1. Jan 18, 2005

### Firefox123

Hey all....

I have a question related to Newtons third law of motion.....action and reaction....

I was discussing what happens in certain action/reaction situations like

1. A gun recoiling
2. A rocket flying
3. A balloon filled with air flying after releasing the open end
4. A water hose recoiling from high pressured water

The other person I was speaking with is trying to look at the reaction as a "push" from the exiting air or liquid....

Let me explain......here is how he sees the "rocket" example...

He believes the rocket moves in an upward motion because the expanding gases leaving the bottom of the rocket exert a net upward force on the rocket itself......

since after the gas goes through the opening there isnt any physical contact with the rocket....he sees the gas (or liquid) exerting equal forces in all directions, but since one direction (the opening for any of the 4 examples) is not against the object itself we get a net force.

Im not sure if this is a good way to think about it......

I believe the explanation has something to do with the nature of mass/inertia itself.....if some system is losing mass in a certain direction with a certain velocity, then that system must act in such a way to balance that loss of mass.

Am I way off here?

I cant picture any way there could be a physical force transmitted (via the air or water being expelled) to an "edge" or side of a rocket, hose, gun, or balloon in such a way to cause it to move in a direction opposite the flow of the fluid.

Im thinking that we probably shouldnt try to think of it that way and that the answer is more fundamental and has to do with the nature of mass/inertia itself.

Hopefully I havent completely confused any readers.....can any one help me here?

What really causes the "reaction" in the 4 examples I have listed?

Russ

2. Jan 18, 2005

### DaveC426913

Your friend is partly correct, but the force is applied inside the rocket, not outside. Specifically, the force is exerted upon the inner front wall of the expansion chamber.

Say the chamber is cube-shaped. The expansion of the fuel pushes in six directions equally. It pushes against the 4 side walls, but these forces are equal and opposite, and so cancel out. It pushes against the front wall of the chamber. But there is no rear wall - the gases escape. Since the forward push is not balanced by a rearward push, the rocket moves in the direction of the only force that's not cancelled - forward.

3. Jan 18, 2005

### Firefox123

I can see how this line of reasoning can easily be extended to the recoil of a gun......but what about the balloon or water hose?

Can we think about the gas giving the balloon a net force upward because the force downward is reduced by the escaping gas?

What about the water hose? Can we say that the pressure exerted by the water in the direction of the opening is reduced and so the water pushes the hose back by forces inside the hose?

Can we think about it this way?

I thought it was due to a more fundamental rule of inertia and mass rather then due to a lack of forces in a particular direction......maybe I was wrong and overcomplicating the situation.....

Russ

4. Jan 19, 2005

### SGT

Both you and your friend are right. As DaveC explained, you can think the movement is due to an imbalance of forces, but you can think that the cause is the conservation of momentum. If mass is expelled in a certain direction with a certain velocity, the remaining mass must move in the opposite direction, in order to conserve momentum.

5. Jan 19, 2005

### DaveC426913

"... what about the balloon or water hose?..."

Yes. I presume you're talking about an air-filled balloon (a helium-filled balloon involves properties of bouyancy that might complicate the situation). Yes, the push against the front of the balloon pushes it in the opposite direction.

How much push? Well, how much push is *not* being cancelled by the rearward wall? One might think that's rather small, considering the tiny opening it has to escape through, but realize that the push is quite strong - strong enough that the air rushes out so fast as to create as jet - and a phblthpblthpth! sound!

Note that the force does not merely 'start' the moment the gas starts to exit the balloon. That force has been there as long as the balloon has been inflated. The forces are always pushing against the walls. In fact, it is what is keeping the balloon's shape - that's how much force is being applied inside the balloon.

"Can we think about the gas giving the balloon a net force upward because the force downward is reduced by the escaping gas?"

Yes, but again, don't confuse it with a helium balloon. The balloon will be just as happy to go sideways or even straight down, provided it's pointed that way.

"What about the water hose? Can we say that the pressure exerted by the water in the direction of the opening is reduced and so the water pushes the hose back by forces inside the hose?"

Yes.

"I thought it was due to a more fundamental rule of inertia and mass rather then due to a lack of forces in a particular direction......maybe I was wrong and overcomplicating the situation....."

No, you're not wrong - that is an equivalent way of looking at it.

"...but you can think that the cause is the conservation of momentum. If mass is expelled in a certain direction with a certain velocity, the remaining mass must move in the opposite direction, in order to conserve momentum."

Yes, I was just choosing to explain in a way that doesn't invovle the rather mysterious property of inertia, since that's the crux of the confusion.

Last edited: Jan 19, 2005
6. Jan 19, 2005

### Firefox123

Hey SGT......

I guess the two ways are really the same thing....

Im sometimes overly skeptical of what I would call "billiard ball" explanations....it seems my skepticism was misplaced in this case...

Thanks for the response.

Russ

7. Jan 19, 2005

### Firefox123

Right.....I was referring to an air filled balloon and was neglecting such things as bouyancy....

True.......good point....

Agreed....

Right....

Okay.

After a little thought I found myself becoming more convinced that this was the case....

Got it.

Thanks.

Cool.......Thanks for the replies DaveC426913....I appreciate it.

Russ