Name: Matthew H.
How do rocks skip across water?
As the rock hits the water, it must be tilted up somewhat or at least the
front edge must be bevelled. Otherwise, it can dig its way into the
water, ending its skipping career.
Then when the rock strikes the water, it pushes the water down. By
Newton's Third Law (For every force there is an equal and opposite
reaction force) the water exerts an upward and slightly backward force on
the rock. The upward force pushes the rock back up into the air and the
backward force slows the rock, hopefully slightly.
Usually the rock is given a strong spin so that, like a gyroscope, it
tends to keep its orientation. I used to be reasonably skilled at
skipping stones (due to much practice). My record, I believe, is 10 skips...
Best, Dick Plano, Professor of Physics emeritus, Rutgers University
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Update: June 2012