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How do boats float?

Multicolored illustrated boats float in blue water.
Tetiana Lazunova

How do big cargo ships and ferries float, even though they are so heavy? Why do boats float but stones sink? How do paddles make boats move? What’s inside those enormous container ships? We learn about the physics of floating with Fahad Mahmood, professor of physics at the University of Illinois Urbana-Champaign. If you do any of the activities we mention in the episode, send us your videos!

Download our learning guides: PDF | Google Slide | Transcript

  • Physics is the study of matter and energy. 
  • Most stones are very heavy. They have a lot of mass for their volume, meaning they’re very dense. Stones are dense. (Mass is not the same thing as weight, because weight depends on gravity, while mass is consistent whether you’re on earth or in outer space, but if you’re unfamiliar with mass and volume you can think of them roughly as weight and overall size.)
  • Boats are heavy, but their weight is spread out over a larger volume. The overall density of the boat is not as high.
  • If you’ve ever sat down in a bathtub full of water, you may have noticed that the water level rises. Whatever object you put in water is displacing that same amount of water. 
  • With something like a boat, the displaced water is exerting force upward against the boat. The upward force is equal to the weight of the boat, so the boat floats. (Whereas the rock is small and heavy, and doesn’t displace (push aside) enough water to allow it to float.)
  • The hull of a boat–that’s the bottom part that is usually under water–is mostly empty and full of air. That helps reduce the overall density of the boat. 
  • A boat moves because something is pushing it forward through the water. For a sailboat, it’s the wind pushing against the sail and moving the boat forward.
  • Other boats have a propeller underneath the water that is pushing the boat forward using a motor. A propellor is like a fan, and when it spins it is pushing water backward. 
  • When you use a paddle you are also pushing water back which moves the boat forward. But with paddling, you’re using your own energy instead of gasoline or electricity. 


Physics Van

Condensed Matter

Jane Lindholm is the host, executive producer and creator of But Why: A Podcast For Curious Kids. In addition to her work on our international kids show, she produces special projects for Vermont Public. Until March 2021, she was host and editor of the award-winning Vermont Public program Vermont Edition.
Melody is the Contributing Editor for But Why: A Podcast For Curious Kids and the co-author of two But Why books with Jane Lindholm.

But Why is a project of Vermont Public.

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