a. Newton's third law tells us that the two pucks feel the same force, but in opposite directions. Using Newton's second law (page 63), the acceleration of each puck is the (same) net force, divided by the (differing) mass. So the double-mass puck has half the acceleration of the other, for the same amount of time. Both pucks start from rest, so the heavier puck's change in speed, its final speed, is only half as much as the lighter puck's final speed. b. $ 0.2 m/s$. Use the result just derived.