Which puck will reach the bottom faster?

Question

http://i5.tinypic.com/4r1o407.jpg

Two pucks slide down inclined plane.
Puck A starts from rest.
Puck B is given initial sidewise velocity
(that is in horizontal direction) Vo = 5m/s.

Which puck will reach the bottom (marked as blue line) faster?

Coefficient of friction 0 < μ < tan(α) is insufficient to prevent sliding.

Answer 1

Nice drawing, great question.

Puck B will reach the bottom faster. Sliding Friction is a force based on the normal. It is constant and not related to velocity. This means that both pucks will experience the same force of friction, but for Puck A it is in a direction opposite that of gravity, but for Puck B, a portion of the friction is perpendicular and slowing Puck B's horizontal velocity down with the remainder slowing down Puck B's vertical velocity. This means that Puck B will experience greater net acceleration downwards.

(Note: I originally thought Puck A because Puck B will pick up some rotation from friction as it travels its arc downward. I thought that some potential energy will be translated into kinetic energy of rotation -- however, with greater thought, I realize that the spin is created by loss of the horizontal kinetic energy and is therefore not relevant)