Upward Pressure Force Demonstration

Here is another demonstration that you can do to try to convince yourself that air pressure really does push upward as well as downward.  Not only that but the upward force is fairly strong.

Actually this is not part of the demonstration.  Gravity pulls downward on the water balloon in the picture above and the balloon will fall.  Air pressure is pushing up, down, and sideways on the balloon, but they have very little effect on the falling object.  Even though the forces produced by air pressure are large, they are nearly all in balance and mostly cancel each other out.  The downward gravity force is virtually unopposed.

Here's a little bit more detailed and more complete explanation of what is going on. 

The figure at left shows air pressure (red arrows) pushing on all the sides of the balloon.  Because pressure decreases with increasing altitude, the pressure from the air at the top of the balloon pushing downward (strength=14) is a little weaker than the pressure from the air at the bottom of the balloon that is pushing upward (strength=15).  The two sideways forces cancel each other out.  The total effect of the pressure is a weak upward force (1 unit of upward force shown at the top of the right figure, you might have heard this called a bouyant force).  Gravity exerts a downward force on the water balloon.  In the figure at right you can see that the gravity force (strength=10 but don't worry about where this number came from) is stronger than the upward pressure difference force (strength=1).  The balloon falls as a result.  This is what you know would happen if you let go of a water balloon, it would fall.


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In the demonstration a wine glass is filled with water.  A small plastic lid is used as a cover.  You can then turn the glass upside down without the water falling out. 


All the same forces are shown again in the left most figure.  In the right two figures we separate this into two parts.  First the water inside the glass isn't feeling the downward and sideways pressure forces (because they're pushing on the glass, this is shown at the right side of the figure above).  Gravity still pulls downward on the water but the upward pressure force is able to overcome the downward pull of gravity.  The upward pointing pressure force is used to overcome gravity not to cancel out the downward pointing pressure force.