Re: Do ocean waves have mass?
- From: "George Dishman" <george@xxxxxxxxxxxxxxxxx>
- Date: Sat, 17 Jun 2006 15:24:15 +0100
"Rob" <rloldershaw@xxxxxxxxxxx> wrote in message
news:1150470598.187895.291430@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
George Dishman wrote:
Consider a slight variation on your question, what
about a sealed box have full of water. If this is
motionless it has some mass. If we change that
so that there is a standing wave in the box, it
has extra energy so the box and contents would
have greater mass.
Looking inside, at any instant some of the molecules
are moving due to the passage of the wave. The
extra mass they contribute is due to their kinetic
energy.
So if the system of box and contents has increased
mass but the mass of the inividual components is
the same whether the wave is present or not, the
extra mass must be attributed to the wave. You
could extract the energy stored in the wave and
the increased box of the box is E/c^2.
Hi George,
Hi Rob,
Very interesting! So let's see if I get it. The wave's energy is
passing to the water molecules and increasing their kinetic energy ..
"Wave" is a name we give to a variation of height
on the surface which is moving in some way. There
is also a variation of potential energy as David
mentions and variation of pressure at the bottom
of the tank. The wave has no separate existence
other than as those combined effects.
and
that KE somehow gets turned into added mass. Would you please remind
me how kinetic energy gets turned into mass? I think I know, but I
want to hear someone else verify it.
It's not a question of turning one into the other,
mass is another name for energy bound in a system.
For example the mass of a proton is not the same
as the sum of the quarks that compose it.
The other thing that is important to me is: What is the relative
magnitude of the ratio of the wave's starting energy to the amount of
energy converted into mass.
As I said above, if the energy in the wave is E
then the extra mass is E/c^2.
If the box weighs something like 500 lbs,
how much would the mass increase if a 1 ft wave were created
David cited a page where you can get a formula
for the energy. While the wave is present, that
energy forms part of the mass of the system.
and allowed to dissipate.
If the box is insulated then as the wave dissipated
through friction, it will heat the water. Eventually
you have motionless water at a higher temperature.
The energy E is still in the box but in the form of
heat rather than bulk motion but since the energy
hasn't been lost, the mass still includes E/c^2.
Also, where does to mass disappear to, since the
box has to go back to its original mass eventually? Crude
approximations allowed.
If the sides of the box aren't perfectly insulating
then the heat will leak out into the surroundings.
The mass of the box decreases while that of the
surroundings (whatever that is) increases by the
same amount so that the total is conserved.
George
.
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