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  • 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.

This is a water jet pack... but no, that's not me flying it. This is me. It's harder

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Jetpack Rocket Science

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    VoiceTube posted on 2014/01/09
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