a) A balloon filled with air and its mouth untied/is released with its mouth in the downward direction, it moves upwards because of the air that is present in the balloon rushes out in the downward direction. This is the equal and opposite reaction in the downward direction as the air pushes the balloon upwards.
b) Mass of the unloaded truck, m1 = 2000 kg
Acceleration, a1 = 0.5 m/s2
Mass of loaded truck, m2 = 2000 + 2000 = 4000 kg
Acceleration, a2 = ?
m1×a1 = m2×a2
a2 = 0.25 m/s2
