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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?