Skip to content

Water pushes up

Air and fluids · level 2, lesson 2 of 6

A question to start

Try pushing a beach ball under the water in a pool. It fights back and shoots up as soon as you let go. What is pushing it?

The idea

Water pushes up on anything you put in it. Scientists call this push upthrust.

  • Gravity pulls a thing down.
  • The water pushes it up.

If the water's push up is as big as the pull down, the thing floats. If the pull down is bigger, it sinks.

The more of something you push under the water, the harder the water pushes back. That is why the beach ball fights you more and more as you push it deeper.

push downwaterpushes uphand

Worked examples

1. Lifting a friend. In a swimming pool you can easily lift a friend you could never lift on land. The water is holding up most of their weight.

2. A stone on a string. Hang a stone from a spring or an elastic band, then lower it into water. The band gets shorter. The water is pushing up, so the stone pulls down less.

3. Floating on your back. Lie still in the water, with your lungs full of air, and the water holds you up.

Try it

  1. Which way does water push on things in it?
  2. Why is it easy to lift a friend in a pool?
  3. What happens to the beach ball when you let go under water?
  4. A stone hangs on an elastic band. What happens to the band when the stone goes into water?
Answers
  1. Up.
  2. The water pushes up on them and holds up most of their weight.
  3. It shoots up, because the water's push up is much bigger than its weight.
  4. It gets shorter, because the water pushes up on the stone.

Common mistake

"Things that sink feel no push from the water." Water pushes up on a sinking stone too. The push is just not big enough to hold it up.

Where it leads