Observe: Lighter Than Air

Have you looked into the sky at hot air balloons and wondered how something the size of a house can float on air?

Well, consider this! You know something about balloons.

  • If you fill a balloon with helium, it floats into the air, and unless you hold that string, it will float away to the ceiling or into the sky when you are outdoors.
  • If you blow up a balloon with your breath, it will bounce, but it will sink to the floor or ground.

What is the difference between the two balloons? 

  • The helium is less dense (or lighter) than air, which means the balloon is able to float on the air.
  • Your breath has almost the same density as the air around you, so it will not float. 

Hot air balloons work on a similar principle.  Heating the air inside the balloon causes it to expand and become less dense.  This is the key to producing buoyancy that lifts the balloon against the pull of gravity.  Hot air balloons are so large because they need a large amount of hot air to have enough buoyant force to lift the basket with passengers in it. The heavier the load the balloon is carrying, the more hot air it needs to cause it to rise.

  Notebook: How can hot air balloons float on air?

  1. Click here to download this activity. 
  2. Complete the activity using the links provided (in the activity and below).
  3. Save to your Flight Notebook.    

Activity Links:


  Check Your Answers

Topic: Buoyancy

(What is the website about?)

Main Ideas: Complete the blanks with the missing words
Buoyancy
1.  What is buoyancy?
  • Buoyancy is an upwards force exerted by all liquids and gases.
  • Objects that are less dense will float.
2.  What makes a hot air balloon float?
  • Warm air is less dense than cold air is.
  • When the balloon is filled with hot air, it becomes less dense than the air around it; therefore, it can float (due to buoyancy).
Click "Inflate Balloon".
3. Why does the balloon start rising?
  • The helium is less dense than the air around it.
  • Higher air pressure under the balloon pushes it up.
4.  Why does the balloon stop rising?
  • As the balloon goes higher, air pressure begins to decrease (so does buoyancy).
  • When the weight of the balloon equals the buoyancy of the air around it, it stops rising.
Lift: Lighter than Air ( Watch the video.)
5.  What happens when the balloon is filled with cold air?
  • The balloon falls.
6.  What happens when the balloon is filled with hot air?  Why?
  • It rises because the hot air is less dense than the air around it.
  • Buoyancy pushes the balloon up.
7. Why does the balloon fall down?
  • The air inside cools and becomes more dense.
  • As the balloon becomes heavier, gravity pulls it down.