Understanding Newton's Third Law: The Cause of Balloon Movement

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SUMMARY

The discussion explains Newton's Third Law in the context of balloon movement, emphasizing that when a balloon filled with air is released, it moves in the opposite direction of the escaping air due to thrust. This thrust is generated by the elastic force of the balloon attempting to return to its original shape while counteracting the internal gas pressure. The principle of conservation of momentum is highlighted, illustrating that the momentum of the balloon and the expelled air must remain constant, resulting in a kickback force experienced by the balloon.

PREREQUISITES
  • Understanding of Newton's Third Law of Motion
  • Basic principles of momentum conservation
  • Knowledge of elastic forces and their effects
  • Familiarity with gas behavior under pressure
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  • Research the principles of thrust in rocket propulsion
  • Explore the relationship between pressure and volume in gases (Boyle's Law)
  • Study the applications of Newton's Laws in real-world scenarios
  • Investigate the mechanics of elastic materials and their properties
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Students of physics, educators teaching mechanics, and anyone interested in the practical applications of Newton's Laws in everyday phenomena.

member 529879
If you fill a balloon with air and let it go, since the initial momentum was 0 and the air in the balloon is pushed in one direction, the balloon must move in the other direction. What is the cause of the force that causes the balloon to move this way?
 
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The force that causes the balloon to move the other way is thrust, the same force pushing rockets into orbit. The particular thrust here comes ultimately from the elastic force of the balloon trying to compress back to its original shape, but having to work against the pressure of the gas inside.

Thrust itself comes from conservation of momentum.
If you throw something away from you (for the balloon, this would be a stream of air), you experience a kickback force (i.e., thrust), because the total momentum of you plus the thrown object must add up to the same value before and after the throwing event (assuming no other external forces).
 
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I'd say that the air in the balloon pushes on the inside surface of the balloon, but the balloon skin is missing at the hole through which the air escapes, so the push on the inside of the balloon is more on the side opposite the hole.
 
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