Activity: Skimming stones

Oliver and his collection of skimming stones.

On SCOPE's water sports episode, you saw Oliver skipping stones across water. Here’s how he did it. Watch the clip.

What you need

• A collection of small, flat, round stones
• Somewhere big, calm and watery to skim them.

What to do

1. The stone has to hit the water at a really flat angle – about 20 degrees usually gives good results. Throw from a little above ankle height directly away from your body.
2. For best results flick your wrist before release to get the stone spinning horizontally.
3. What kind or rocks gave the best results?
4. Did you get near the world record (40 skips or bounces)?

What's happening?

In lots of water sports surface tension plays a big role. But you don't need to be playing a water sport or be a scientist with specialised equipment to conduct an experiment to do with surface tension. Skimming stones may be a great pastime, but it's also a perfect example of science at work.

First things first – your stone. It needs to be as smooth as possible with an oval shape. Anything jagged or too spherical will sink, not skim. It should be light enough that you can throw it accurately, but heavy enough that it is not affected by breezes or turbulence.

So what's actually happening to make the stone skip across the water?

When the stone hits the water, the surface tension (which acts like a flexible membrane) exerts a reaction force against the stone giving it an upward lift. The spinning makes it less likely that the stone will actually break the surface tension. But there are forces working against you too: water friction (or drag) and air resistance. These will slow your stone and change its angle. Surface tension will be overcome and your stone will sink.

The same forces which make a stone bounce off the surface of a lake can also affect other moving objects. When a spacecraft is entering a planet's atmosphere, it is possible for it to skim off the top of the atmosphere and bounce back out into space.