Question

When an object is immersed in water why does its weight go down?

Answer 1

When a body is partly or completely immersed in water (or any other liquid), then:Loss in weight of body = Weight of water (liquid) displaced by the body = Buoyant force or upthrust exerted by water (any liquid) on the body. It was Archimedes who first observed that bodies lose their weight when immersed in water.

Answer 2

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An object in water looses "weight" because the water column beneath it pushes the object upwards. Now an object in water experiences a downward pull of gravity we call weight but the water follows newtons third law and exerts an upward force, which pushes the object upwards.