A suction cup is simply a flexible piece of material that has wide edges. If this is placed (very gently) against a wall, the pressure pushing out and the pressure pushing in will be equal and the unmatched force of gravity will “win” causing the suction cup to fall.
If, on the other hand, the suction cup is pressed against the wall the situation changes. Pressing the suction cup against the wall flattens it. So, the volume of the space behind the suction cup decreases, thus increasing the pressure of that small amount of air. This high pressure air can then force the edges of the suction cup away from the wall and escape. The net result is that much of the air behind the suction cup is removed.
When the suction cup is released, the material (rubber, silicone, plastic, etc.) returns to its original shape (this is why we don't make suction cups out of steel or wood). When that happens the volume of space behind the suction cup goes up and the pressure goes down. This pressure is now much less that the pressure of the atmosphere. (Remember that the two pressures were the same, but we have lost much of the air that was originally trapped.
As a result, the atmosphere presses the suction cup against the wall much harder than the air behind the suction cup pushed out and the cup is held against the wall with enough force to fight gravity.
No comments:
Post a Comment