Physics help (action and reaction forces!)?

Question

hi i am wondering what the reaction forces are to

gravity
and the force of a balloon pushing air backwards

for the balloon will it be the force of propulsion?
any help much appreciated, thanks :)

Answer 1

I will assume you mean a ballon inflated with air.

The elastic ballon stores energy as it expands (without breaking) to contain the pressurized air.

When the little stem is released, the air escapes, which acts as a propulsion for the balloon. To fly around it must work against air friction and gravity will play a factor as the balloon will deflate and fall to the ground.

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Answer 2

This is not simple problem. Are you referring to a hot-air balloon or an elastic fabric balloon filled with compressed air.

The expelled air depends upon:

1/ The temperature and thus density of the hot-air in the balloon. Heating the air produces a decrease in density therefore it will rise due to the greater density of the outside atmosphere. No hot-air supply and the force of gravity mg will cause the balloon to descend. The rate of descent is affected by the frictional drag of the atmosphere on the balloon fabric.

2/ In a simple elastic balloon, the diameter of the neck, the elasticity of the balloon material and initial pressure of the compressed air will affect the expulsion force. In addition as the balloon deflates the force decreases as the pressure drops.

Again, the downward force on the balloon is mg but the frictional air resistance (drag) becomes a variable as the size and shape changes.

From Newton's Laws of Motion:

To every force an equal and opposite force reacts.

In both cases above the forces are variable.

Answer 3

I guess we're talking Newton here - to every action there is an equal and opposite reaction.

When you say gravity, you must mean the weight of the balloon (due to gravity) and the reaction to this would be the force stopping the ballon from dropping through whatever it was lying on (the ground, a table, etc.).

The action-reaction I think the question is asking is the force exerted by the balloon in expelling the compressed air gives an equal and opposite force acting on the balloon (which will accelerate it up/down/wherever, opposite the opening).

The value of the force is given by the conservation of momentum. If you give a velocity to a mass in one direction it gains momentum (m*V) so the thing causing this gains an equal but opposite momentum (-M*v).

e.g. A canonball with small mass given large velocity while the large mass of the canon recoils with a smaller velocity.

Answer 4

...you know the answer why did you ask it then.