Subtitles section Play video
-
This is a water jet pack... but no, that's not me flying it. This is me. It's harder
-
than it looks, ok? 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 3rd 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.
-
Or that's the theory anyway.
-
If you look closely, you can spot the exact moment I realize 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 gasses exit the nozzle at 3 to 4 km/s, ejecting
-
an amount of mass of 9000 kg/s. This creates thrust equal to 30,000,000 N or the equivalent
-
of about 2 million decent fire extinguishers.
-
Now imagine you are an astronaut preparing for launch in the space shuttle. You'd be
-
seated not vertically but horizontally, perpendicular to the acceleration.
-
That's because the human body is a bit like a water balloon where the water represents
-
your blood and the balloon represents your harder parts like your skeleton.
-
Now, if you are 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 are 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 five to eight meters per second squared - that's less acceleration
-
than an object in free 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 three g's -- that's
-
three times the acceleration due to gravity or about 30 meters per second squared.
-
Now the term g-force has been invented to give a sense of the amount of force experienced
-
by astronauts, in multiples of the force we experience everyday. Right now you are experiencing
-
one g-force, probably on your butt if you're sitting down -- can you feel that force?
-
But accelerating at three 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 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 28,000 km/h. So most of the acceleration of
-
a spacecraft, in orbit anyway, is horizontal.
-
So how is this like a jet pack? 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 jetski pumps water out of the lake and up that hose at a rate
-
up to 60 litres per second. And then right on these nozzles here, the
-
water changes directions. 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 pushed 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 1800 Newtons of thrust, that's
-
roughly equivalent to 150 decent fire extinguishers.
-
This could accelerate me at up to 1.5 g's
-
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...
-
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 jetski.
-
I got a pretty fat lip from doing this but thankfully all my teeth were intact.
-
When the thrust is equal to my weight plus the weight of the 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 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 are pushing off is the propellant, so even without the
-
air around a water jetpack would still work because you're pushing off the water that
-
is coming out 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 and 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 ok and you don't need to adjust anything. You just need to trust that the
-
jetpack will get you out of any trouble.
-
It's a pretty incredibly experience, feeling the power of that water rushing passed you.
-
It's the closest I've gotten to flying really. That's the power of physics.
-
Now many of you may not know that I have a second channel
-
called 2 Veritasium 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 Veritasium 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.
-
Alright, thanks for watching.