Both spring balances show the same reading because the force exerted by spring balance A on balance B is equal in magnitude and opposite in direction to the force exerted by balance B on balance A. This illustrates Newton’s third law of motion.
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Both spring balances show the same reading because they are each experiencing the action-reaction pair of forces according to Newton’s third law. The gravitational force exerted by the object downward on each spring balance is equal in magnitude to the upward force each balance exerts on the object, resulting in identical measurements on both scales.