MadSci Network: Physics |
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 measurement. 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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