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Float or sink?

Air and fluids · level 2, lesson 1 of 6

A question to start

A big wooden log floats down a river, but a tiny pebble sinks straight to the bottom. Surely the heavy thing should sink?

The idea

When you put something in water, it either floats at the top or sinks to the bottom. You can test it.

Get a bowl of water and some things to try: a cork, a coin, an apple, a stone, a plastic toy, a metal spoon, a crayon. Guess first, then drop each one in.

floats: cork, apple, ducksinks: stone, coin, spoon
Floats Sinks
cork coin
apple stone
wooden log metal spoon
plastic duck glass marble

Look at the table. A log is much heavier than a coin, but the log floats and the coin sinks. So being heavy does not decide it. What a thing is made of matters, and so does its shape. You will find out why in this level.

Worked examples

1. The apple. Most people guess it sinks. It floats, because it has lots of air inside.

2. The crayon. A wax crayon floats or only just sinks. Wax is close to being as light as water for its size.

3. A bottle. An empty bottle with its lid on floats. Fill it with water and it sinks lower. Fill it with sand and it sinks to the bottom.

Try it

  1. Does a cork float or sink?
  2. A log is heavier than a coin. Which one floats?
  3. Is "heavy things sink" always true?
  4. An empty bottle floats. What happens when you fill it with sand?
Answers
  1. It floats.
  2. The log.
  3. No. A big log is heavy, but it floats.
  4. It sinks.

Common mistake

"Big things sink, small things float." A ship is enormous and floats. A grain of sand is tiny and sinks. Size alone does not decide it.

Where it leads