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Cup and Card

This activity explores the force of air on objects. The particles of air that are all around us push in all directions--that's called air pressure. The card sticks to the rim of the cup because of the adhesive property of water, but it stays in place because of the force of the air pushing into it.

Water is incompressible, which means that you can't squash it to make room for air. When we tap the jar, the force of gravity pulls water molecules down, leaving a region of low pressure in the cup. The force of air moves from the high pressure all around us, into the region of low pressure.

Objectives

  • Explain how air pressure works

Materials

  • Cup or Jar (with ring lid)

  • Square or circle of hard thin plastic (a yogurt tub lid, for example) or cue card (thick paper)

  • Jug of water

  • Square of window screen or mesh (for Part 2)

Key Questions

  • How does the water stay in the jar?
  • Part 2: What direction does the air move when you tap the jar? Why?

What To Do

Part 1:

  1. Pour water into the cup
  2. Hold a piece of plastic over the mouth of the jar
  3. Quickly flip the cup over so the cup is upside down
  4. Remove your hand from under the plastic!

Part 2:

  1. Cover the mouth of the jar with a piece of screen and use the ring lid to keep it taught and in place
  2. Repeat steps 2 and 3 above
  3. Remove your hand from under the plastic.
  4. Without wiggling or tipping the jar, remove the plastic entirely!
  5. Holding the jar over the jug of water, tap the jar to observe what happens.