Given the following question:
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 include which of the following?
I) Angular Momentum
II) Linear Momentum
III) Kinetic Energy.
My guess was one and two — there's no net torque on the system, and as I looked at it, no net force either, as there's not an external actor. Apparently, linear momentum isn't conserved according to the answer set I have. Why is that, in this case?
Best Answer
The author of the question may be expecting you to recognize or assume that the axle of the platform is supported by the floor (probably by way of some kind of superstructure), in which case linear momentum would not be conserved for a system defined as the student and the platform because of the reaction between the platform and the floor.
It is, in my opinion, a less than optimal question but not actually atrocious.