MadSci Network: Physics

Re: Why doesn't a scale submerged in water register the weight of the water?

Date: Sat Feb 26 10:36:19 2005
Posted By: Richard Bersin, Other (pls. specify below), Senior Technical Staff Member, Emergent Technologies
Area of science: Physics
ID: 1109396495.Ph

Dear Tom:

The principle of operaton of a bathroom spring scale is based on the 
measurement of the net downward force on the scale platform.   When the 
scale is sitting in the room with nothing on it, the air in the room is 
still over the scale and pressing downward with a preassure of 14.7 pounds 
per square inch, atmosphberic pressure, on the top;   however, the same 
air is underneath the scale and pushing up from the bottom with the same 
atmospheric pressure so there is no net force pushing the platform down to 
give you a reading.

Now when you put the scale in water (remember the ecale is completely open 
to the water-the top and bottom surfaces; it is not sealed!)   That means 
that no matter how much water you put on top of the scale, because the 
scale is not sealed the pressure upward from the bottom of the platform 
will be exactly the same as the downward pressure from the top, just as 
with the air in the room; and you get no reading on the scale.

If the mechanism of the scale were sealed so no water could get inside to 
the undersurface of the platform, then as you add water above there would 
be a net downward force on the spring in the scale and you would get a 

If you put a jar on the scale and fill it with water you get a reading.   
Why?  There is a net downward force on the platform because the pressure 
upward from the bottom has not changed with the water present, but the 
pressure down from the top has increased beause of the water in the jar.

I hope this anwsers your question OK.

Let me know if not!

R. Bersin....

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