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Name: Patti
Status: Other
Age: 6-8
Location: NY 
Country: United States
Date: March 2006

For a science experiment my 10 year old son modified a demonstration from his scuba class. He inverted a hard plastic pitcher (marked with measurements) with a floating frog inside, and placed it at the bottom of the pool at varying depths (using a net with weights).

He noted that the water inside the pitcher rose 2 centimeters for every 2 feet of increased depth. Is there an easy way to measure the water pressure, air pressure, air volume, and compression rate?

Dear Patti,

If the pitcher were a cylinder with a ruler that you could easily read from outside, you could easily make the measurements you desire.

The air volume is just the cross sectional area of the cylinder times the height of the air column. The air pressure is identical to the water pressure at the bottom of the cylinder and is easily calculated using the equation pV = constant. Here p is the pressure (atmospheric pressure is roughly 15 pounds per square inch) and V is the volume. When the cylinder is above the water, the air pressure is atmospheric. As the cylinder is lowered into the water, the water level rises and the volume decreases indicating an increase in pressure. Plotting the pressure increase versus the depth will give you the compression rate.

Good luck!

Best, Dick Plano, Professor of Physics emeritus, Rutgers University

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