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[Phys-L] Re: buoyancy



-----Original Message-----
From: Forum for Physics Educators [mailto:PHYS-
L@list1.ucc.nau.edu] On Behalf Of Anthony Lapinski
Sent: Tuesday, February 01, 2005 10:53 AM
To: PHYS-L@LISTS.NAU.EDU
Subject: buoyancy

Objects float in water because the weight of the displaced
water equals
the object's weight. I put a beaker of water on a scale and ask
my
students what will happen to the reading if I dip my hand in.
Most say it
will stay the same, but are surprised to find the reading
actually
increases -- by the weight of displaced water my finger pushes
away. I
ask
if a live fish were placed in the water, would the reading
change? Yes -
-
by the weight of displaced water once again. But what if the
fish dies
and
goes to the bottom of the beaker? The scale reading should
reflect
this.
It should increase by the actual weight of the fish. But does
the weight
of displaced water matter?


Actually, when you put a live fish in the water the scale reading
increases by the weight of the FISH, which is the SAME as the
weight of the water displaced. When the fish dies, the scale
reading doesn't change until the fish decomposes and the gases
bubble out of the water.

Bob at PC