Drew drops a snowball from the roof of a building, attempting to hit the top of a student's head. Evaporation causes the mass m of the snowball to decrease at a rate proportional to the snowball's speed as the snowball falls, so that m' = k | y' | , where y denotes the vertical distance from the snowball to the student and k is a negative constant. Derive the differential equation governing y(t), neglecting all physical phenomena except the pull of Earth's gravity on the snowball and the variation in the snowball's mass. If Drew wants to drop a rock in addition to the snowball and have both strike the student simultaneously, which should he drop first? Justify your response.