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Question 6: When you release an inflated   (پھولا ہوا ۔۔۔ پڑسیدلے) but untied balloon, why does it fly across the room?


Two concepts need to be understood; pressure and conservation of momentum.

Pressure results from the density of molecules of the air (number of molecules per unit volume). As the number of molecules is more inside the balloon than the exterior surrounding (sure, we talk of number of molecules per unit volume), therefore, pressure inside the balloon is more than in the external areas. Again, air molecules rush to the area of low pressure, like water flows from a higher place to a lower one. (And this is the main cause of climatic winds!). So when the balloon is untied, the gas molecules speedily rush out to the low pressure area.

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.

Observation: Initially, the balloon pushes back with greater velocity and slows down. This is because due to high pressure in the balloon, the molecules escape with high velocities and number in the beginning. Therefore, the total momentum is larger. As a result, the balloon also pushes back with a high speed. When the number of molecules decreased in the balloon, its pressure also decreases. And the number of molecules escaping the balloon and their speeds decrease. This decreases the total momentum of the molecules. This way, the momentum of the balloon also decreases.


  1. Ammarah Rehman

    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.

    • admin

      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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