![]() You would need an immense force pulling down on the plate to stretch the water to any significant degree. Water has such a high bulk modulus that we can approximate it as incompressible. If you pull the plate down slightly (and no air leaks in) the volume inside the glass must increase. Either the volume of the water in the glass must increase, to allow the plate to move down, or air must flow into the glass at the contact line between the glass and the plate.Ĭonsider the first of these. For the plate to fall down one of two things must happen. Start with you holding the plate in place, and consider what happens when you release the plate. One is the atmospheric pressure, and the other is the surface tension of the water. There are two unrelated effects at work here. Is there a good description of this phenomena? Why does the trick continue to work with lower density fluids and why does placing the plate in the water cause it to fall? So, how does the pressure of water and atmosphere change? If we put the bottle and plate into water slowly, when the plate and part of bottle are in the water but part of the bottle still above the water, will the plate fall down?Ĭontinuing question 2, when both bottle and plate are in the water I know the plate will fall down. So, if there is a threshold density of liquid make the plate between falling down and not falling down? But, if we continue to chose the smaller density liquid until the liquid has density like air the plate will fall down. So, If we use other liquid instead of water, this liquid has smaller density than water, like alcohol, the plate will not fall down. Because the atmospheric pressure, the glass plate will not fall down.Īs we know, if we didn't put water into the glass bottle, just air in the glass bottle the glass plate will fall down. Slowly, release the hand which hold the plate. Hold the glass bottle and plate, and invert them. Prepare a glass bottle, fill with water, put a glass plate on the bottle, make sure there is no air in the bottle, just water. When I was in high school, my teacher did an experiment to show the power of atmospheric pressure.
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