The Science of Jetpacks and Rockets!
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The Science of Jetpacks and Rockets!

August 8, 2019


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.

Only registered users can comment.

  1. Rocket science is really, really simple. The difficult part is rocket engineering. Trying to get an appreciably sized payload to orbit or wherever else without running out of fuel is the tough part.

  2. Yes, you'll continue moving with constant velocity, but you still need a small constant force to maintain it, because air friction, and friction with the hose on the water will eat away at your velocity.

  3. i have a question, every time a spaceship fly it push the harth rigth? so if its fly in the day, the world get away from the sun(a very very small amount) and making the day longer? and if that is right, every time i jump in the day the world get away from the sun(a REALLY SMALL AMOUNT). its that right?

  4. 4:19 "Stacked on top of you – OHH OHH YEAH – hey how're you DOWN BELOW HUH? – AH you guys ARE HEAVY, OH MAN PHEW" #HaHGayyyyyyyyyyyyyyyy  

  5. Think the nozzle should be explained a little more. The nozzle increases the velocity of the fluid, converting internal energy into kinetic energy, lending itself to a higher momentum force at the nozzle. Along with the increased velocity at the nozzle exit, yielding a lower pressure than right before the nozzle, also giving an upward force. Nozzles are what makes all this possible

  6. About the trust of the jet pack…
    I think that the majority of the trust does not come from the water passing through the curve that redirects the water from going up to going down.
    If the water would be fed in from the top of the unit, you would probably still be lifted out of the water.
    So I think there is something else that provides the trust.
    My guess is that the water is accellerated because the the nozzle has a smaller diameter than the main tube.
    The acceleration exerts a force on the unit according to f=m*a and that's the main force that lifts you out of the water…

  7. Wait?
     3:22 Mass decreases?
    Is this due the burning of fuel, having less fuel, less mass, so accelerating faster?
    Or is this due being further away from earth, having less attraction?
    Or is this because it's easier to accelerate when he's already moving?

  8. 4:40 never been on a plane but sometimes someone felt the need to kick the gas pedal in a car and while not insanely wild it was noticeable a bit like a roller coaster.

  9. 2:15 So that's why when I go up in War Thunder, My screen gets dark. And if I dive, my screen gets red…

    Thanks for the explaination

  10. I love that you showed the failed experiment with the skateboard and fire extinguisher, failure does not mean the science is bad, it might means the engineering is bad. The reverse is also true, bad engineering can lead to validation of bad science, like the many peoples that think they have achieved free energy/perpetual motion. I'm thinking of the people who genuinely believe they have achieved those things, not the scammers.

  11. Correct me if I am wrong, but I was under the impression that force equals mass times acceleration not mass times velocity?

  12. How rockets change direction and control speed in space? I mean there is no air to apply force and get the reaction force to increase or decrease the speed or turn left or right.. so how did they do it? @veritasium

  13. Your annotation at the end of the video for Veritasium 2 directs to as site selling instagram followers.

  14. 3:30 – Even better, the thrust actually slightly increases, because the Isp (specific impulse) will increase as atmospheric pressure decreases, while fuel flow remains roughly constant. Even more g's for the astronauts 🙂

  15. So is it possible to duct tape 2 million fire extinguishers together and fly to the moon (assuming there's an ample amount of mass in the extinguishers)?

  16. It's not very polite to disguise a bogus link as a link to subscribe to your second channel. Disliked & Unsubscribed.

  17. When I click the MinutePhysics link in the summary, it brings me to a place to buy instagram followers too.

  18. 0:16 someone please tell me where this music is from , i knowni heard it somewhere . my guess is a game (a cute one) or maybe a ringing tune. i think its a game bgm, please someone

  19. a normal fire extinguisher doesn't have CO2 in it. It has charged nitrogen. If it had CO2 it would choke you while using it and it wouldn't function at negative temperatures. A little fact I noticed in the video that was wrong. Small and unimportant but still wrong! The white powder is the extinguishing factor in an ABC Fire extinguisher. CO2 extinguishers exist but are only in Kitchen scenarios for grease fires.

  20. I comment rarely … just wanna show off that I knew most of that science ….. though I never specifically read or learned about it …. just my imagination with the equations ..😅😅😎😎😎

  21. If contact with an external mass is not necessary, can you then "swim" on space? Isn't moving one's own limbs physically analog to a jet?

  22. the flyboard air from zapata is the next big thing and i believe soon zapata soon will be demonstrating a working flying bike

  23. you never explained the genius of water jetpacks. air is light, so it takes a huge amount of exhaust speed to get thrust. as a rule of thumb for a helicopter, it takes 1hp to lift ONLY 5 pounds. water is heavy, so it takes less exhaust speed to get the same thrust.

  24. isnt this basically the law of conservation of momentum
    the exhaust gases fired out of the back have momentum and so in order for momentum toy remain constant the shuttle experiences a force in the opposite direcion changing its velocity
    this is exclusively for the shuttle in orbit as on liftoff the exhaust gashes can push against the launch pad

  25. Don’t Rockets also go faster the higher they are because of less air resistance due to decreased air density so that also explains the exponential curve

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