# Is it possible to solve this?

I was planning on making a compressed air gun using a culligan water jug. I'd put a bicycle valve stem in the bottom of the jug and us a valve on the top with a barrell attached that has the diameter to shoot golf balls. I know this has been done before but I've never seen a water jug this big used. My question is this, is there a formula you can use to solve for feet/second by plugging in weight of the ball (45.93 grams) and psi level used to shoot the ball? I don't know the psi the jug is capable of holding yet but could someone solve it by plugging in 100psi and then again by using 200psi. This may be a very easy problem for most of you. I think I would have been able to figure it out a couple months ago in school when learning this type of stuff, but I forgot it allready. Thank you.

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I'm not sure if this will help to solve but the total surface area of the ball is 8.76sq.in. Also I know the answer won't be exact due to barrell length and friction, but I would just like a rough estimate. thanks

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marcusl
Gold Member
I wouldn't put 100 psi in a plastic water bottle, let alone 200 psi. Check out the steel tanks used on portable air compressors for tools or auto shop use. They run at these pressures.

I wouldn't put 100 psi in a plastic water bottle, let alone 200 psi. Check out the steel tanks used on portable air compressors for tools or auto shop use. They run at these pressures.

In school we made water rockets by filling 20oz. water bottles 1/3 full of water and then filled them to 140psi and they held the pressure fine.

marcusl
Gold Member
Small soda bottles are designed for use as pressure vessels. Culligan jugs aren't.

So if a pop bottle can hold higher psi then a culligan jug, then it should be able to shoot the ball further? or does the volume of air have an effect on it?

russ_watters
Mentor
Only if the barrel is of insignificant volume compared to the water bottle will the bottle be able to propel the golf ball very far. Otherwise, as the air is released, the pressure will drop fast.

For your acceleration calculation, though - force is pressure times area and acceleration is force divided by mass. How's your high school physics...?

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So if a pop bottle can hold higher psi then a culligan jug, then it should be able to shoot the ball further? or does the volume of air have an effect on it?
I could be wrong, but the volume of air is only important in so far as there is a positive pressure behind the ball the entire duration of the barrel. After that, I believe pressure is king to increase velocity.

Use pvc pipe... back in highschool I made a pvc pipe potato gun and it worked great until we tried to use acetylene and oxygen (the mix for a professional blowtorch).

Before we used that, we used brake cleaner and air. I don't know the pressures we were generating in there, but I wouldn't be surprised if it exceeded 100psi by a good margin(we were able to shoot stuff that weighed about a pound the length(ish) of a football field).

rcgldr
Homework Helper
You could make a smaller version of a pumpkin air cannon.

http://www.saroff.com/shows/pumpkin_chucking_2006/15.php

You'll find a few hits with a web search for pumpkin air cannon. With long 10 inch diameter barrels, the pressure doesn't need to be that high. The cross setional area of a 10 inch diameter is 78.5 in2 so just 51 psi would translate into 4000 lbs of force acting on 10 lb pumpkin, which is 400g's (rules require the pumpkin to leave intact). Distance can be over 4000 feet if the barrel is long and the pressure tank is big enough.

I recall tennis ball can cannons used to shoot tennis balls, although I mostly used mine (1 can "injector", 5 can tube) as a noise maker; a 3 inch diameter and who know how long a volume of supersonic air can make quite a boom. These used the fluid from wiker based lighters, which is relatively tame even when mixed very well with air. We were smart enough not to use something truly explosive like gasoline. The most powerful fuel based home made golf balll cannon I saw was a 8 foot long section of PVC pipe using ether from a spray can (the original purpose of these was to spray into the caurberator of an engine to get it to start), which shot golf balls about 3/8's of a mile (or more, but we'd never find those). Unfortunately (or fortunately for those safety minded), you can't find tennis ball cans (one of the few cans meant to hold some amount of pressure), or ether in a can any more. (I've still got my old tennis ball cannon, but it's just decorative now).

Only if the barrel is of insignificant volume compared to the water bottle will the bottle be able to propel the golf ball very far. Otherwise, as the air is released, the pressure will drop fast.

For your acceleration calculation, though - force is pressure times area and acceleration is force divided by mass. How's your high school physics...?
My high school physics is good, probably my favorite class. They are good teachers though that may be why I like it. My teacher was going for her masters in physics and one of the other teachers had a degree in rocket engineering. I learned a lot, it's just that I forget a lot too.