# The Science of Jetpacks and Rockets!

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
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.

## 100 thoughts on “The Science of Jetpacks and Rockets!”

1. Ryan Sheffield says:

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

2. Bigman74066 says:

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…

3. randy says:

dude dude you have another channel
tell us earlyer
i missed some vedios
i gotta go subscribe in the outher channel quick

thank you for making science SOOO fun:)

5. xFuaZe says:

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?

6. Righteous says:

Are you always stoned?

7. matszz says:

I still have no idea how the thing works.

8. TWENTYFO5 says:

Jet ski to da face!

9. Riley Moulton says:

How much thrust would u need to fly in a jet pack

10. Riccardo Conti says:

ahaha epic crash!

11. Sigma207 says:

Where did you get the picture of that fighter jet?

JET FUEL DOESNT MEALT STEEL BEAMS

so…. i could double jump off of air.

14. 4798alexander4798 says:

Love.your.videos

15. WaterAid says:

I want to ride that water jet pack SOO badly!

16. SuperVegitoFAN says:

4:30 astronauts have to be fit atleast partially because of this no?

17. SuperVegitoFAN says:

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.

18. Star Dragon says:

JETPACK!!!!!!! I'M COMIN' FOR YOU!!!!!

19. Kernels says:

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

20. sine moderamine says:

An' it starts right MEOW!!!!

21. kalle banan says:

4:26 ( ͡° ͜ʖ ͡°)

22. SlyPearTree says:

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.

Wanna feel 1.5G? I know a certain water slide that can do the trick.

24. Darlyne Maxy says:

Amazing lol

25. Luis Javier Castro says:

The force is strong with this one.

26. Spartan D.N.A.35 says:

27. ZaKarIa JO says:

looooooooool

28. PauLtus B says:

SUPER MARIO SUNSHIIIIINE!
woohoo!

29. Xiaoke Ding says:

Billions of micro organisms passing through the hose and pump had awesome roller coaster ride.

30. jcoronet2000 says:

7:31 AHH! Derek has a second head!

31. Metacom Football says:

It's not really wasting water right?

32. Stacy Nunez says:

HAVE A GREAT DAY

33. Engin Yeğnidemir says:

Friction = Fail

34. LubeYourCube says:

He's so lucky he gets to all this

35. Splash Mountain Power says:

this reminds me of F.L.U.D.D from super mario sunshine

36. Stickmaster1100 says:

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

37. Hamu_ Nz says:

Don't scream PROTIP all the time

PROTIP

38. Swaniket Chowdhury says:

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

39. England is my city says:

5:33 look at his eyes hits weed

40. Hamza Shoaib says:

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

41. LaserCake says:

We have somebody that owns one of those water jetpacks here!

42. The Cheaterman says:

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 🙂

43. Matthew K. says:

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)?

44. Handsome Hitler says:

when i clicked veritasium 2 it sent me to a website called buy instagram followers XD

45. Gugglewolf says:

AQA physics… 2016… P2…

+Veritasium Thanks for risking your bright shiny teeth for making this video.

47. jhuiwensley belizaire says:

i understand everything

48. MusTheBunneh says:

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

49. TroyDejayzoo says:

im making one of these haha, its on my channel 🙂

50. Jaime Wilson says:

Why do you say F=mv?

51. digitalmonster604 says:

4:26 I really enjoyed that 😂😂😂

52. S H says:

I know my smokers and you sir, are as high as a kite. Veteran by the looks of it.

53. SirDeimosKnightoMars says:

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

54. Nova.86 says:

1:42 that's my Physics Professor from SBCC! LOL checkout SBCCPhysics at youtube

55. Brooks Silber says:

"1.8 kilo newtons" that's more thrust than the Saturn V's third stage MMH RCS blocks!

56. Plop Plop says:

2:33 How did you make the balloon look like that? like with the ears or whatever is on the balloon

57. AJay Gupta says:

did anyone else heard Ric flair theme song in this video.

what song is it in the skateboard scene

welcome to chillis

i subbed to unsub kid

61. shikamaru nara says:

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

62. Mme G et Co. says:

63. Jacksonvillage says:

I call it a wetpack

64. Sanjay Bankhele says:

how does mass of rocket decreases

65. Jack Hughes says:

Even the birds are laughing at your epic fail

66. Mempo says:

wtf your link to vertisam 2 is a side where you can by instagram followers LUL

67. Zom Bee Nature says:

Once I buy a jet pack I will buy a jet car and then a Jet Li movie.

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.

69. Zipra says:

IM (possibly) NEWEST FIRST!!! I mean… the current newest one is a month old…..

70. Sherlock Holmes says:

This man teaches me things I never would have learned in my life…

71. WatchThat Clip says:

Man, you really need to find a job

72. Noorquacker says:

I should totally go water jetpacking sometime in my life…

73. Origins Light says:

Now i can now that i can bleed from my eyes. Cant stop thinking about the sight of that. Who agrees?

74. James says:

75. George Higgins says:

Slow mo balloons are so weird.

76. Garrett Blanton says:

Speed run4 music from roblox

77. yash verma says:

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 ..😅😅😎😎😎

78. Petitio Principii says:

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?

79. Tiisiphone says:

Wow! Boba (or Jango) Fett!

80. t0msan says:

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

81. HyperCuriosity says:

You just can't fly if you don't Fart….

82. atte dau says:

is this why u have fake teeth

83. Robert the Robot says:

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.

84. plezx29 says:

This is hardly "jetpack" more like "pumpwaterfromawatersource pack"

85. tariq nazeem says:

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

86. alex jervis says:

Jet Pack on the Moon , 6 times more effective!?

87. Jonah Powley says:

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

88. Juan Diaz says:

I forgot that we were talking about a jet pack

89. A Draven Main says:

"You know the feeling when you take off on a plane?"

Cant relate😢

90. ThePrufessa says:

I'm guessing the science that seems pretty obvious is wrong and you're about to blow our minds.

91. ThePrufessa says:

I always thought the science was the same as rockets

92. Kareem Salessi says:

1800-Newtons==183 kg-force, probably just over twice your gross weight, thus Thrust-Weight-Ratio==2. However, you had NO acceleration, but were hovering the whole time. You had minimal initial, and momentary, acceleration which lifted you above water and into the air where your thrust-force became sufficient to only hold you in suspension (Zero-G), no more!!! For an acceleration-related video youtube::: ("Solving-Apollo-Enigma-2") & thanks for the cool video!!!

93. Kareem Salessi says:

Based on this Chinook-Helicopter, lifting each 2.3 kg weight demands one-horsepower of engine force:: Youtube::: (The CH-47 Chinook Proves It Can Lift 50K Pounds)

94. andrin1248 says:

I'm a couple years late but anyway, 3:10 the force in the rocket is not 5-8m/s but 15-18m/s, you have to add the gravitation. Also a free falling object has actually a acceleration of 0, not 9.81. It's only realtive to an observer 9.81, in truth, not the falling object is accelerated, but rather the observer standing on the ground is accelerated upwards. This is the concept of relativity and matters especially in the context of g-forces exerted on human bodies.

95. Ronak vijayvergia says:

F = m.v is wrong it should be m.a

96. pratheep kumar says:

Hey
mv is momentum
ma is force

97. John VanAsselberg says:

I never understood why we use the antiquated system of rocketry. We could build a curved tube simular to what's at the local bank with a larger payload than any rocket to date & be shot out using compressed air. Now, the best payload would be water! Why because the water could be converted to breathable air & fuel after it was used to support the astronauts bodies during takeoff. 200-300 miles of tube would be needed & it would need to exit a tall mountain top, but it could be solar powered.

98. Christian Czarnowsky says:

All of your teeth were intact?

UV light determined that to be a lie

99. Salman Mushtaq says:

Why "Check out 2Veritasium!" link is Social Media viewers buying service? Are you doing some experiment?

100. Lo Hi says:

What to do if I have some ideas to glow everyone's head?