Question

Can you explain how the balloon generates an upward thrust?

Answer

This question can be answered at different levels.

PCOM

The fact that the exhaust air is gaining downward momentum means that the balloon must be gaining upward momentum, in order for the total momentum to remain as zero.

Newton’s Laws

By Newton’s 2^nd Law, the fact that the momentum of the exhaust air changed towards the downward direction means that the balloon must be exerting a downward force on the exhaust air. In fact, the magnitude of the thrust force is given by the expression v dm/dt where v and dm/dt are the speed and rate at which air is being ejected from the balloon respectively.

By Newton’s 3^rd Law, the exhaust air must be exerting an upward force on the balloon.

Pressure

When the balloon is not leaking air, the pressure forces exerted by the air on the skin of the balloon cancel one another out.

When it is leaking air, there is no more downward pressure forces acting at the “hole” at the bottom of the balloon. This means that the pressure forces exerted by the air on the balloon is now a net upward force.

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