Author: BIGS 2001 (C. Bluck, J. Gans, Prof. W. Heimbrodt, Dr. Maddocks, S. Stallmann, M. Totzeck)
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This is explained by the law of action and reaction. In this case, the buoyancy force on the body immersed in the water glass is accompanied by a "down force" of the same magnitude acting on the container and therefore also on the balance.