1. Assertion (A): A man standing on a completely frictionless surface can propel himself by whistling. 2. Reason (R): If no external force acts on a system, its momentum cannot change.

1. Assertion (A): A man standing on a completely frictionless surface is initially at rest. To propel himself, he needs to exert a force on something. By whistling, he expels air forwards. By Newton's third law (action-reaction), the expelled air exerts an equal and opposite force on the man, pushing him backward slightly. Although the force is small, on a frictionless surface, any net force will cause acceleration. So, Assertion (A) is true. 2. Reason (R): The statement expresses the principle of conservation of linear momentum: If no net external force acts on a system, its total momentum remains constant (cannot change). This is a fundamental law of physics. So, Reason (R) is true. 3. Explanation Check: Consider the man and the expelled air as a system. Initially, the total momentum is zero. When the man whistles, he exerts an internal force to expel air. The air gains momentum in one direction, and the man gains an equal amount of momentum in the opposite direction, keeping the total momentum of the system zero (conserved), as no *external* horizontal force acts. The principle of conservation of momentum (Reason R) explains why expelling mass (air) in one direction results in the man moving in the opposite direction (Assertion A). 4. Conclusion: Both A and R are true, and R is the correct explanation of A.