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Suction cups

Air and fluids · level 3, lesson 4 of 6

A question to start

Press a rubber suction cup onto a window and it sticks so hard you can hang a towel from it. There is no glue. What holds it on?

The idea

When you press a suction cup flat, you squeeze the air out from under it. As the rubber springs back, the small space under it is left with very little air.

The air outside now pushes on the cup much harder than the little air underneath pushes back. The air outside holds the cup on.

air pushes it onwindowlittle air under the cup

A suction cup falls off when air leaks back in under it. Then the air pushes on both sides equally, and nothing holds it.

Worked examples

1. A rough wall. On brick or wood, air leaks in through the tiny bumps, so suction cups soon fall off. They work best on smooth glass or tiles.

2. Wetting it. A drop of water around the edge seals the tiny gaps, so the cup holds longer.

3. Lifting the edge. To take a suction cup off, lift its edge a little. Air rushes in, and it comes away easily.

Try it

  1. What do you squeeze out when you press a suction cup on?
  2. What holds a suction cup on a window?
  3. Why do suction cups not work well on brick?
  4. Why does lifting the edge make it easy to take off?
Answers
  1. The air from under it.
  2. The air outside pushing it on.
  3. Air leaks in through the rough surface.
  4. Air gets in under it, so the push is the same on both sides.

Common mistake

"The cup sucks onto the glass." Nothing pulls. The air outside pushes the cup on, because there is so little air under it to push back.

Where it leads