06x03 - Whips, Jumping People and Swings

[bunniefuu]

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

Yes, this is the show where we pull up
the rubber gauntlets of science,

to sift through
mounds of stupidity.

We learn our
lessons the fun way.

While others learn
theirs the hard way.

Then we reveal,
exactly what went wrong.

And why.

It might be they haven't understood
gravitational potential energy.

Possibly, they didn't consider
the transfer of kinetic energy,

or maybe they've forgotten all about the
centripetal acceleration.

But we haven't.

Don't try any of this yourself.

Watch out.

It's the Science Of Stupid.

In this show, we'll explore
the slowing effect of drag.

The hydrodynamics
of plunge pools.

And sudden deceleration.

[man] Yes Andy!

[Dallas off-screen]
But first, this.

There can be no greater pleasure in
life, for any parent,

than pushing your
child on a swing.

Unless your swing
looks like this.

The tallest swing set in the world is a
whopping 288 feet eight inches high.

This isn't even it.

And this looks more like a nightmare
than my idea of a fun time.

But if you think sitting on
a swing might be dangerous...

You should try pushing one.

Pushing a swing is one of those things

that sounds like it should be child's
play,

and yet it holds some pretty
deceptively complicated science.

Our man and his swing
form a pendulum.

Converting gravitational potential
energy into kinetic energy,

and then back again.

But a pusher, can give a swing
additional kinetic energy.

When the swing slows at the
top of its arc,

he applies a force with
his muscles,

which accelerates the swinger.

Allowing him to get
higher and higher.

As long as he can hold on.

Well that all seems fairly
straightforward,

but I still think we'd
be better starting

small, especially if
you're a beginner.

As the important thing when
you're learning any new skill.

[woman] Rebecca,
what day is today?

[Rebecca] Happy Mothers Day.

[woman] Thanks.

[Dallas off-screen]
Is not to get distracted.

Rebecca here, hasn't got the hang of
this application of force business.

Oooh.

Don't worry she's fine, and she's learnt
some valuable physics.

Now this swing isn't being pushed side
to side, but around in a circle.

Let's see if that
makes any difference.

No, not really.

[boy] I'm good!

[boy] Ow! My elbow hurts.

[Dallas off-screen] This swing still
acts as a pendulum, experiencing

centrifugal force,
pulling it outwards.

When this kid grabs hold of the
chain, his weight is also pulled

to the outside and he
experiences about 1.5 G's.

For a while anyway.

[boy] I'm good, I'm good.

[Dallas] I know what you're
thinking, because let's face it,

we're all thinking
exactly the same thing.

Do the same rules
apply on a rope swing?

Well the answer is yes.

But not always in
the ways you think.

This guy is using that rope,

to transfer kinetic energy to his friend
for the swing.

But it's that same kinetic energy

that allows the rope to snare the
pushers leg,

transferring some
energy back to him.

Two friends this time, adding kinetic
energy at either side of the swing.

This is a really efficient
way, to get sore shins.

So, there you have
it, pushing a swing.

More dangerous than you'd think.

Are you the sort of person who longs to
experience the thrill of water skiing,

but just hates standing up?

If so, I may have found
the sport for you.

[Dallas off-screen] Knee boarding,

it's a great way to feel the wind in
your hair.

And the water up your nose.

The good news is, all you
need is a few inches of water.

And something to pull you.

You can do it anywhere.

The bad news is.

You really shouldn't.

Yes, knee boarding is an
aquatic toe sport,

a bit like wakeboarding or water skiing,

but because you've got a lower center of
mass and a larger surface area,

it is an easier option.

But that doesn't mean there isn't any
tricky science, ready to trip you up.

[Dallas off-screen] A knee board has a
smooth, flat bottom, to help it skim

over the surface
of the water.

If he maintains rigid body tension,

the forces from the cable will pull the
board

and rider as one.

His main concern is a nose dig.

If the front end goes under water, a
sudden increase in drag,

can act as a brake.

This braking force combines with the
pulling force from the cable.

Resulting in a torque,
and flipping him forward.

For a thrilling knee board ride,

you want a wake that's about twelve
inches high,

and for that, you and your boat need to
be going around twenty miles an hour.

But we're getting a
little ahead of ourselves.

[Dallas off-screen] Let's just
start with getting started.

[woman] Ready to go.
Lean back.

[Dallas off-screen] She's right, you
need to lean back

and maintain body tension.

[woman] Gav, you've really got to lean
right back, really right back.

[Dallas off-screen] Well Gav, you can't
say you weren't warned.

The wet sand forms a seal with the
smooth board,

making starting from the
beach harder

than starting in the water.

[female] You've really
got to lean back back.

[Dallas off-screen]
When the jolt comes,

the pull force from the cable results in
a torque flipping him forward,

with a force equivalent to
double his own body weight.

And leaves a nasty
taste in his mouth.

This guy has got started
and is doing a good job.

Looks like he was paying
attention to the science.

Although apparently
not all of it.

Sitting towards the back of the board,
keeps the nose up,

which reduces hydrodynamic drag.

But when the nose digs into the water,
it suddenly has a lot more drag,

which causes massive
deceleration to it.

And then to him.

Once you've mastered the basics.

The low center of mass and large
surface area,

means that even
a novice can pull off tricks.

Like that.

Good body tension and remembering to
keep the nose up,

means that he can flip,

land and ruin that
nice lady's day.

[man] First time baby, yeah!

[Dallas off-screen] Yeah, and
hopefully the last time too.

What a lovely day for a slip
and slide. Here's team Nitro.

[man] Team Nitro!

[Dallas off-screen] But what
science will our slider reveal?

[Dallas off-screen] Did you guess the
science this slippery speedster

is about to
demonstrate?

[man] Team Nitro!

[Dallas off-screen] That's right, it's
the co-efficient of friction.

A smooth plastic sheet
covered in lubricating water,

has an extremely low
co-efficient of friction.

And so, he maintains his momentum and
he hits twenty miles an hour.

And then a table.

Bit early for lunch
mate, come back in five.

[Dallas] In 1947, Chuck Yeager flew at a
top speed

of 700 miles an hour and became

the first person to go through
the sound barrier.

But, he was a long way from being the
first person to break the sound barrier.

Whips date back longer
than written records.

[Dallas off-screen] And in the
right hands,

you can use them to break
the sound barrier,

time and time again.

Which I think calls
for a harmonica solo.

Oh yeah!

Of course, in the wrong hands,
they're just six feet of pain.

[boy] My ear!

[Dallas] Now, whip cracking is a
dangerous hobby

and it's not one that I
recommend.

Unless, you know your science.

[Dallas off-screen] Our cow girl flicks
the whips handle back and forth rapidly,

creating a loop that passes
energy along the whip's length.

As the whip tapers, the energy traveling
down it is conserved, but the mass

of each section decreases.

This means the velocity increases

until the end of the whip is traveling
faster than

the speed of sound.

So, the loop tightens and breaks
the sound barrier,

producing a pressure wave that we hear
as a

small-scale sonic boom.

The crack.

This was all discovered by other
animals, well before humans.

The two-inch p*stol shrimp

snaps its claws to produce a similar
cracking shock wave.

But why start small.

[Dallas off-screen] The longest whip
ever cracked was nearly 330 feet long.

[man] Holy ****, that
scares the hell out of me.

[Dallas off-screen]
This one is only 80 feet.

[man] ****!

[Dallas off-screen] But it
still has a bit of a kick.

The force of the pull overcomes the
whips friction with the ground,

and it loops back towards him.

In quite a painful way.

[man] Well that is by far...

...one of the stupidest things
I've ever done in my life.

[Dallas off-screen] These two
look a lot more professional.

They've even got a routine,

and it looks like they're building up to
a big finale.

They call that one the show
stopper, for obvious reasons.

As their whips taper, the wave added at
the handle

increases in velocity until
the thin end

is going faster than


Fast enough to snap the tip.

So that is going
to leave a mark.

[man] Let's see how
fast I get in trouble...

...cracking this
whip inside the house.

[woman] Err! What did you break?

[man] Oh ****!

[Dallas] Alright, settle
down please class.

I hope you've all got your notepads at
the ready, because it is time

for today's science lesson.

The part of the show where we focus our
microscopes on one particular,

scientific principle.

So, do you know what these
three things have got in common?

[Dallas off-screen]
This reluctant ride.

This swift stopping skater.

And this reckless rally driver.

Don't worry, they
were fine, eventually.

Okay, hands down everybody.

Yes, they're all examples
of course, of acceleration.

We're all probably thinking of the same
thing when we say acceleration.

A change in velocity over time.

But do you know about
negative acceleration?

And, what about
centripetal acceleration?

Well, prepare to be schooled.

As this remote-controlled car speeds
off, energy is supplied by the engine

and it experiences
positive acceleration.

And as it brakes, it experiences
negative acceleration.

A velocity change in
a negative direction.

Since velocity incorporates both speed
and direction, as the car turns, even at

a constant speed,
it's accelerating.

This is called
centripetal acceleration.

And, when it takes off from a jump, it's
experiencing the constant gravitational

acceleration that brings
it back down to earth.

Right, now let's see who's been paying
attention, with a little pop quiz.

Question one, how is
acceleration defined?

[Dallas off-screen] That's right, it's a
change of velocity over time.

In this case, quite a lot of
velocity, in quite a short time.

That was an example of a positive
acceleration, but question two is,

what does a negative
acceleration look like?

Like that.

This time the bike negatively
accelerates when it hits that ramp and

he negatively accelerates.

[man] Yes Andy!

[Dallas off-screen]
When he hits the bike.

[man] That was epic!

[Dallas] Question number three,

when is maintaining centripetal
acceleration most important?

[Dallas off-screen]
That's right, on a corner.

Speed remains constant, but
in a more sideways direction.

Unlike that.

So, that's acceleration.

Alright, class dismissed.

[Dallas] Of all of
nature's wonders,

maybe the most universally
admired is the mighty waterfall.

[Dallas off-screen] From those permanent
ones like, Victoria Falls in Zimbabwe

which dumps around two million cubic
feet of water each and every minute.

[man] I don't think
I have ever seen...

...anything quite
as cool as this!

[Dallas off-screen] To temporary ones
like the Grand Canyon

after a flash flood.

[man] The land of a
thousand waterfalls!

[Dallas] We all know that water likes to
take the easiest path,

and therefore the steepest gradient, but
the science is worth exploring more.

[Dallas off-screen] In a flowing river
abrasion by sediment particles,

gradually
erodes the river bed,

and the underlying rocks
can form steep drops.

Gravity accelerates falling water,

and this extra energy erodes the bottom
even more,

resulting in deep plunge pools.

Then as descending water mixes with the
water on the lower level,

it creates turbulence.

This can form a vortex near the surface
that makes water flow, back upstream.

Now, while most people think the Angel
Falls in Venezuela

is the world's highest waterfall,

that honor technically goes to the
Denmark Strait Cataract,

which is three times taller,
at more than two miles high.

But, as it's under water it's a lot
harder to put it on postcards.

A lot less suitable for
a marriage proposal.

[woman] Oh my God!

[Dallas off-screen] Yes, I'm
going to take that as a, 'No'.

The water in the plunge pool
is deep because of erosion,

and chaotic because of the water

hitting the bottom, so that ring
is going to be tricky to find.

You're probably better off using falls
for something less dangerous.

Like white water rafting.

Yep, still less dangerous
than getting married.

[woman] Nice!

[Dallas off-screen] When the nose of
this raft hits the chaotic water of the

plunge pool, turbulent vortexes push it
backwards, so it turns upside down,

giving everyone inside a closer
look at the wonders of nature.

[woman] Nice!

[Dallas off-screen] Even a small
waterfall can form a stable vortex.

Now, what was that science
about what a vortex does?

Ah yes, that's it, it makes
water flow backward upstream.

[man] Get off it.

[man] Get off the boat.

[Dallas off-screen] As well
as anything floating on it.

By now, you'll have a
handle on the physics.

Unlike this waterfall.

Look, if you're not going to play by the
rules, I'm going home.

There are a few things in life,

more annoying than being stuck behind a
group of slow walkers

that you can't get around.

But I think, I may have finally found
the solution

that we've all been looking for,

and it is ever so simple.

Jumping.

Sure, it'll probably take a bit
of practice,

but I really think that
this could be the answer.

[screams]

[man] I'm really sorry.

[Dallas off-screen] But
I have been wrong before.

As soon as there are two people
involved in any endeavor,

you've doubled the amount
of people who can mess things up.

Which is one of the
reasons that this is hard.

First, he runs to build
up horizontal velocity.

The velocity of his center of mass on
take-off

determines his trajectory and
his leap follows

a predictable curved parabola.

For a bigger jump, he needs
to maximize his impulse.

That's the force he applies to
the ground,

multiplied by the
time over which it's applied.

But because of the time it takes
to jump,

he only has a fraction
of a second to reach his maximum

force, to clear his friend.

Maximizing impulse and
parabolic trajectories.

That was a bit more
complicated than I'd imagined.

So, let's just start simply.

And what could be more simple that
jumping over your swinging brother?

Well, probably most things.

Her brother presents a tricky moving
target

and she only has about 0.2
seconds to maximize

her impulse, so she needs
to time her jump perfectly.

Which she completely
fails to do.

What you want is a nice
static target for your jump.

And for that target,
to stay static.

On take-off, the jumper gets enough
velocity

for a center of mass to clear
his friend whilst

he's crouching, but not
enough when he's standing.

Shame.

These guys look like they're
professionals and ambitiously,

he's aiming to jump five people.

[Dallas off-screen] But,
the more people you add,

the greater the chances
someone will mess up.

As there are five people
in a row,

the jumper needs a
parabolic trajectory that's both

higher and longer than
when he's just jumping one.

Or at least, slightly
higher than that.

This is hard, but
not impossible.

Alexander Pope once said, 'A little
learning, is a dangerous thing,'

and over 300 years later, this lot still
seem determined to prove him right.

[music plays through credits]

[woman] What did you break?