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?
I tried this with a golf ball, each weighed (massed) separately. When I
dropped the ball in, the scale only increased by the ball's weight. There
is still a buoyant force upward, and this balances the (downward) weight
of displaced water. So the reading should only increase by the object's
weight. Is this correct reasoning?