Ask A Scientist© Physics Archive

 
name John
status student
age 10

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?
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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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