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.
What makes things float?
- 51% Density
- 5% Gravity
- 17% Displacement
- 1% Water wings
- 25% All of the above!
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!
"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!
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.
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!
What's Water's Weight?
Water's density varies by temperature.
When it boils into steam, water expands. When it freezes, it expands too! That means there's more mass (and weight) in a cup of water at room temperature than a cup of ice.
Water's weight also varies depending on what's dissolved in it. Saltwater is heavier.
That said, freshwater at sea level is about:
1 kilogram / litre, or
8.35 pounds / U.S. gallon
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.
Explanation: Why Pumice Floats
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
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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!
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!