Monday, May 10, 2010

Air Pressure: A Cup, An Index Card & Some Water

This is a pretty well-known, classic experiment when it comes to air pressure.

If you aren't familiar with it, you should be - it's easy to do in any setting.

Fill a cup part way with water. Place an index card on top (make sure the card is large enough to completely cover the cup).

Pick up the cup, placing one hand on top of the index card.

Turn the cup over, holding the index card up with your hand.

Ask your students what they think will happen if you remove your hand that's holding the index card.

Then do it!

There really is water in the cup, I promise! My hand got in the way of the water line...

Air pressure doesn't just push down, it pushes on things in all directions. In this case, air molecules are pushing on the index card. They exert more pressure than the water pushing down does, so the card stays up.

If something breaks the seal, or the index card gets too wet (as it will after enough time has passed), the card will fall and water will gush out!


  1. You could be introducing a misconception if you neglect to mention the vacuum above the water in the inverted cup that reduces the downward force of the water.

    I really like your blog! I'm a first year science teacher and enjoy learning about and being reminded of these fun activities.

  2. I've seen that before, and always think it looks so cool. I also like being able to put a cup of water upright in a pool or bathtub, and it doesn't fill up, under the same principle.

  3. would the index card fall if the cup were turned sideways?

  4. Thank you for the story.
    With your permission I will use it in my Stress Mgmt Workshop tomorrow !