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When you push air out of the sucker, the air pressure inside the sucker drops.
So what happens is that there is a net force, the force acts on the sucker from the outside, because the air pressure outside is greater than the air pressure inside.
Thus, it is difficult to pull the sucker directly from the window (perpendicular pulling). You will find, however, that it is easier to slide the sucker along the window.
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And if you're really too lazy:
12.3.5.3 Plumber's force cup, suction cup, suction disc, Lift a stool
See diagram: 4.227.4
1. Press the force cup against a flat surface such as the top of a stool. Try to lift the stool with the force cup. The force cup works better if it is wet. Press the force cup against a flat surface such as the top of a stool. The force cup works because some air between the force cup and the stool surface was pushed out when the force cup was pushed on to the stool surface. Thus the air pressure inside the force cup is less than outside and as a result atmospheric pressure pushes together the cup and stool. A rubber suction disc stays on a smooth window because there is no air between the disc and the window. It is kept there by the pressure of the atmosphere on the rubber. You can use a force cup to clear a blockage in a drain.
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