Final answer:
The momentum of the top half of the watermelon is half the initial momentum of the entire watermelon, assuming the slicing does not affect the speed of the watermelon.
Step-by-step explanation:
The student has asked how the momentum of the top half of a watermelon compares to the initial momentum of the entire watermelon after it is sliced in half by a ninja, with the assumption that the cutting doesn't change the speed of the melon at all. Momentum is defined as the product of an object's mass and its velocity (p = mv). If the watermelon is moving leftward at 10 m/s and is sliced exactly in half, each half will continue to move leftward at the same speed, since the cutting does not affect the speed. Because the mass is now halved, the momentum of each half of the watermelon will also be half of the initial momentum of the entire watermelon.
Therefore, the momentum of the top half of the melon is half the initial momentum of the entire melon. The given options (60 mph, 120 mph, 40 mph, and 80 mph) are speeds, not measures of momentum, and thus are not relevant to the question. The momentum is measured in kilogram meters per second (kg·m/s) and will not be measured in miles per hour (mph).