The correct option is (c) 2 and 3 only.
Explanation
The elongation of a suspended string is governed by the tension generated within it, which balances the external forces acting on the suspended object. In this scenario, the interplay between gravitational force and buoyant force determines the net tension and resulting elongation.
- Statement 1 is Incorrect: The elongation of the rubber string is caused by the gravitational force (weight) of the stone acting downwards. This force creates tension in the string, stretching it. Unless explicitly stated that the stone is magnetic and interacting with an external magnetic field, magnetic force is not the cause of elongation.
- Statement 2 is Correct: When the stone is immersed in water, it displaces a volume of water. According to Archimedes' principle, the water exerts an upward force on the stone. This reduces the net downward force (apparent weight) supported by the string. Since the tension in the string decreases, the elongation of the rubber string also decreases.
- Statement 3 is Correct: The specific force responsible for the decrease in elongation is the buoyant force (or upthrust). This force acts vertically upwards, opposing the gravitational pull. The net force acting on the string becomes the actual weight of the stone minus the buoyant force ($T = mg - F_B$), resulting in reduced extension.
Key Takeaway: The extension in a suspension string is directly proportional to the tension. Immersion in a fluid introduces an upward buoyant force that reduces the tension (apparent weight), thereby reducing the extension.