The clay block provides the force of resistance in the photo, since the weight it exerts on the right end of the ruler on which it is supported makes the system stay in balance. The weight of the spring balance (object A) is transferred to its point of support on the ruler, while the wood and clay cube only exerts one point of support, but does not exert any force on the ruler. The weight exerted by the clay block downward counteracts the weight of the spring balance.
