Simple, everyday science

[El_Machinae]

I know you think I'm wrong. What, exactly, then is 'pushing' the atoms out the back and what are those atoms pushing against?

The individual atoms are pushing against each other - separate from the rocket. Some of the atoms go one way, and some go the other. You're thinking of objects, when rockets are propelled by atoms.

Here's another example: if you dropped a bomb out the back of the rocket and exploded it, would the rocket move? Yes, it would. It would move because of the momentum imparted by the bomb-atoms hitting it. The principals of a rocket are the same, whether you're exciting the atoms due to a chemical reaction (which I believe is the case), or with radiation, or if you're releasing them due to pressure.

 

[Renata]

Of course the rocket would move, from the pressure wave. It wouldn't be very efficient, though, since the explosion would be spherical and the rocket occupies only a tiny part of that sphere. Also, if the pressure wave were intense enough, the rocket might just crumple.

But anyway. The hypothetical explosion "pushing" the rocket and rocket propulsion are not the same thing. Rocket propulsion is more analagous to "if you dropped a bomb out of the back of the rocket", without the explosion. Only, if you just dropped the bomb, with negligible velocity, the rocket would also move very little.

Think about it this way. Let's assume a rocket could contain an actual explosion, as you keep misidentifying the combustion reaction involved. Completely contained, no gasses escape. Will the rocket go anywhere? No. Atoms are pushing all over the place, but there is no net motion. Only when you open a channel and direct some of that gas away from the rocket in a particular direction does it move. No matter what else is happening inside the rocket, the speed at which the rocket moves (multipled by its mass) is exactly equal to the speed and mass of the escaping gas. How can "some of the gas pushes back on the rocket" account for this?

I'd also like to see you explain the recoil of a gun using your theory.

As for what "pushes" the gas out the back of a rocket, the gas does that all by itself. Rocket fuel is a condensed liquid or solid. When it burns, it is conveted to gasses, which take up an immensely larger volume. It escapes out the nozzle due to the pressure differential. The body of the rocket is just along for the ride.

Seriously, can someone with a good understanding of Newtonian physics back me up here? I feel like I'm spitting into the wind. And I don't have the time to figure out how best to explain this.

 

[nonconformist]

Don't the particles bounce around inside very fast, some hitting the front of the combustion chamber, and others blasting out the back?

 

[El_Machinae]

Pressure wave? What's it composed of? There are no waves in space, unless the wave is made of either atoms or energy. And I'm entirely sure that a solar sail operates because photons impart some momentum to a sail when they hit, and then again when they leave (via the theory you're talking about) at a lower wavelength.

I don't really want to argue, we just have a different way of looking at things:

When the gas expands - what is it expanding against? Itself. The molecules are repelling each other (kinda). If you took away half the atoms (going one direction), in that instant the other atoms (going the other way) would continue to be moving. They would eventually hit the wall and bounce off. Is there a momentum transfer ... yes

I think the difference is that you're looking at a cloud of expanding gas as if it's a discrete object. I'm trying to explain what the pressure is, that's pushing on all sides, except for the back.

 

[El_Machinae]

Okay, one last question. If the atom is separating from the back of the rocket, what is the cause of the energy pushing the atom one direction, and the rocket the other ... what is the 'arm doing the throwing' in my analogy?

 

[Che Guava]

I beleive that 'push' is the release of energy from the fuel igniting and expanding...

 

[Bozo Erectus]

No, I'm sorry, but this entire paragraph after the first sentence is wrong, and I'm not sure you understand the first sentence correctly.

Thats beautifull You really screwed up that time El. Reminds me of when Baldrick spelled Christmas without getting any of the letters right.

 

[El_Machinae]

Clearly someone's wrong, or someone is having trouble communicating. I just don't see how I've screwed up.

The pressure is caused by an exothermic reaction. This energy propels the molecules away from each other. The molecules functionally travel in all directions equally. They apply a pressure equally against all surfaces, imparting momentum to these surfaces. Parts where the momentum balances, there's no net motion, so the momentum is either transfered back to the molecule or absorbed by the surface (likely in the form of heat), or some sharing of momentum in between. Parts where the momentum doesn't balance, the rocket moves.

the speed at which the rocket moves (multipled by its mass) is exactly equal to the speed and mass of the escaping gas

That's because functionally 'half' of the atoms involved (assuming the atoms that don't contribute are taken out of the description) go out the back, their momentum is uninterrupted, the other 'half' impart their momentum to the rocket.

 

[Bozo Erectus]

Heres another enduring mystery: Why does electricity make machines perform tasks? Including our own muscles? Is it just because its an energy source, and any other sort of energy would do, or is there something special about electricity?

 

[Sophie 378]

Electricity is just one that we've found easy to use and convert into other kinds of energy: electricity, magnetism and movement are cool: if you've got two, you can cause the third. You might as well ask why biology uses chemical energy...

 

[Bozo Erectus]

Electricity is just one that we've found easy to use and convert into other kinds of energy: electricity, magnetism and movement are cool:
if you've got two, you can cause the third. You might as well ask why biology uses chemical energy...

I was going to ask next if its known how exactly the brain produces an electric field. Its converting chemical energy into electricity I guess, like a battery does?

 

[CartesianFart]

Electricity is just one that we've found easy to use and convert into other kinds of energy: electricity, magnetism and movement are cool: if you've got two, you can cause the third. You might as well ask why biology uses chemical energy...

Chemical energy?what does this mean when the field of biology using the word energy and then chemistry using the same word as well?is it because they both use the same method of describing what energy is?

 

[nonconformist]

Because elctricity, unlike other forms of energy, is so easy to strore.

 

[nonconformist]

Chemical energy?what does this mean when the field of biology using the word energy and then chemistry using the same word as well?is it because they both use the same method of describing what energy is?

Chemical; i.e stored in food, and "fuels" such as glucose etc.

 

[Bozo Erectus]

Because elctricity, unlike other forms of energy, is so easy to strore.

Can you really 'store' electricity? Or isnt it more like storing the potential to generate it?

 

[nonconformist]

Can you really 'store' electricity? Or isnt it more like storing the potential to generate it?

Electricity is the flow of electrons around a circuit of different potentials. All you have to do is keep the cell at a same potential or with a broken circuit, and it's effectively stored.

 

[El_Machinae]

I was going to ask next if its known how exactly the brain produces an electric field. Its converting chemical energy into electricity I guess, like a battery does?

It's similar to how a battery prepares itself to create a voltage difference. With a battery, you put an acid on one side, and a base on the other.

With the brain, we have molecules being specifically transported to one side a membrane to another, to build up a 'charge'.

Electricity and chemistry get very interrelated at the biological level.

 

[Renata]

Clearly someone's wrong, or someone is having trouble communicating. I just don't see how I've screwed up.

I'll try one more time. This is the part that's getting me to believe that you really don't understand what is going on, because though much of the rest seems to indicate some at least some understanding of Newton's laws, this part is completely wrong:

That's because functionally 'half' of the atoms involved (assuming the atoms that don't contribute are taken out of the description) go out the back, their momentum is uninterrupted, the other 'half' impart their momentum to the rocket.

I think you are getting confused by "every action has an equal and opposite reaction" into believing that the rocket propellant is somehow split into lots of parts. Some of it's lost to heat, some of it goes out the back (apparently uselessly?), but the important part of the propellant is the bit that goes "forward"; it pushes the rocket the way you might push a handcart or wind might push a sail. Right?

But that's not the case. Your everyday intuition is failing you. You can't get your car out of the ditch by throwing stuff out the back, so you assume that can't be happening for rockets either. Rockets, you seem to think, must somehow have to be "pushed" by something traveling the same direction you want the rocket to go. But rockets are not designed to operate in the realm of everyday objects, and everyday intuition doesn't necessarily apply.

But sometimes it does. I'm not sure why you keep rejecting the "throw a baseball on roller skates" analogy. It's a perfectly apt illustration of "every action has an equal and opposite reaction", and it's absolutely comparable to what happens in a rocket. You exert a force in one direction on a baseball, and in turn, you are moved in the opposite direction. A rocket pushes exhaust gas out the back; in turn, it is moved forward. No part of the baseball has to turn around and whack you in the gut in order to get you moving, and no part of the exhaust gas does either.

Here's a link on rockets from one of my favorite sites on the net, howstuffworks.com. Linkie.

Good luck figuring it out.

 

[El_Machinae]

I appreciate your time.

You exert a force in one direction on a baseball, and in turn, you are moved in the opposite direction. A rocket pushes exhaust gas out the back;

What, exactly, is going in-between the gas and the rocket and pushing them apart? That's the crux of my confusion with your position. In the roller skate analogy, my arm is what's 'in between' me and my ball.

In my statement, the thing that's 'in between' the two atoms is the explosion from their reaction, or the energy from their excited states.

 

[Truronian]

Can you really 'store' electricity? Or isnt it more like storing the potential to generate it?

Capacitors, which work as nonconformist described. Computers use these to store vital information in the event of a popwer cut (ie. Power cuts, capacitors 'release' electricity so Windows can store settings etc.)