Cocentric Water Rings
Name: Darrin C.
Please explain to me why when you throw a rock in water
it continues to make rings in the water for a while instead of just one
ring (which seems logical since there was only one penetration)? How do
they keep generating from the same spot?
When the rock penetrates the water it creates a cavity -- a low spot in the water
surface. When the displaced water rushes in to fill the cavity, it collides with
itself and causes the water surface to have a bump on it -- a high spot in the surface.
When the bump collapses, it pushes water away, thereby creating another (smaller)
cavity. The process repeats itself until the energy of the original displacement
dissipates through intermolecular friction between water molecules.
Good observation. When the rock falls into the water it displaces a volume
of water (approximately equal to its volume, since water is essentially
non-compressible) in the vertical direction above the surface of undisturbed
water, increasing the potential energy of that volume of displaced water
P.E. = mgh. Gravity "pulls" this volume of displaced water back down, but it
overshoots the level of the undistrubed water because the potential energy
is converted into kinetic energy K.E.= 1/2 mv^2. Then a restoring force
"pulls" the water back up again because the potential energy in the "trough"
is less than the potential energy at the undisturbed level of the water. The
cycle then repeats several times. These oscillations die out because water
has a viscosity that dampens out the displacements generating heat (not very
much). It is this viscous flow that causes the ripples to expand away from
the point of impact.
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Update: June 2012