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Balloon and Cup Attraction

In this demonstration, students discover that variances in air pressure can be exploited to create suction.

When the demonstration balloon is small, it fits in side the cup, it is curved and takes up a lot of room in the cup. As the balloon inflates, less of the balloon is in the cup, increasing the volume available to the air molecules trapped inside the cup. This reduces the air pressure inside the cup since the air molecules now have more space to move around. Meanwhile, the higher pressure air outside the cup pushes the cup into the balloon.

In other words, the "suction" is really the pressure of the air outside the cup pushing the cup into the balloon and causing it to stick there.

A great visual is to introduce the idea of a "push of war" (instead of a "tug of war"). The (higher) pressure of the air outside the cup pushes the cup into the balloon harder than the (lower) pressure inside the cup pushes out.

Objectives

  • Describe the characteristics of air.

  • Explain how air pressure works.

  • Discuss how air pressure affects our daily lives.

  • Describe the how air pressure explains suction.

Materials

  • Per Demo or Student:
    a balloon
    a plastic cup

Key Questions

  • What is in the cup?
  • Can any more air get into the cup?
  • What’s happening to the volume of the space inside the cup?
  • Explain with arrows how the difference in pressure causes the cup to stick to the balloon. Hint: air always flows from a high (pressure) area to low (pressure) area.

What To Do

  1. Blow up the balloon so that it is the size of a grapefruit or softball.
  2. Place the cup upside down on the upper side of the balloon.
  3. Continue blowing up the balloon.
  4. Pinch the neck of the balloon and ask the students what would happen if you turned the balloon over.
  5. Rotate the balloon, without letting any air escape, so that the cup is on the bottom.
  6. The cup will stay attached to the balloon.

Teacher Tip: Slightly squeeze the sides of the cup as you attach it (forces some air out to create better suction)

Extensions

  • How big does the balloon have to be to make the cup stick?How many cups can ‘stick’ on one balloon?
  • How many cups can ‘stick’ on one balloon?
  • Try this experiment with a coffee mug or a cup with water in it. Try to dislodge the cup with your finger.