Department of Energy Argonne National Laboratory Office of Science NEWTON's Homepage NEWTON's Homepage
NEWTON, Ask A Scientist!
NEWTON Home Page NEWTON Teachers Visit Our Archives Ask A Question How To Ask A Question Question of the Week Our Expert Scientists Volunteer at NEWTON! Frequently Asked Questions Referencing NEWTON About NEWTON About Ask A Scientist Education At Argonne Why Things Float?
Name:  Grace R.
Status: student
Age: 6
Location: N/A
Country: N/A
Date: 2000-2001

Which items float in water, which do not and why?


Did you ever notice that when something floats in water, part of it is actually under water? As it sinks (even a little bit) it pushes away the water until that amount of water weighs the same as the thing that is floating. If the thing you try to float is too heavy, it cannot push away enough water to be the same as how much it weighs. If that happens, the thing will sink.

Ask an adult to help you with an experiment (a test) that can show you how this works: Float a small plastic boat in water and notice how deep the boat sinks when it is empty. Then add pennies to the boat and watch how the boat sinks deeper and deeper the more pennies you add. The pennies make the boat weigh more and more. If you add enough pennies, the boat will sink deep enough so that water reaches the top and then the whole thing sinks.

Blow up a balloon and float it on water. It will not sink very far because it is not very heavy. If you look really close, where the balloon touches the water, you can see a little dent in the water under the balloon. That's the place where the water is pushed out of the way. If you try this test with a ball that is exactly the same size as the balloon, the ball will sink deeper before it floats. Because the ball is heavier than the balloon, it has to push more water out of the way before it can float.

It is not just how heavy something is that makes it float or sink. Look how heavy real boats are -- and they still float. Floating or sinking has to do with the amount of water pushed out of the way. Any boat will sink if you put enough stuff inside it -- just like your experiment showed. Small, heavy things like a marble or a rock cannot float because they cannot push enough water out of the way to be the same as how much they weigh..

So remember, anything that floats weighs the same as the water pushed out of the way.


The object is buoyed up (pushed up) by a force that is equal to the weight of the water that the object occupies that was previously occupied by the water. If you shape the object is in such a way that it occupies a volume of water whose weight equals that of the object, the object will float. If it occupies a volume of water whose weight is less than the weight of the object, the object will sink.

You can show this to yourself by taking a piece of aluminum foil and making a water-tight boat out of it. If you carefully put the boat in a dish or pan of water, you will see it float.

Now take the aluminum foil boat and crumple it up into a ball and put it back on the water. It sinks! There is the same amount of aluminum foil in both cases, but in the case of the boat, you shaped it so that it displaced a lot of water compared to the amount of water that is displaced when you crumpled the aluminum foil into a ball.

Vince Calder


Objects that are heavier than the same amount of water will sink. Things that are lighter than the same amount of water will float. If you push an empty bottle under water, you push a lot of water out of the way. That water would weigh much more than the bottle. The bottle will float. A rock put under water also pushes a lot of water out of the way. That water would weigh less than the rock, so the rock can sink.

Dr. Ken Mellendorf
Illinois Central College

Whether an item will float in water has to do with a property called density. Density relates to the weight of items that are of a specific size.

Things that have a lower density than water will float in water. This is because the item weighs less than the water that it displaces. Because the water is pulled (by gravity) towards the earth with more force than the item it flows around the item and pushes it out of the way so that it is closer to the earth. The result is that the item is further from the earth and thus floating on top of the water.

You can "model" this result using a shallow can full of small beads or BBs and a polystyrene foam packing peanut. Bury the peanut in the beads so you can't see it from the top. Now gently 'bounce' the can on the floor (drop it a small distance, like 1/2 inch) several times so that the beads move around, just like water molecules do. The peanut should come to the surface.

Greg Bradburn

Click here to return to the Physics Archives

NEWTON is an electronic community for Science, Math, and Computer Science K-12 Educators, sponsored and operated by Argonne National Laboratory's Educational Programs, Andrew Skipor, Ph.D., Head of Educational Programs.

For assistance with NEWTON contact a System Operator (, or at Argonne's Educational Programs

Educational Programs
Building 360
9700 S. Cass Ave.
Argonne, Illinois
60439-4845, USA
Update: June 2012
Weclome To Newton

Argonne National Laboratory