Imagine this little though experiment with me for a moment.

A piston sort-of contraption - like a bicycle pump, but without any nozzle to push air out of. One foot tall, one inch width/depth. Lets say the pump is completely bottomed out. As such, there is no air at the bottom. It's also completely air tight, and if I pull on it, no air can slip in to displace it. ASCII explanation:

 ======             ||
   ||               ||
   ||               ||
  ||||             ||||
  ||||    ==>      |  |
  ||||             |  |
  ||||             |  |
  ----             ----

In a vacuum, I would be able to pull the pump out, fairly easily. No forces acting other than friction and gravity.

However, on earth, air pressure is acting on it strongly. Being that the top is 1 inch by 1 inch - it should be 14-15 pounds of force, correct? This seems too easy. I can lift that amount quite easily. Am I missing something? I have enough force to create a vacuum so easily?

My guess is that it gets more difficult very quickly, with pulling out the piston further. But a vacuum cant be even more of a vacuum, so that's not what's going on. The exposed bit of the pump has more surface area, does that cause more pressure on the piston? Or is it simply a linear 15 lbs the whole way through?


1 Answer 1


You are correct. With just a 1 in cross section you would be able to pull as much vacuum as you wish. (most pumps are larger than this.

As a counter example, just think of a mercury barometer. It takes about 26 to 32 inches of mercury to draw the vacuum and hence we can measure the ambient pressure. If we made this barometer 100 inches tall it would not take any more mercury, we would just get 70 inches of vacuum.


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