The Physics Police

The Physics Police

Wednesday, March 4, 2015

A soda jetpack is physically impossible

Today I saw this funny viral video.

We see a young man appear to lift off the ground propelled by 6 1-liter name brand soda bottles strapped to his feet. We are presumed to believe the carbon dioxide pressure in the bottle produces sufficient thrust to lift an adult human. Most people will intuitively decide the video is fake. After all, he's probably being lifted by a hidden wire.

I'll use some rocket science to show it must be a fake, not just to debunk this silly video, but to provide an example application of physics.

The force to lift a person off the ground must exceed the force of gravity pulling them toward the ground. If we assume the fit young man is 70kg we can calculate this force.

F = m * a = (70kg) * (9.8m/s^2) = 686 newtons

The six 1-liter bottles mass 1 kilogram each for a total of 6 kilograms. You can see on the second attempt they empty in about 3 seconds. Using the ideal thrust equation we can find the minimum exit velocity needed to lift the person.

V = F / ṁ = (686 N) / (6 kg / 3 s) = 343 m/s

That's faster than the speed of sound (340 m/s)!

Since soda bottles don't make sonic booms upon opening, we can conclude this video is fake.