06x04 - Kayak, Bridge Swing and Trampoline

[bunniefuu]

[Dallas off-screen] This
is the Science of Stupid.

Yes, this is the show that dissects
stupidity to find science.

We fling open the cage
of misguided guinea pigs.

Risking life and limb in the
name of scientific research.

[man] [Bleep].

[Dallas off-screen] We'll
reveal what went wrong.

And why.

With the help of principles
like buoyancy, trajectories,

and impact.

Science is not a game.

Mess with its rules and
it'll mess up your day.

Look out, it's the
Science of Stupid.

In this show we'll look at...

The co-efficient of friction.

The pitfalls of building
on cast terrain.

And how to experience
weightlessness, and how not to.

But first this.

Now, call me old fashioned, but I've
always seen bridges

as a method of
getting safely from

one side of something
to the other.

But there are some people who like to
use them for, well,

something quite
different.

[Dallas off-screen] This
is a bridge swinger.

Someone who loves nothing more than
tying a rope

to a bridge and swinging
off it.

Sheer madness if you ask me.

But it turns out
it's quite popular.

[man] Get set, go.

[screams]

[Dallas off-screen] Here's 135 bridge
swingers hanging about in Russia.

I bet there's a real
problem with tangling.

But it could be worse.

That could happen.

Bridge swinging can be extremely
dangerous, and is widely ill*gal.

But, if you're at ease with broken ribs
and a hefty fine then listen up,

because it all starts with
gravitational potential energy.

[Dallas off-screen]
The higher he starts,

the more gravitational
potential energy he has.

As he falls, this is
converted into kinetic energy.

The rope applies a centripetal
force swinging him in an arc,

allowing his kinetic energy

to be turned back into
gravitational potential energy.

As he swings, he'll be gently slowed by
air resistance, hopefully.

It's that centripetal force that
makes a swing a swing,

helping to convert
kinetic energy back

into gravitational
potential energy.

Otherwise it's just falling.

[speaking foreign language].

[man] Ow!

[Dallas off-screen]
Like that.

By launching halfway up a vertical rope,
he couldn't convert much gravitational

potential energy into kinetic
energy until he let go.

Is it any safer to start
higher than the bridge?

No, it's worse.

Air resistance will slow him down, but a
bridge will do it even quicker.

[screams].

This chap
is an abseiler.

And now a bridge swinger.

[woman off-screen]
[Bleep], Charlie.

[Dallas off-screen] A true pendulum
motion we see gravitational potential

energy converted into
kinetic, and then back.

Although I'm not sure that's
what he was trying to do.

[woman off-screen] Charlie.

[Dallas off-screen] No, bridge swings
are just too dangerous, even if you

master the science there's just so much
other stuff you have to worry about.

See what I mean?

I do see kayaking as a relaxing
way to spend an afternoon.

But I always find getting
in a bit, well, wobbly.

[woman off-screen] Careful.

[Dallas off-screen] He
finds it wobbly too.

But, luckily, thanks to
science, there is a solution.

[Dallas off-screen]
The slide launch.

Find a handy slope like this drainage
ditch

and speed your way to the water's
edge at more

than 40 miles an hour.

And if you find that a bit scary, you
can always try something gentler.

[screaming]

A textbook launch.

Slide launchers may be exhilarating, but
kayaks are designed for water.

They don't cope so well with land, and
that's largely due to friction,

directional
preference, and drag.

[Dallas off-screen] In water
a kayak's long narrow hull

helps it resist changes and
direction as it glides forwards.

But on land this shape lacks
that directional preference,

which means it can slide
in any direction.

A smooth bottom also means a low
co-efficient of friction,

so the kayak can build momentum quickly
on a slope.

Hit the water at an angle fast

and a hydro dynamic drag can cause the
center of mass to

topple over the base.

Result, wet socks.

So, kayaks are slidey, they're prone to
careering sideways

and even when you do
hit the water,

they're likely to flip.

So, not much to go wrong then.

Well, as long as you start
off with a nice easy slope.

[woman] Oh man.

[woman] This is so dumb.

[man off-screen] I would
lean back as you go over.

[Dallas off-screen] Good idea.

[laughter]

Well, she leant back, keeping her center
of mass low and stable,

but the kayak turned to one side

so momentum and drag turned her to one
side too.

[woman] Oh, I'm soaked.

[Dallas off-screen] Alright,
how about a slidier surface?

Snow has a very low coefficient of
friction,

because it's covered with a
thin layer of

lubricating water.

[screams]

So, lots of slidey
for this g*ng,

until they all come
back with hypothermia.

[girl] They should
have taken the paddle.

[woman off-screen] Oh my.

[Dallas off-screen] Yeah,
they're not coming back.

Icy surfaces are similar.

[man] Ooh.

[Dallas off-screen] Only
bumpier, and more painful.

May have undershot
the water there.

[man off-screen] I'll go
in front of you, Denis.

[Dallas off-screen] These
experimenters are in a canoe.

[man off-screen] She's going.

[Dallas off-screen] The
same low friction bottom,

the same lack
of directional preference.

[man screams]

In layman's terms, it's
very hard to control.

[man off-screen] [Bleep].

Watch out, watch out,
watch out, watch out.

[girl] Oh.

[man off-screen] I got her, I got her, I
got her, I got her, I got her.

[Dallas off-screen] Don't
worry, she was fine.

But, if you do manage to nail the
launch,

you can relax and be at one with
nature.

Or get eaten by it.

[woman] Happy birthday
from Atlanta, Zachary Michael.

[Dallas off-screen] Ah, that's sweet,
but can you guess what

scientific principle this birthday
well-wisher is about to demonstrate?

[woman] I'm enjoying your
birthday without you.

[woman] The best New York
City birthday ever.

[Dallas off-screen] Did you guess what
science our pool-based

well-wisher is about to show us?

[woman] This time last year...

Oh God, oh God.

Oh no, oh no.

Hold on.

[woman] Not the phone!

[Dallas off-screen] It is, of
course, center of buoyancy.

[woman] Enjoying your
birthday without you.

[Dallas off-screen] This
is the center of mass

for the submerged
part of the inflatable.

[woman] happy birthday and I hope you
have the best birthday ever.

[Dallas off-screen] As long as she can
keep her center of mass directly over

the center of buoyancy
she'll be balanced.

[woman] This time
last year you were...

[Dallas off-screen] But
she leans too far back.

[woman] God, oh God, oh no.

[Dallas off-screen] And she
gets a birthday surprise too.

[woman] Not the phone.

God darn it.

I just done my hair and makeup.

[Dallas] I'm afraid to say some rather
distressing news

has come to my
attention.

It's regarding trampolines.

[Dallas off-screen] Tired of years of
abuse by scientifically inept families.

-[man] Oh.
-[boy] Oh.

[Dallas off-screen] To say
nothing of their pets.

[woman off-screen] Oh God.

[bleats]

[Dallas off-screen] These bouncy
funsters are making a break for it.

Sorry kid, it's gone
to a better place.

Number 23.

So, why not give the big guys a break
and learn with a trampoline

better suited to the untrained amateur.

[Dallas off-screen] Like
the mini trampoline.

You can still pull
impressive tricks.

Nice, but they're safe
for people of all ages.

Except his.

[woman off-screen] Oh.

[Dallas off-screen] Anyone
going to pick him up?

Look, mini trampolines
really are safer.

You've just got to learn how to harness
elastic potential energy.

[Dallas off-screen] First, he
builds horizontal velocity.

As he hits the center, the trampoline
stretches converting kinetic energy

into elastic potential energy.

Which is converted back into kinetic
energy, as it springs back into shape,

giving him just enough vertical velocity
for sufficient airtime to flip out.

A mini trampoline's size means it can't
store and release as much energy

as a full-sized trampoline.

But it's still plenty enough to flip
your way to mini gymnastic stardom.

[Dallas off-screen] You've just got to
put some velocity into it.

[man] Ooh.

[Dallas off-screen] But a
little more than that.

A run up can boost your horizontal
velocity, sometimes too much.

Her foot had a nice
soft landing though.

Build enough horizontal velocity, to
convert into vertical velocity

and you can accomplish the impossible
like Kerim Daghistani,

the first person
to forward flip

trampoline slam dunk from over 26 feet
and a half feet.

[Dallas off-screen] And started a bit
like this, but didn't end like this.

For maximum elastic potential energy,
the sweet spot's in the center,

not the edge.

That's just metal.

These physicists are experimenting with
a little trampoline

and a big
trampoline,

but he's not the main event.

[boys] Turps, Turps,
Turps, Turps, Turps...

[Dallas off-screen] Turps is.

[boys] Turps, Turps,
Turps, Turps, Turps...

yes.

[Dallas off-screen]
Yeah, nice one, Turps.

That one's not so nice.

Turps converted just enough of
his horizontal velocity

into vertical velocity for a nice

flip but kinda cheated
by using his hands.

The other chap wishes
he'd used his hands.

It's nice to have
friends, isn't it?

[laughter]

Quiet at the back,
class, please.

Thank you very much.

It's time for your science
lesson where we poke our noses

into a particular
scientific principle.

Can you guess what today's
lesson is from the following?

[Dallas off-screen]
A flying lady.

A bouncing boy.

And a man in a
helmet on an air bag.

And now on the floor.

Don't try this yourself.

[man] You alright?

[man] No.

[Dallas] Yeah, today's science
lesson is on weightlessness

and it might not be
what you think it is.

Yes, astronauts
experience it in orbit,

but you too can feel it down
here on earth just by falling.

[Dallas off-screen] Gravity pulls her
towards the earth's surface,

but the balloon

counteracts this by exerting
a contact force on her.

She experiences this
force as her weight.

Without that contact force she
feels weightless,

and the only force controlling
her motion is gravity.

So, it's being in contact with something
that gives you the feeling of weight,

but there is an exception.

Drag acts like a contact force,
and increases with speed.

Confused?

Well, this might help.

[Dallas off-screen] A
skydiver at terminal velocity,

about 120 miles an hour, feels
their full weight because

at that speed air resistance is
pushing back on them

as much
as a solid object would.

Okay, question one.

What is it that gives you
the feeling of weight?

[Dallas off-screen] The contact force
from his seat

means this sleepy head
feels weight.

[laughter]

Then he didn't.

Contact force, weight.

No contact force, weightless.

Contact force, sore head.

[man] Did my *******
head hit the ceiling?

[Dallas off-screen] No,
it was just a dream.

Question two,

when you're weightless there are no
contact forces but there is a

non-contact force that
controls your trajectory.

Can you remember what that is?

[man off-screen] Oh my God.

[Dallas off-screen]
It's gravity.

[man off-screen] Oh my God.

[Dallas off-screen] With no contact
forces affecting it, this car's

trajectory is subject only to
gravity which pulls it down.

-[man] Is he alive?
-[man 2] Yeah.

[Dallas off-screen] Yep, he might be,
but I can't say the same for his car.

And now for my third
and final question.

If there are no solid objects exerting
forces on you,

does it always mean
you're weightless?

[Dallas off-screen] No, the large amount
of drag

produced by this skydiver's
canopy

provides a contact force through his
harness, giving him a sense of weight.

Oh, here comes
another contact force.

If that's his car, then
that's an impressive landing.

[Dallas] Do you ever get that feeling
you wish the ground

would just swallow
you up?

Well, thanks to geology, it can.

[Dallas off-screen] That's a
sinkhole, but it's not so bad.

If everyone just
stays well back.

[screams]

Right dog, you're
paying for that.

Sinkholes can swallow up
far more than your car.

In 2010, a 300-foot-deep hole in
Guatemala City

devoured a three-story
factory.

The culprit, a particular kind of
terrain which makes up

a tenth of the world's
land surface.

[Dallas off-screen] Cast terrain is land
where underlying layers of soluble

bedrock can be dissolved away by
subterranean water.

This can leave a precarious cap of
undissolved soil

or rock suspended above
the void.

In urban areas, a thin layer
of rigid man-made material

like concrete can be left unsupported

until the force of its weight,
or the weight of something else,

means its tensile
strength is exceeded.

The deepest known sinkhole is the
Xiaozhai Tiankeng in China, which is

over 2,000 feet to the bottom.

Deep enough to fit one and a
half Empire State buildings.

[Dallas off-screen] Whereas
this sinkhole in Florida.

[man off-screen] She's going.

[Dallas off-screen] Is just enough to
fit the front room in.

A terrifying 20% of land
in the US is cast terrain.

[woman off-screen] Oh my God.

[Dallas] In urban areas, sinkholes can
be triggered by buildings

and roads focusing

rainwater onto particular patches of
ground, where it seeps in,

erodes the land beneath, and opens up a
whole world of trouble.

[Dallas off-screen] A
stroll in Shaanxi, China.

[screams]

A hole in Shaanxi, China.

This lady survives a fall 19 feet

through a layer of low tensile strength
pavement.

And then deep into a
water eroded chasm.

Roads are also made of low tensile
strength material,

like asphalt or
concrete.

So, a little force and
they can easily give way.

A bit like that car.

So, that's sinkholes for you.

Always turning up when
you least expect them.

So inconsiderate.

I've been working all day on my closed
Delilah and I can't wait to show you.

The man in the moon's not quite ready
yet, but that's aerial hoops for you.

[Dallas off-screen]
So many tricky poses.

All demanding core
strength, athleticism,

and grace.

Well, at least your
face hit the mat.

Shame about the knee.

Yeah, learning those aerial hoop poses
can lead

to more than just a nasty crick
in the neck.

So, why not allow science to fast track
you painlessly

to the hooping hall of
fame.

[Dallas off-screen] Maintaining
body tension helps her

control the position
of her center of mass.

The closer that center of mass
is to the line of the rope,

the more gravity works to pull her

onto the hoop and the
more stable she'll be.

With enough tension in her hands, and
her center of mass perfectly aligned,

she can hang from
just four fingers.

Get all that right and you can be like
this ruler and hammer party trick.

As long as the center of mass here near
the hammer's head

is directly under the
point of

support here, it'll
always balance.

I go to some wild
parties, I can tell you.

Okay, how about an example in this form
of a classic pose, the heel hang?

[Dallas off-screen] I've
got those exact pants.

[woman] Ooh.

[Dallas off-screen]
Never done that in them.

Well positioned center of mass, hammer
like balance,

but when the tension in
her ankles goes,

more of a dropped hammer.

Yeah, you've got to
earn those pants.

With high body tension, and center of
mass aligned,

she'll be able to balance
off the

smallest of body parts.

Except her hip.

She can't do that.

Ah, now, this looks
like the French gazelle.

[woman] Ow.

[Dallas off-screen] Yeah,
I don't know that one.

The French gazelle should bring the
center of mass

closer to the hoop for
balance,

and there should be more
tension and less impact force.

I'm not sure you should
be doing that in church.

Just make it quick.

Thanks.

My guess is she was hoping to swing up,
grab the hoop and rotate through,

but the lack of tension in her
hands couldn't handle the load.

We'll ask her about it
when she comes around.

Well, that's all the scientifically
induced agony we have time for.

So, please do not attempt to recreate
any of the dangerous stunts you've seen.

Just leave all that to this lot.

[man off-screen] Heads up,
watch out, watch out, watch out!

[man off-screen] Oh my god!