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Grades 3–5 · Everyday Life

What keeps a suction cup on a smooth surface?

A suction cup has a flexible rim that can seal against a smooth surface. Pressing the cup flatter pushes some of the air out from beneath it. As the flexible cup tries to recover its shape, the trapped space grows and its air pressure falls.

J53 visual explanationThe supplied inside-to-outside pressure ratio is 0.5; the normalized pressure difference is 0.5. A pressure difference presses the cup against the surface. Holding load and wall friction are not calculated.Outside air presses the cup into placeA cutaway of the flexible cup and its rimSealed rimAir beneathLower pressureOutside airpushes inwardSurfaceThe pressure difference presses the cup toward the surface.

In the diagramOutside air presses the cup into placeA cutaway of the flexible cup and its rimSealed rimAir beneathLower pressureOutside airpushes inwardSurfaceThe pressure difference presses the cup toward the surface.

The cutaway shows outside air pressing a sealed cup toward the surface more strongly than the lower-pressure air beneath it presses outward.

Air outside the cup still pushes on it. When that outside pressure is greater than the pressure underneath, the difference presses the cup toward the surface. The cup does not pull itself on with invisible glue, and the space underneath need not be completely empty.

The rim is essential because it limits air leaking back in. A rough or porous surface can let air enter. As the inside and outside pressures become equal, the pressure difference disappears. A cup on a wall also depends on friction at its contact with the wall to resist sliding downward.

About this explanation

Starting knowledge: Understand that air occupies space and can push on surfaces.

Sources

  • Schmalz — Vacuum suction cups
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