Veritasium makes a cool video here where he tests a water jetpack.  At the same time he uses physics to explain what is happening.   Here is how he describes his experience:

Despite the fat lip I had a great time. I think knowing a bit about physics actually helps fly the jetpack. It works on the same principle as a rocket (Newton’s 3rd law) but unlike the shuttle, you don’t carry your own propellant with you. Instead, water is pumped out of the lake by the jetski at up to 60 litres a second. It is then fired out of the nozzles at around 15 m/s creating 1800 N of force, the equivalent of about 150 fire extinguishers. On me this can produce acceleration of about 1.5g’s.

Let’s watch him use this neat device in the video on page 2

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