Re: Single mother in need of explanation

Dumb_Blonde wrote:
Thank you in advance for your time. I found a neat project to do with
my 14 year old son, but would like to know the science behind it so it
will be educational.

Here is the video link.

http://www.metacafe.com/watch/470767/firecracker_rocket_project_experiment/

It is a fire cracker experiment, and I know he will love it, but I am
clueless as to how this makes my car run.

This is not so much a fire cracker rocket is it is a fire cracker cannon.

A rocket gets its propulsive force by having a high pressure gas accelerate out the back. Any mass being accelerated requires a force. The force generated in the rocket engine that accelerates the exhaust out one direction, also puts the same force on the rest of the rocket, pushing it in the other direction.

A cannon builds up gas pressure in a small volume inside a barrel, and that pressure pushes the projectile out the end of the barrel, while also pushing the other way on the cannon. There are action and reaction forces in both cases, but a rocket normally does not involve the containing barrel that holds the pressure till the projectile escapes the muzzle.

In this case, the telescoping outer and inner can form the containing barrel, and the water forms a seal between them. The fire cracker goes off, essentially instantaneously (before the top can can move or the water get pushed out pf the way), increasing the pressure in the small volume above the water, inside the inverted can. That pressure is what accelerates the can upward as it slides out of the water.

If you had a wire mesh or some other loose and open structure to hold the top can in place with just a bit above the water line, the result is almost the same, since it is mostly the mass of the water inside the top can the pressure pushes against while the top can takes off, not so much the sides of the bottom can. But proving that this is the case by trial would make for an interesting experiment. So don't take my word for it.

You could probably get a much more detailed explanation in the sci.physics newsgroup.
.

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