Assertion (A): A man standing on a completely frictionless surface can propel himself by whistling. Reason (R): If no external force acts on a system, its momentum cannot change.Codes:
✓ Correct answer: b) Both A and R are true but R is not a correct explanation of A
ExplanationAccording to Newton’s first law of motion, “ A body at rest will remain at rest, and a body in motion will continue in uniform motion in a straight line unless acted upon by an external force”.According to Newton’s second law of motion, “The rate of change of momentum of a body is directly proportional to the applied force and occurs in the direction of the force”.Assertion (A) is true: When the man whistles, he expels air from his lungs.This expelled air has mass and velocity, creating momentum in the forward direction and thus creating reaction force which pushes the man backward on the frictionless surface.This is based on Newton’s second law of motion.Reason (R) is true: The statement that ‘ if no external force acts on a system, its momentum cannot change’ is clearly stating Newton›s first law of motion which is clearly not explaining the Assertion (A).Thus, Reason (R) merely states Newton’s First Law of Motion and doen’s explain the Assertion (A) which is based on Newton’s Second Law of Motion.232
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