Science Projects for Big Kids

So, Jake and I have spent the last few days messing around building air cannons. PVC, automatic sprinkler valves, a handful of parts from Radio Shack, and next thing you know, we are shooting golf balls 250' into the water hazard out back.

Then, we saw this link, and got some even better ideas. A little poking around on Craig's List, and the next thing you know, we have hooked up a vacuum pump, and sealed off the end of the firing tube.

We needed a target to test out our new contraption, so we lined up the gingerbread houses (all the candy had been picked clean). We placed a bag of rocks behind the targets to stop the ball, and then propped up a flat cardboard box with plywood inside, just for good measure--don't want to hit the neighbors' house. Here is the "before" photo.

We put in 4 golf balls, just for fun. Turned on the vacuum pump, charged the pressure tank to 130psi, made sure there are no cars or joggers . . . FIRE!

It fired through all the gingerbread houses, embedded one of the golf balls in the middle of the bag of rocks, and blew the bag of rocks clean off the table.

Maximum theoretical speed = 760mph
Force: F = PA = P(pi)r2 = (634,407 Pa)(3.14)(0.019 m)2 = 719.5 N = 161 lb

Acceleration: a = F/m = (719.5 N)/(0.038 kg) = 18,933 m/s2
Velocity: v2 = 2ax = 2(18,933 m/s2)(3.3 m) or v = 339.7 m/s = 760 mph or 1,115 ft/s

Our estimate, around 450mph.