Science Buddies: "Ask an Expert"

My expirement is to shoot off a water rockt at different angles and record distance traveled and hangtime, however when the Psi is lower it shoots further with less hangtime. Why is this?

I suggest you look at the following website for information on the effect of compressed air psi on distance.

http://bradcalv.customer.netspace.net.au/wrbook.htm

PSI means "pounds per square inch", or "pressure" in normal language.

Translated into what you are doing, more PSI means more force is available to push your water rocket. It's essentially the same thing as using a stronger rubber band on a slingshot, a stronger spring in a pogo-stick or, to get closer to home, turning the spigot on your garden hose from low to high and seeing how far the water sprays.

Steven,

It may be that there is a "sweet spot" or particular psi at which you can maximize the distance traveled and minimize hangtime. Often times in science and engineering, there is such a sweet spot. Anything more or less than this results in poorer performance. I don't know what sorts of pressures you are working with, but, for example (THIS IS AN EXAMPLE, NOT FACT!) it may be that you find a maximum distance when the pressure is at 15 psi. If your were to shoot at, say, 10 psi, the rocket would not go as far, and if it were shot at 20 psi, it would not go as far.

I'm not clear on the exact setup of your experiment, so I can't really comment of the physics going on, but I would not be surprised if there were an optimum pressure. It may also be that the pressure that gets you the longest distance traveled may be a different pressure from the pressure required to maximize hangtime.

Try playing around with the pressure to figure out the optimum pressure for distance traveled and for hangtime. If you can provide more detail about your exact experimental set-up, we'll be able to explore more of the physics involved.

Out of curiousity, what angle did you find had the largest distance traveled?

Interested in you reply!

To reply to Terik Daly's post, our water rocket went the furthest distance (70.25 feet) when launched at 75 degrees, however it had the longest hangtime(3.39 sec.) at o degrees. The average Psi was somewhere in the range of about 25-45. Thank you
Steven