Question:
What is the scientific explanation of how a ripple is
formed when something is dropped onto water? Why not just one wave?
Replies:
The ripple effect is similar to a spring. Unless a spring is carefully
"damped" the weight on the end of a spring will oscillate up and down. A
similar thing happens when a weight is dropped into a pool of liquid. The
weight "pushes" the liquid down, but because the liquid has been displaced
from its equilibrium point it recoils back up. The exchange of potential
and kinetic energy back and forth causes a ripple in the liquids surface
back and forth until friction causes the waves to decrease in amplitude.
Vince Calder
Dear Susanne,
The train of ripples is caused by the oscillation of the water at the point
the stone is dropped. Initially, the stone depresses the water under it.
The depressed water is then lifted by the buoyancy of the water around it.
Since the top of the depressed water is under the top surface of the rest of
the water, the buoyant force on it is greater than its weight and so the
water is rapidly accelerated upwards. When it reaches its original level
(lined up with the top of the rest of the water), it is moving upward and
will continue some distance above the rest of the water. Eventually, of
course, its unsupported weight causes it to stop and start falling down. It
will again overshoot its equilibrium position and continue downward some
distance. This oscillation continues until the energy lost to the viscosity
of the water finally brings it to rest in its original position.
The up and down oscillations of the water which was originally under the
stone cause the waves to form which propagate outwards in a circular
fashion.
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