Physics: Conceptual Question (angular momentum)

Question

At a dilapidated playground, people have stolen the support bar to a merry-go-round leaving a flat disk that can rotate freely. While pondering conservation of angular momentum, a curious physics student wonders if he left his physics book on the ground and jumped onto the merry-go-round, and started walking, will he ever move away from the physics book, assume he is less than half its mass?

 

a. No, the merry-go-round will have angular momentum in one direction and he in the other, they will cancel and he’ll stay right above the book.

b. No, because the initial angular momentum is zero, so the final must be zero too, causing his angular velocity to be zero, so he’ll stay right above the book.

c. Yes, angular momentum is not conserved so he’ll move around away from the book.

d. Yes, because his moment of inertia is smaller than the merry-go-round’s, so his angular velocity will be bigger, so he’ll move around away from the book.

Answer 1

If we assume the merry-go-round is moving and in the phrase "assume he is less than half its mass?" that "its" refers to the merry-go-round despite the nearest possible precedent being "the book" then the answer is d.

 

a.  the momenta are perpendicular and do not cancel out.

b.  if the merry-go-round is stationary, he still walks away from the book.

c.  angular momentum is conserved.