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Name: John
Status: student
Age: 10
Location: N/A
Country: N/A
Date: 1999 


Question:
I need information on how air pressure works in a water gun that is pumped up like the super soakers. Why? Well, I am a 10 year old doing a science project on figuring out the ideal number of pumps to use for the best distance and water soakage. I have gathered all of my data and made my charts and eliminated all the variables I possibly could (strapping down my waterguns, shooting 5 times at each pump with two guns that were the same and brand new, windless as possible day, and the same person measuring and timing throughout the experiment). Now I need back ground information on air pressure or compressed air that would relate to this project and I am having a real tuff time finding it. Could you please help me?


Replies:
Well, it sounds like you have done a pretty good job on the experiment. The principles of fluid flow are not really complicated, but you can have some complicated equations to predict your fluid flow. Understand that when I say a fluid, I mean water, air, ketchup, etc. Almost anything can be considered a fluid if it has to have a container to have a defined volume. In simple terms, you will have fluid flow when there is a difference in pressure between two points. For instance, the pumping action of your water guns creates a pressure inside the water/pressure chamber. I don't know how much pressure because that depends on how many pumps you make, how well the vessel holds pressure, the efficiency of your pump, just to name a few variables. Now, where you live, the air pressure around you is around 14.7 pounds per square inch (psi). So now you have a pressure difference, say 20 psi in the vessel and 14.7 psi at the discharge location (end) of the squirt gun. Now, this pressure difference causes a flow of water and thus a velocity of the water. The greater the pressure difference, the greater the velocity. Velocity is the measure of distance per time (like miles per hour in your car). If you assume that the water falls the same distance (you said that you strapped the guns down, so I assume the height of the guns is the same), then the time of flight of the water is the same for each gun. So therefore, the higher the velocity, the farther the water travels over a set time. Theoretically, the more pumps you make, the farther it should go. HOWEVER, there is a matter of how good your pump is and how well your vessel can keep the pressure. Typically the kind of pump built onto the squirt guns and the vessel are going to have an upper limits on how much pressure you can pump in, not to mention, a stronger person may be able to get more pressure in because he/she is stronger. It sounds like you have done a good job in trying to eliminate as many of these variables as possible, so chances are you are proving how good (efficient)the pump is per squirt gun. Now, there are many books out there on this principle, called fluid mechanics. Most of the books in your public library in the adult section may be over your head when trying to explain this stuff. Heck, it is even over my head alot of the times! Maybe you should see your local librarian to inquire if they have some juvenile books on how things like pumps and fluid flow systems work. Explain that you have done this experiment and realize that you need to know more about fluid flow, hydraulics, and pumps. See if she can help. If not, maybe give us here at Newton another e-mail. Good luck with the experiment and keep up the good work. You sound very intelligent and inquisitive. You'd make a good engineer when you get older.

Dr C. Murphy



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