Why does the balloon fly across the room, then? We know about the law of conservation of momentum. (State it). The escaping air molecules and balloon constitute an approximately isolated system. The momentum must be conserved! When the air molecules escape, they get a momentum, say pm. As initially, the system has a zero momentum, therefore, the law of conservation of momentum demands the momentum of the whole system to be zero. The balloon with the remaining number of molecules pushes in a direction opposite to the escaping molecules acquiring a momentum pb. This momentum of the balloon must be equal to the total momentum of the escaping air molecules. Therefore, the balloon fly across the room.
As an isolated system is that in which no force enters or exits then if the air escapes out of balloon then it will escape the system so please clear my confusion.
I shall be highly greatful to you.
Ammarah Rehman
A perfectly isolated system may not exist in our surroundings. At least gravity is there and acts on every body having mass. So there might be other forces as well. However, we explain the answer with the conservation of momentum point of view.
When the balloon is tied, the air molecules inside it are randomly moving, and probably, the net momentum of these particles is conserved. When the balloon is untied, a great number of molecules due to high pressure in the balloon rush out the balloon through the hole. Consequently, the momentum increases in this direction inside the balloon. In order to conserve the momentum, the balloon goes in the opposite direction. When the mass is out of the balloon, then it is not part of the isolated system of the balloon and the momenta of the air particles do not affect the motion of the balloon.
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