Kind: captions
Language: en
[Applause]
[Music]
This is a water jetack. But no, that's
not me flying it. This is me.
It's harder than it looks. Okay, but to
understand how it works, we need to
first talk rocket science. Rocket
science is meant to be one of the most
complicated things in the world, but the
basic principle is incredibly simple.
It's just Newton's third law. All forces
come in pairs, which are equal and
opposite. To demonstrate this, I'm using
a fire extinguisher on a skateboard. As
the carbon dioxide is forced out the
back of the extinguisher, it puts a
force forwards on me, causing me to
accelerate.
[Music]
Or that's the theory. Anyway,
if you look closely, you can spot the
exact moment I realized this is a fail.
So, what was the problem here? Well, the
force applied to me by the carbon
dioxide is equal to the rate of mass
ejected out the back of the fire
extinguisher, call it m dot for short,
multiplied by the velocity of that
exhaust gas. So, in this case, the
carbon dioxide wasn't ejected fast
enough to create a big enough force and
overcome the small frictional forces to
get me to accelerate.
But it can be done as has been
demonstrated many times on YouTube. When
the space shuttle lifts off, exhaust
gases exit the nozzle at 3 to 4
kilometers/s,
ejecting an amount of mass of 9,000
kg/s.
This creates thrust equal to 30 million
newtons, or the equivalent of about 2
million decent fire extinguishers.
Now, imagine you're an astronaut
preparing for launch in the space
shuttle. you would be seated not
vertically but horizontally
perpendicular to the acceleration. And
that's because the human body is a
little bit like a water balloon where
the water represents your blood and the
balloon represents your harder parts
like your skeleton. Now, if you're
accelerated up really quickly, then your
skeleton accelerates up at that rate,
but your blood tends to stay where it
is, and this results in the blood ending
up in your feet. Now, since there's not
enough oxygen going to your brain, you
would black out. But fighter pilots face
an arguably worse fate when they
accelerate down too fast because then
the blood all rushes to their head and
they suffer something called a red out
where the blood actually comes out of
their eyes, nose, mouth, and ears.
But back to astronauts, since you're
reclined, at worst, the blood will end
up in the back of your body and the back
of your head, but your brain will still
have enough oxygen to remain conscious.
Now, as the spacecraft lifts off and
starts speeding up, the acceleration is
initially a very reasonable 5 to 8 m/s
squared. That's less acceleration than
an object in freef fall here at the
surface of Earth. But as the spacecraft
continues to burn fuel, its mass
decreases while the thrust remains
essentially constant. Now, Newton's
second law says that the acceleration of
an object equals the net force applied
to it divided by its mass. So, as the
mass decreases, the acceleration
increases and it increases at an
increasing rate. So much so that at the
end of the rocket burn, the thrust has
to actually be limited in order to keep
the acceleration from going over 3gs.
That's three times the acceleration due
to gravity or about 30 m/s squared. Now,
the term gforce has been invented to
give a sense of the amount of force
experienced by astronauts in multiples
of the force we experience every day.
Right now, you're experiencing one G
force, probably on your butt if you're
sitting down. Can you feel that force?
But accelerating at 3 G's, you would
experience three G forces. So, the force
between your back and the chair would be
the same as if you had two of you
stacked on top of you.
Hey, pipe down below, huh?
You guys are heavy. Oh, man. You know
that feeling when you're taking off in a
plane and it feels like you're pressed
into the seat? Well, really it's the
seat pressing into you. But if you
imagine that feeling times 20, that's
what it would be like to be taking off
in the space shuttle. Now, an
interesting little side note is that we
think of the space shuttle's
acceleration as being mainly vertical
because that's what we see when it lifts
off. But that's actually not true. Once
the space shuttle exits the thicker part
of the atmosphere, it turns horizontal
and accelerates up to its orbital
velocity of 28,000 kmh.
So most of the acceleration of a
spacecraft in orbit anyway is
horizontal. So how is this like a
jetpack? Well, unlike the shuttle, you
don't carry your own propellant with
you. And also, there's no chemical
reaction releasing energy that drives
the propellant downwards. Instead, the
jet ski pumps water out of the lake and
up that hose at a rate up to 60 L/
second. And then right on these nozzles
here, the water changes direction. So it
goes from coming up to being fired out
the bottom. And that change in momentum
as it goes over the bend is what
actually pushes the jetpack up because
the jetpack's pushing down on the water.
So by Newton's third law, the water has
to push up on the jetpack generating
1,800 ntons of thrust. That's roughly
equivalent to 150 decent fire
extinguishers. This could accelerate me
at up to 1.5 gs. And you use your hands
in order to steer, lifting up to drive
yourself upwards, moving your hands down
to accelerate forwards, and pretend like
you're turning a big wheel very gently
in order to turn side to side. One thing
you don't want to do is try to explain
the physics of the jetpack while in the
air. That's what I was trying to do
here.
[Music]
I didn't want to happen.
While you're learning, your thrust is
controlled by your instructor. So, if he
sees you doing something stupid, he'll
just turn off the thrust and drop you
into the lake so you don't hurt
yourself. That's generally a good idea
unless you're on a collision course with
the jet ski.
I got a pretty fat lip from doing this,
but luckily all my teeth were intact.
When the thrust is equal to my weight
plus the weight of water in the hose,
then I can hover or move with constant
velocity. It's a common misconception
that you need a little bit of unbalanced
force to move with a constant velocity.
In truth, if the forces are balanced,
you will just continue moving with
whatever constant velocity you have. The
other common misconception about rockets
is that you need something to push off
like the atmosphere. In reality, what
you're pushing off is the propellant.
So, even without the air around, a water
jetack would still work because you're
pushing off the water that is coming out
of those nozzles. If you want to go
jetpacking, I recommend you go easy on
the controls. I mean, the worst thing
you can do is overcompensate, which I
think is a typical human reaction
because you're reacting to where you are
and how fast you're moving, then you're
not reacting to acceleration, which is
the real thing that you can control. So,
even if you're coming down towards the
water quite quickly, you may be slowing
down. So, it may be okay and you don't
need to adjust anything. You just kind
of need to trust that the jetpack will
get you out of any trouble. It's a
pretty incredible experience
feeling the power of that water rushing
past you.
It's the closest I've gotten to flying.
Really,
it's the power of physics.
Now, many of you may not know that I
have a second channel called Two
Veritassium, and I've been posting more
videos on there recently. So, if you
want to check them out, then click on
this annotation or the link in the
description. If you ever want to
download a Veritassium video, you can do
that now via iTunes by clicking this
link. And that's a service provided to
me by Science Alert, which is one of the
greatest Facebook pages on science that
exists. So, click on this link if you
want to check them out. All right,
thanks for watching.