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Heavy for its size

Air and fluids · level 2, lesson 3 of 6

A question to start

Hold a ping-pong ball in one hand and a golf ball in the other. They are about the same size, but one feels much heavier. Which one floats?

The idea

What matters is not how heavy a thing is, but how heavy it is for its size.

Compare a thing with the same amount of water:

  • If it is lighter than the same amount of water, it floats.
  • If it is heavier than the same amount of water, it sinks.

A ping-pong ball is mostly air inside, so it is much lighter than a ball of water the same size. It floats. A golf ball is solid, and heavier than a ball of water the same size. It sinks.

woodwatermetal lighter: floatsheavier: sinks
Same size as a cup of water Heavier or lighter than the water? In water
a cup of cooking oil a bit lighter floats on top
a cup of wood lighter floats
a cup of ice a bit lighter floats
a cup of sand heavier sinks
a cup of metal much heavier sinks

Worked examples

1. Ice cubes float in a drink. Ice is a little lighter than the same amount of water, so it floats with most of it under the surface.

2. Oil and water. Pour oil into water and it floats in a layer on top. It is lighter than water for its size.

3. The log and the coin. The log is heavy, but lighter than a log-sized lump of water, so it floats. The coin is small, but heavier than a coin-sized drop of water, so it sinks.

Try it

  1. A ping-pong ball and a golf ball are the same size. Which floats?
  2. Why does ice float?
  3. You pour oil into water. Where does the oil go?
  4. Is a thing that is heavy for its size likely to float or sink?
Answers
  1. The ping-pong ball.
  2. It is a little lighter than the same amount of water.
  3. It floats on top.
  4. Sink.

Common mistake

"Heavy things sink." Heavy for their size things sink. A big, heavy log floats because it is light for its size.

Where it leads