-[man, off-screen] Whoa!

[Dallas]
...within limits.

[man, off-screen]
Whoa!

[Dallas]
Different watercraft use different hulls,

but they're all designed
to do one of two things:

either displace water,
or ride on top of it,

what sailors call "plaining."

Two examples of plaining types
are flat bottoms

and V-shaped bottoms.

Bear with me.

Here's the science of bottoms.

V-shaped hulls allow a boat
to cut cleanly through choppy waters,

giving a smooth ride.

But they produce a large amount of drag,

and therefore need powerful engines.

A hydrofoil can lift a boat's hull
out of the water,

meaning less drag and better efficiency.

Flat bottomed hulls are more stable
than V-shaped hulls,

because when they're tipped,
there is a larger righting moment.

This is the force
causing the boat to right itself.

So, that's hulls.
Very much horses for courses

when it comes to design,

but how does this translate
into the real world?

This kite surfer has a hydrofoil,

which should allow him
to do some amazing tricks,

or at least it would have done,
if he had kept hold of his board.

[splashing]

Just like an aircraft's wing,
a hydrofoil creates lift,

which then raises the hull
out of the water,

and the less of the hull
that's in the water, the less drag,

which means he can go faster.

Perfect for raising his hopes
and crashing disappointments.

[splashing]

The hulls of these kayaks
are flat bottomed,

designed to be extremely stable,
and they work very well

when gently paddling on flat water.

-[splashing]
-[people chattering]

But less well when your mates can't steer.

-[woman 1, off-screen] No, are you joking?
-[woman 2, off-screen] Oh, no.

[Dallas]
A high center of mass

and a sudden force applied to the hull

can only mean one thing.

-[splashing]
-[woman] Oh, Lisa!

[Lisa, off screen]
My sunglasses, my sunglasses.

[Dallas]
And nothing could be worse than that.

Here we have a classic V-shaped hull,

great for going fast over choppy water,

but this one's got no driver,
leaving it spinning in circles.

[crashing]

Well, that stopped that nonsense.

[glass shattering]

[electricity crackling]

[shattering]

Mark Twain once said,

"All you need in this life
is ignorance and confidence,

and then success is sure."

By that measure,
this is a show full of winners.

[lively fiddle music playing]

[clattering]

-[snaps]
-[screaming]

[woman]
Aah!

-[crashing]
-[splashing]

[man yells]

-[thuds]
-[grunts]

[thuds]

[snaps]

[thuds]

[crashing]

[thuds]