Super Science Saturday October 26, 2002 Potato Launcher Demo


Write “Spud-nik” on the side of the launcher. Also write on/off next to nozzle, and PSI rating on barrel.

Walk in. Fire launcher. Show audience potato pieces. “What just happened?”

“Spud-nik is a pneumatic launcher. It is powered by compressed air. At the back of the launcher I have a tire valve, just like on the tires of your car or bike. With the ball valve closed, I fill the reservoir with air. The PVC pipe is rated to 200 PSI (pounds per square inch), but I only go to about 100 PSI. But first I load a potato into the front.”

Load potato. Charge the launcher.

“ The pressure on the barrel side of the potato is just the pressure in this room, which is the pressure of the atmosphere from here to outer space pushing down on us. We call that pressure 1 atmosphere, for obvious reasons, and 1Atm is about 14.7 PSI. So there is about 7 times more pressure in the reservoir than on the barrel side of the potato. When we open the valve, this difference in pressure will exert a force on the potato and push it out the end of the barrel.

“Pressure differences are responsible for a lot of common forces of nature. Who can think of one?”

“Has anyone heard of Sir Isaac Newton’s second law? (F=MA) It tells us that the force on the potato will be converted to acceleration of the potato. The potato will go from resting in the launcher to flying out the end of the barrel at high speed. The potato will be at its highest speed when it leaves the launcher. Once it leaves the launcher, the drag of the air will slow it down, but not enough for us to tell in the short distance it has to go.” Turn the valve. Boom. “Walla, instant French fries”.

“Speaking of French fries, how come that happened? Shouldn’t the potato have bounced, like a tennis ball?” Let them speculate.

“What if I shot a tennis ball out of the potato launcher? Would I get tennis ball French fries?”

“What if I threw a potato at the tennis racket? Would it bounce?” After the show, I will unmount the racket and anyone who wants can throw a potato or a tennis ball at the racket.

Newton’s 2nd law continues to work on the potato when it hits the racket. At this point, it is decelerated, or slowed down. That deceleration is proportional to a force in the opoosite direction to the motion of the potato. Since the tennis racket is made of strings, only those strings push on the potato. So the part of the potato impacting the strings slows down and the part of the potato between the strings keeps going. If that difference is enough, it will cause the potato to be ripped apart by the strings, like a cheese slicer cuts through cheese.”

“Let’s try to cut the potato with a knife.” Have an audience member cut the potato.

“Now try to cut a tennis ball with a knife.” Have audience member cut tennis ball. “How does it compare? Why is it different? Do you think we could use our potato launcher to make tennis ball French fries? How?”

“So, the variables are the speed of the object, and how hard the object is. The speed is related to the acceleration, and that’s proportional to the force, which was generated by the pressure difference. And now you know a little about Newton’s second law. Shall we launch another potato?”

Directions to build a “potato gun”

Items needed:

-these adapters start out one size and open up to a larger size -1 piece 4 inches in diamater- 4 to 5 inches long -1 piece that transforms the end into a cap that can be srewed on -1 screwable cap to fit on the 4inch diameter end -ABS glue -drill and 1/2inch drill bit

Steps for constuction of “potato gun”:

-Glue one end of the 2 foot piece of ABS (2 inch diameter) to the adapter (2inch diameter end being glued, leaving the 3 inch diameter end open) -if you have 1 adapter from 2 inch to 4 inch you will only glue the one end and go to the * -you will then glue your second adapter (from 3 inch diameter to 4 inch diameter, leaving the 4 inch end open) -glue the cap adapter on the end leaving the threaded portion open for the screw on cap. -about 21/2 inches from the end of the cap adapter drill a hole with 1/2 inch drill bit (in the 4 inch diameter ABS pipe) -some people believe that if you sharpen the end of the 2 foot ABS pipe it allows you to insert the potato much faster and easier (instead of having a flat pipe end, simply file down the corners so that you have a “<” instead of “[“…..but this does take time)

Steps on how to operate the “potato gun”:

- take a potato and insert it in the end of the 2 foot ABS pipe (2 inch diameter) you will have to use force, so, place yourself on top and slap it in (this will trim any excess). - you will then need a tool (broom handle works well) to push the potato in as far as the 1st adapter (until you get used to it, look from the back open end to make sure you don’t push it in too far) the potato should be completely in the 2 inch diameter pipe, but as far at the edge as possible. - place your finger over the drilled hole, to cover any leaks - you then need a flammable gas (hairspray works well), you spray in the open end about 3~5 seconds, depending on the bang, distance or experience you want - you should then try to close the cap as fast as possible (keeping your finger on the drilled hole) - in your other hand you would have a b-b-Q lighter with the long shaft (about 1~2 inches from the end of the b-b-q lighter use some sort of tape, duct tape works well, and wrap it around until the tape covers the hole completely when inserted) - insert the b-b-q lighter (this is where you want to do a countdown, when the crowd hits “0”) pull the trigger, the spark should ignite the gas (hairspray), pushing Mr. Potato Head through the air to his destination ??? (here at Science North we aim the potato gun prior to launch, directly at a huge cauldron type pot fastened sideways on a stand. - this contraption should be fastened as well to a stable work bench of some sort (because it does has quite a kick…be carefull) • if you have any questions, please feel free to contact me at (705) 522-3701 ext. 253 • I will be sending you some pictures of our potato gun within the next week. • HAVE FUN!

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