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Skipping stones

By Pat, 21 February 2013 Activity

Have you ever wanted to know how to skip stones across a lake? Wonder no more! with our handy guide you’ll be skipping in no time!

Container of small stones.

You will need small stones of different shapes.

outdoor hazard iconSafety: You’ll probably have to travel to find a suitable spot for this activity. Take an adult with you. Before you go, check the weather forecast and dress for the conditions. Protect yourself from the sun by wearing a hat, shirt, sunglasses and sunscreen. Wear insect repellent to avoid bites.

You will need

  • Small stones of different shapes
  • Body of flat water – such as a lake or pond

What to do

When following these steps keep careful watch so that you do not throw stones at people or animals.

  1. Drop a stone into the water. What happens?
  2. Take another stone and throw it into water. What happens this time?
  3. Hold a third stone in the crook of your index finger.
  4. Hold your hand above your head.
  5. Facing the water, swing your arm down and throw the stone so that it spins. Can you make the stone skip across the water?
  6. Try different shaped stones and different throwing techniques – how many times can you make a stone skip? Leave a comment below telling us your record and stone skipping tips!
Hand holding a pebble over water.

Drop a stone into the water. What happens?

What’s happening?

Water is a liquid, but objects that hit the surface will still feel quite a force. Water molecules on the surface are attracted more strongly to other water molecules than the air. This creates what is known as surface tension.

In many situations, objects will still fall through the surface of the water anyway due to gravity. In some other situations, such as skipping stones, they don’t. There are a few things that need to happen for a stone to skip across the surface. Firstly it has to hit the water at an angle – around 20°. Secondly, the stone has to be spinning. The spin stabilises the stone and means it moves more smoothly through the air, helping it keep the same angle. A Frisbee works on the same principle.

Hand holding a stone in the crook of the index finger.

Hold a stone in the crook of your index finger.

The trailing (or back) edge also has to hit the water first. When the stone hits the water, the trailing edge skids along the surface and drags some water with it. This drag creates a small crest of water that the stone then collides with. With enough speed, the ‘sticky’ water doesn’t have enough time to move aside, sending it back into the air. Gravity once again pulls the stone down towards the water and the same thing happens. The more the stone spins and the more speed it has, the more likely it is to skip further. The world record – set in 2007 – is an astonishing 51 skips!

Every time the skipping stone interacts and bounces of the water, it loses energy. As it loses more energy, its spin slows down, it drags more water and eventually it is slow enough to fall through the surface. Unless it skips to the other side of course!

Hand throwing a stone into the water.

Try to spin the stone as you throw it.

Applications

Similar things can happen with other fluids, including air. When a spacecraft re-enters the Earth’s atmosphere, it does so at an angle so drag from the atmosphere slows the craft down. The angle it hits the atmosphere is important – if it isn’t just right, the craft might simply bounce off the atmosphere, like a skimming stone.

Drag from the atmosphere also generates heat through friction. Given the speeds of space craft, high temperatures can be generated. Again, if the angle of entry is wrong, the speeds will generate temperatures that are too high, that may damage the craft and endanger the safety of those inside.

Stone kipping off the surface of a lake.

Can you make the stone skip on the surface of the water?

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2 comments

  1. I threw a stone once that we all agreed on as doing 15 skips. How could you possibly get one to skip 51 times?

      Reply
  2. More on the skipping stones story: Dr Carl said in one of his books that a common problem is throwing the stone from too low is no good, and one should throw it from shoulder height. This doesn’t work for me: I throw them from as close to water level as possible

      Reply

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