The following letter was published in last week’s New Scientist in response to the question “How do pebbles skim on water?”. Clearly the advice contained in this letter needs to be studied and acted on by anyone intending to take part in the forthcoming Stone Skimming Championships on Easdale Island. We therefore reproduce the letter in full below.
“• In a paper published in the journal Nature in 2004, the French physicist Lydéric Bocquet and his collaborators revealed some of the secrets of successful stone skimming. They found that the optimum angle of attack is 20 degrees. So, even when the stone is thrown horizontally, the leading edge should be 20 degrees higher than the trailing edge. This maximises the number of jumps by limiting the contact time between the stone and the water, which is proportional to the energy dissipated.
The thrower also imparts a spin to the pebble, providing a gyroscopic effect that stabilises its flight and preserves the original angle of attack when it bounces. In the absence of spin, the water would impart a torque on the stone and, because the trailing edge is the first to make contact with the water, this would tend to make it tumble.
The actual physics of stone skimming is not yet perfectly understood. However, the bounce could be understood as a result of the conservation of momentum and Newton’s third law: when the stone exerts a force on the water, the water exerts an equal and opposite force on the stone. This lifting force is proportional to the density of water, the surface area that is wetted and the square of the forward speed of the stone.
[Hello?? Are you still awake at the back there? Ed.]
Also, the bow wave created ahead of the stone when it strikes the liquid might act like a waterski jump, helping to launch the next hop. This minimises the contact time between the stone and the water, which in turn maximises the number of jumps.
Although ensuring the optimal angle of attack as the stone strikes the water, and imparting just enough spin to maintain stable flight are important, there are other factors. Selecting the correct size and shape of stone and having a fast throwing arm are examples.
Given that the urge to skim stones has been with us for thousands of years and the rules – getting the greatest distance or number of bounces – have remained unchanged since the ancient Greeks, perhaps this should become an Olympic sport.