A block with mass M, on the end of a string, moves in a circle on a horizontal frictionless table as shown. As the string is slowly pulled through a small hole in the table:
A) the angular momentum of M remains constant
B) the angular momentum of M decreases
C) the kinetic energy of M remains constant
D) the kinetic energy of M decreases
E) none of the above

Question

A block with mass M, on the end of a string, moves in a circle on a horizontal frictionless table as shown. As the string is slowly pulled through a small hole in the table: 
A) the angular momentum of M remains constant 
B) the angular momentum of M decreases 
C) the kinetic energy of M remains constant 
D) the kinetic energy of M decreases 
E) none of the above

Quizplus · Accepted Answer

The angular momentum of the block M remains constant because there is no torque acting on the system due to the absence of friction or any other external force. The tension in the string changes direction but it always acts at right angles to the velocity of the block, so there is no change in the angular momentum of the block when the string is pulled through the hole.

A Block with Mass M, on the End of a String