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Name: Darrin C.
Status: student
Age: 15
Location: N/A
Country: N/A
Date: 6/30/2004
Question:
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?

Replies:
Darrin, 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.

Regards,
ProfHoff 859

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.

Vince Calder


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