A puck moves on a horizontal air table. It is attached to a ...

[Arii_bell]

A puck moves on a horizontal air table. It is attached to a string that gradually winds itself up around a round peg in the center of the table. This shortens the string, so the puck gradually spirals in towards the center.
 
  What happens to the magnitude of the puck's angular momentum?

Question 2

What is the energy of a photon that, when it is absorbed by a hydrogen atom, excites an electron from the ground state of the hydrogen atom to the n = 2 state?
 
  A) 10.2 eV
  B) 1.63  10-18 J
  C) 1.63  10-18 Nm
  D) All of the answers are correct.
  E) None of the answers is correct.

[nickk12214]

Answer to Question 1

The tension in the string is perpendicular to the path of the puck, so there is no work done on the puck, but there is a retarding torque on it about the center of the table because the string pulls in a direction tangent to the peg, not towards the peg's center. Thus, its energy is constant but the magnitude of its angular momentum decreases.

Answer to Question 2

D