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Balloons

Air and fluids · level 2, lesson 5 of 6

A question to start

A balloon you blow up yourself falls to the floor. But a helium party balloon floats up to the ceiling. What is the difference?

The idea

Air pushes up on things, just like water does, only much more gently. Air is a fluid: a stuff that flows, like water.

Things float in air if they are lighter than the same amount of air.

  • Your breath is air, so a balloon you blow up weighs a little more than the air around it (it has the rubber too). It falls.
  • Helium is a gas much lighter than air. A balloon full of helium is lighter than the same space full of air, so the air pushes it up.
  • In a hot-air balloon, a burner heats the air inside. Hot air is lighter than the same amount of cold air, so the balloon rises.
breath: fallshelium: riseshot air: rises

Worked examples

1. A helium balloon that floats away keeps going up into the sky, until the air gets too thin to push it higher.

2. The next morning, a helium balloon often sinks. Helium leaks slowly out through the rubber.

3. The burner. A hot-air balloon pilot turns the burner on to go up, and lets the air cool to come down.

Try it

  1. Why does a balloon you blow up yourself fall?
  2. Why does a helium balloon float up?
  3. How does a hot-air balloon go up?
  4. Why does a helium balloon sink after a day or two?
Answers
  1. It is full of air like the air around it, and the rubber makes it a little heavier.
  2. Helium is much lighter than air, so the air pushes it up.
  3. The burner heats the air inside, and hot air is lighter than cold air.
  4. The helium leaks out.

Common mistake

"Helium balloons are pulled up." Nothing pulls them up. The air around them pushes them up, because they are lighter than the air they push aside.

Where it leads