1. The problem statement, all variables and given/known data A student initially stands on a circular platform that is free to rotate without friction about its center. The student jumps off tangentially, setting the platform spinning. Quantities that are conserved for the student-platform system as the student jumps include which of the following? I. Linear Momentum II. Angular Momentum III. Kinetic Energy 2. Relevant equations F = dp/dt, T = dl/dt 3. The attempt at a solution The answer is only angular momentum. However, I do not understand why angular momentum is conserved but not linear momentum. Since the system consists of the student and the platform, the force of the student on the platform and the reaction force are internal forces. They don't contribute to external force or torque. Am I missing something?