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Why Do Things Float?

What Makes Things Float or Sink?

Why do things float?

Is it true that things float better on saltwater than freshwater? And how can pumice, a kind of rock, float on water?

On this page, I've created some fun videos to help you understand the answer to all these questions. By the time you're done, you'll be a flotation expert!

At the end, I've got a "Why Things Float" self-test. You can take it as many times as you like!

Poll: Why Do Things Float? - Just for fun, let's have a quick poll before we get started.

Video: Why Do Things Float? - A Fun Video That Will Make You a Flotation Expert!

Have you ever heard the story of Archimedes, an ancient Greek scientist? One day, he noticed the water level rising when he got into his bathtub. He was so excited, he hopped up and ran out into the street stark naked, shouting, "Eureka!" ("I've found it!")

What was he so excited about? Watch this video, and you'll understand what he figured out!

Archimedes' Principle

"Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object."

-- Archimedes of Syracuse, 3rd century BC, "On Floating Bodies"

What Are Density, Volume, and Mass? - Stuffing, Space, and Stuff!

What is density?

What is density?

At left: a beaker of green Jell-O. At right: a beaker of green Jell-O with air bubbles in it.

The Jell-O fills the same amount of space, or volume, in both beakers, but the left-hand beaker holds more stuff, or mass. That means it's more dense.

Volume means how much space, or room, something occupies.

Mass means the amount of stuff (matter) in something. Mass is almost like weight, except for one thing: your weight varies depending on gravity, whereas your mass is constant. If you weigh 100 pounds on Earth, you'll weigh 16.6 pounds on the Moon!

Density is the ratio of Mass to Volume. In other words, "how much stuff is crammed into one space?" A bucket full of marbles is more dense than a bucket of ping pong balls, because marbles have more STUFF or matter in the same amount of space.

So what? Things denser than water sink. Things less dense than water float!

FUN LINK! Your Weight on Other Planets!

Do Things Float Better in Saltwater? - Here's a Magic Trick You Can Do at Home!

In this experiment, I used two plastic hair clips and two glasses of water. I stirred a lot of salt into the right-hand glass. See if you can guess what's causing this before reading my explanation after the video.

Why Things Float Better in Saltwater Than Freshwater

Why Things Float Better in Saltwater Than Freshwater

Scroll to Continue

Explanation: Why Things Float Better in Saltwater Than Freshwater

Displacement Strikes Again

Remember displacement and our rubber duckies?

When you put something in water, it has to push (displace) enough water to make room for its entire volume before it can sink.

When you dissolve salt in water, you've just added more STUFF, or MASS, to the same volume of water. Saltwater weighs the weight of the water plus the weight of the salt.

Salt isn't very heavy, but it's enough to make the difference when you drop something that just barely has enough weight to sink in freshwater.

After trying paper clips, medicine bottle caps (they work, too), and a few other household items, I found that plastic hair clips were just barely denser than water (they sink, but slowly), so the salt made the difference.

If I'd wanted to be strictly scientific, I should've weighed the hair clip and then added exactly the same weight of salt. It took a lot of salt, by the way-- this is like Sweet Tea with salt instead of sugar (which should work too, come to think of it.)

And you know what's cool? When boats go from the open ocean into a river, they're passing from saltwater to freshwater, so they settle a few inches -- or even a few feet-- lower!

Video: Pumice - The Rock That Floats on Water! - Why Does This Happen?

See if you can guess why pumice floats before reading my explanation after the video.

Pumice Floating in Water

Pumice Floating in Water

Explanation: Why Pumice Floats

Volcanoes Fart!

When volcanoes erupt, they release gases as well as molten rock. If the lava is thick and sticky, the gases can't escape, so the rock cools and hardens with those bubbles trapped inside.

Pumice is the solidified form of frothy lava that bubbles over -- or explodes! -- like the fizz in a bottle of champagne. Pumice is more gas bubbles than it is rock!

Since pumice is almost all air bubbles, it has a lot of volume, but very little weight. It's more air than it is rock! So a four-inch-square chunk of pumice weighs much less than a four-inch-square chunk of water.

Remember, when something sinks, it has to push water out of the way to make room for itself. It can't push more than its own weight. So it pushes a little water out of the way-- water equal to its weight-- and there it stops, only partway down!

(Note: the videos of the new Krakatoa erupting are at the bottom of this page.)

Quiz: Why Things Float, a Self-Test

For each question, choose the best answer. The answer key is below.

  1. Which is an example of displacement?
    • You can't find your car keys.
    • Ice cream just fell on your shoes.
    • Some jerk just cut in line and took your spot.
    • Your car's GPS gets confused and can't tell where you are.
  2. Which of these is more dense than water?
    • A Rubber Ducky
    • Pumice
    • Saltwater
    • Bubble Wrap
  3. Why do things float better in saltwater than freshwater?
    • The salt in the water makes it weigh more, so it takes more weight to displace it.
    • The ocean is full of 400 million years of fish pee, and things float on top trying to avoid it.
    • Saltwater is oily from all the oil spills over the past few centuries, so it's slippery.
    • Saltwater is warmer than freshwater, and heat rises.
  4. Density is...
    • Height divided by weight (how skinny someone is)
    • Age divided by shoe size (how likely you are to have fallen arches)
    • Mass divided by volume (how much stuff is crammed into one place)
    • Being really stupid (head is full of rocks)
  5. Which of these is the most dense?
    • A piece of wood
    • A duck
    • A witch
    • A piece of gold
  6. What did Archimedes discover?
    • Displacement
    • The square of the hypoteneuse equals the... er... what's a hypoteneuse again?
    • He needed to bathe more often
    • Feta cheese
  7. Volume is...
    • The shape of something
    • How much space it occupies
    • The fluffiness of hair
    • How much mass (stuff) is packed into one place
  8. Why do ships float?
    • They have a lot of open space in them, so they displace a lot of water.
    • Water is heavy. A ship-sized volume of water weighs more than the ship.
    • Gravity pulls down both the boat and the water it displaces, but the displaced water has more mass.
    • All of the above.

Answer Key

  1. Some jerk just cut in line and took your spot.
  2. Saltwater
  3. The salt in the water makes it weigh more, so it takes more weight to displace it.
  4. Mass divided by volume (how much stuff is crammed into one place)
  5. A piece of gold
  6. Displacement
  7. How much space it occupies
  8. All of the above.

Archimedes Screws Up! - Emily Demonstrates Her Archimedes' Screw

Archimedes was a clever guy. He figured out a lot of ways to use water power. He invented this "screw" design as a way to lift water up. Here's a modern version made by a clever girl!

Screw Power!

Many power plants in Europe use a "Reverse Archimedes Screw" to generate clean energy without harming fish! Instead of using a screw to lift water, they put the screw in moving water (a river) and let the water turn the screw. The open space inside the screw is big enough for fish to swim through.

Video: Volcanic Ash on a Lake, Eruption in 2011

This Stuff Is Pulverized Rock -- But It Floats!

Here's a video taken from Bariloche in Argentina, across the border from Chile's Puyehue Volcano that erupted June 4, 2011, throwing a huge amount of volcanic ash (pulverized rock) into the sky. Some of that ash was pumice, and it covered this lake. Yes, this really is a lake covered with rocks.

Videos of Anak Krakatau ("Son of Krakatoa") - A baby volcano that's grown up where the old island sank

As promised, here's a few videos of the new Krakatoa Volcano. You can see the pumice coming down in some shots.

I hope you found my "Why Things Float" lesson helpful. Feel free to leave comments here.

If you've got any classmates trying to understand this stuff, please share this page with them!

Guestbook - Leave Your Comments Here!

uriel on March 14, 2015:


james-joel-33483 on October 28, 2013:

awesome It is very helpful!!!!!!!!!!!!!!!!!! :)

anonymous on September 02, 2013:

These videos are incredible. I look forward to showing them in class in Jesup, GA. Your explanation of why an object floats is so much better than mine. The kids have some experience now and should be able to relate to your explanation in a snap! I can't wait to hear the "aha's" that are going to take place.

anonymous on March 02, 2013:

Lady! You are defintiely a good teacher!

anonymous on January 15, 2013:

This was a terrific website!!! This helped a lot on my science fair project! Ruff Ruff!!!

anonymous on May 30, 2012:

I really did find your work amazing. I am a Spanish Science and Math teacher and my pupils study this subject partially in english. This materia was easy enough for them ti understand Archimedes' principle and why things float. Thanks a lot for sharing it!

David Dove on September 05, 2011:

My brain is beginning to hurt, thank you.

anonymous on June 20, 2011:

I have been looking all over the net for good information on rocks. Thanks for this info about pumice floating. It was exactly what I needed. And the videos are great.

Joan Hall from Los Angeles on June 10, 2011:

So my son wanted to look up information on how things float. I Googled the topic and had the pleasant surprise of seeing that there was a Squidoo lens ranking on the subject. This was followed with a bit of trepidation as I wondered (while the page slowly loaded) who had written the lens and whether it would have worthwhile information. And then delight and relief when I finally saw YOUR bright smile!

And then admiration as we enjoyed your well-presented explanations and terrific videos. JG loved it and got 7/8 on the quiz. Thank you for this wonderful lens.

efriedman on May 24, 2011:

Nice video demonstration of floating in fresh vs saltwater. Good job overall.

Sanam on April 22, 2011:

Quite Interesting.

anonymous on January 26, 2011:

Interesting stuff!

anonymous on January 13, 2011:

great and clear.

thanks why do things float!

anonymous on December 08, 2010:

that was so cool i can't believe it and thanks now i know what scince project i should do thanks very much!!!!!

Ellen Brundige (author) from California on November 14, 2010:

@anonymous: I think you are right. Edited, and thank you for the correction!

I wish I could remember the Greek for "embarrassed."

anonymous on November 14, 2010:

This was interesting. I think number 6 should be A instead of B.

Jeremy from Tokyo, Japan on November 14, 2010:

Clear lesson on floating with really nice videos. I'm going to have my son watch the first one this week. He has to outline an oral presentation so that will be it. Blessed.

Kylyssa Shay from Overlooking a meadow near Grand Rapids, Michigan, USA on September 10, 2010:

You did a great job explaining why things float under all different circumstances. Science is awesome!

Lisa Marie Gabriel from United Kingdom on May 29, 2010:

This is neat - and I didn't do too badly on the quiz either, so you made the points well. Thank you for teaching me something new :)

sheriangell on May 28, 2010:

Excellent effort. I can't wait to have my kids read this.

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