Suppose you are in a hot air balloon with a sandbag that has a certain mass. The hot air balloon is moving upwards at a constant velocity of $15$ $m$.$s^{-1}$. If you throw the sandbag out of the hot air balloon, will the velocity of the hot air balloon change?
I thought that it will increase, because according to Newton's first law, an object will move in uniform motion unless an unbalanced force acts upon it. If the sandbag is released, there will be an unbalanced force. Or am I wrong? Please help.
Best Answer
Yes. The velocity of the balloon is determined by the buoyant force of the balloon, determined by its effective density and volume balanced against the density of the surrounding air, balanced against the aerodynamic drag of the balloon, which increases with speed.
When you drop a weight, the buoyancy of the balloon increases. This will cause the upward velocity to increase until the drag on the balloon matches the new buoyancy.
This ignores the effect of decreasing atmospheric density with altitude, which will also, by limiting the buoyancy, cause the rate of climb of the balloon to decrease with altitude until a maximum altitude is reached.