# Newtons Third Law

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?

jfizzix
Gold Member
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).

member 529879
Philip Wood
Gold Member
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.

member 529879 and jfizzix