A research balloon of total mass M is descending vertically with download acceleration a. How much ballast must be thrown from the car to give the balloon an upward acceleration a, assuming that the upward lift of the air on the balloon does not change? I get m = (2aM)/(g+a), where g is Earth's gravity, M mass of balloon, and m mass of ballast. It doesn't sound right though.. Can someone please help me with this? Ok, and as for the other... (very simple) Two 10-lb weights are attached to a spring scale on opposite sides of each other. What is the reading of the scale? Should it be 10 lbs? Also, what if a single 10-lb weight is attached to a spring scale attached to a wall? Should the scale still be 10 lbs due to equal and opposite forces? Thanks!