Discover: Air Under Pressure

Pressure is the force of something pushing against a surface.

  Video

Watch this ADLC Video Lesson on pressure:

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Gases such as air can be compressed, but liquids such as water, cannot be compressed.

  Try This!

If you take the cap off a plastic bottle, but hold your hand over the opening, you can still squeeze the sides of the bottle together. If you screw the cap on with only air in the bottle and squeeze, what will happen? Now, fill the bottle completely with water, replace the cap and squeeze the bottle. What do you feel now?

When you squeezed the open bottle, you forced some of the air out of the mouth. When you placed the cap on the bottle and squeezed again, there was no place for the air to go, but you were able to squeeze the bottle together. In other words, you were able to compress (or squeeze together) the air inside the bottle. However, when you filled the bottle with water and capped it, you could not squeeze the bottle very much at all because you could not compress the water inside.

As you work through the activities in this section, think about what the air might be doing. Remember that, although you cannot see it, air takes space, and exerts pressure!

  Notebook: How can air be used to lift a book?

  1. Click here to download the instructions for this activity. 
  2. Complete the activity and save to your Flight Notebook.           


  Check Your Answers

By blowing air into a sealed Ziploc bag, you produce enough air pressure to lift a book.  The air particles inside the bag are pushing outwards on the bag trying to get out.  If there is no way for them to get out of the bag, the air pressure continues to build until the bag is full of air.  The pushing of these air particles lifts the book