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Question: A steel nail sinks. But a steel ship floats. How can that be?

Floating is a lot like a shoving match. The boat is pushing down on the water and the water is pushing up, holding the boat up.

If the water pushes up harder than the boat pushes down, the boat floats.

If the boat presses down harder than the water pushes up, it wins the shoving match and sinks. (It's not much of a victory.)

Which ever pushes harder, wins. Water pushing up. Boat or nail pushing down.

A steel boat hull is not as heavy as the amount water that the boat hull pushes away, or displaces. Inagine the boat as making a hole in the water. If the boat weighs less than the water it would take to fill that hole, it floats.

Pushing Water Away - Displacement

Big jar or bowl - soda bottle - water - marking pen

Fill the jar half full of water. Make a mark on the jar where the top of the water is. Fill the soda bottle half full of water and mark it, too.

Look at the marks you made.

Put the little jar into the big jar and push down. See if the the marks still match the top of the water in both the soda bottle and the big jar.

Your soda bottle pushed away the water in the big jar. That is why the water line moved up.

The water in the soda bottle stayed the same.

The waterline in the big jar moved up. The difference between the marked waterline and the new one is important. It's how much water was pushed away or displaced.

Floating vs Sinking

Bucket - aluminum foil - water - hammer - help and permission from a grown-up

Fill the bucket with water. Make a little boat with the foil and put it on the water. What happened?

Take the foil out of the water and make it into a little ball. Then take it outside to a sidewalk and hammer the ball until you cannot make it any smaller. Fold it over and hammer it more.

Put the little foil ball in the water. What happens?

SO WHAT: The little foil boat floated. That is because it was not as heavy as the water it pushed away.

Then you made the foil smaller and it could not push away enough water to float. The ball weighed exactly the same as the boat. But it got so small it was heavier than the water it could push away. So it sank.

Even though a ship is very big and very very heavy, it is not as heavy as the water it pushes away. That is why a big ship made of steel can float! (Don't forget to use the water from your experiments for the plants outside.)

P.S. from Jax: If you live near a lake or river, visit the docks to see the boats. Some boats are made of concrete which is way heavy. Cement boats float because they weigh less than the water they push away.

Copyright © 1999 - 2004 Jok R. Church - All Rights Reserved.

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