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Homework Help: Potato gun

  1. Mar 7, 2005 #1
    For a physics project, I really want to do something with an explosion in it, but I can really only come up with one practicable idea - a potato gun. Practicable meaning, an idea that won't cost me much more than 2 hundred dollars in instruments and materials. I plan to find out how much energy is contained in some sort of propellant, then vaporize it with a spray bottle and calculate the kind of force that the particular charge is capable of producing to find the efficiency of the reaction based on the height that the potato reaches. I am thinking of using white gas in a potato gun made of that black PVC pipe - is this a safe idea? if not, I will use alchohol. I have a very good physics mind - math is ok too, but not my strongest point. Could any of you help me with this project? Or even better, coulda any of you help me think of an idea that involves an even bigger explosion that I could use for a lab? :)

    I know that this is getting awefully close to chemistry, but hopefully the use of projectile motion in here and conversions from joules to newtons will make it close enough for my teacher. Please reply as soon as possible, I already had a completely different idea before this one, but I couldn't get a hold of a 11,000 gauss magnet that I really wanted for it. So, I have 3 days left before the proposal is due. thanks for any and all help in advance.
  2. jcsd
  3. Mar 7, 2005 #2
    We just made a potato gun in our physics class... Ours was made of white PVC pipe (simply because that's what Home Depot had and we didn't feel like changing anything). Black is suggested in many websites, though, so it's a good idea. As for the propellant, we used hairsrpay, but another team used something else, I think. From what learned, the propellant is not as important (it still is important, though) as the timing and temperature. It's better to try it in cooler weather, and you have to be quick. Right after you spray inside, you have to ignite the gun. Otherwise, it won't shoot. The amount you spray is also important. So, if plain hairspray worked, alcohol and whatever else you plan to use should work.

    As for safety, just make sure there is a fire extinguisher present. If anything drips on the ground and a flame is ignited, a fire can obviously spread. Oh, and make sure it's legal in your area, or make sure nobody else finds out.

    Good luck.
  4. Mar 8, 2005 #3
    SelmerSaxMan, have you ever seen/used/shot a potato gun? If not, and you decide to make one, they are one bad ace. :biggrin: Can anyone else vouge for this?
  5. Mar 8, 2005 #4
    I have one, and I don't know where you are getting a $200 cost from.

    All you need is some PVC, PVC glue, the adapters, and hairspray.
    For an igniter, you can go electric or flint. Each one has its upsides and downsides.
    And the trick is not how much fuel, but it is how much backpressure you can create. Be careful though, my friend had his blow apart at the joints, and I have heard of another turning itself into a PVC handgrenade.

    Oh, and for for whoever said cold days are better, that's just because cold air is more dense. Go ask any drag racer about that.
  6. Mar 8, 2005 #5
    The black PVC is considerably stronger, I can't remember what the real name of it is, but that is why I was planning on using it - I know that potato guns made of regular PVC can be used with hair spray regularly survive, but gasoline has a LOT more energy in it than alchohol does.

    I have use done before - other than that, 1+1, I don't know what you are saying - sorry, I think I am kind of out of it.

    And Igeek - I estimated the pvc and adhesives to cost around 40 bucks and I was planning to use the ignition off of a ride in lawn mower with a blown engine which was 140. Then I suppose gas, pure alchohol, wadding and potatoes would cost around another 20. I know I don't need gas, that alchohol would do fine, but I want this thing to make some noise (not to mention...hit god in the ass with my potato.) - I considered sticking an M80 in a steel pipe launcher, but decided this would be too dangerous and costly.

    Well, I talked to my pyhsics teacher about it, and he said there was too much chemistry in it. I wanted something to explode, so he suggested I launch a M80 from a trebuchet - now we're talking!!!

    I am probably still going to build this, (for fun) So I will probably be back later. Sorry for the spelling today - i'm walkign dead today I think.
  7. Mar 9, 2005 #6
    Do you really think you need a $140 igniter? The potato gun I helped build before only used a $3 barbeque lighter and it worked fine.
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