planetf1.com

Nakojo wrote:Ah, wings don't bend?

Again, there is no accelaration without friction. None.

Nakojo wrote:Ah, wings don't bend?

Again, there is no accelaration without friction. None.

Nakojo wrote: In a vacuum it's slightly different, but it's still all about friction.

There can be no friction in a vacuum because the definition of a vacuum is a space in which there are no particles. Without particles to rub past each other, there can be no friction.

The heated gas expands the exhaust, this tries to return to it's original shape -molecular bindings and cooling effect by it's surroundings-, thereby creating friction (between the gasses and the exhaust).
As the rocket meets no environmental friction, this is enough for thrust.
Where else are the expanding gasses and rocket going to go?

Friction is not the root cause for molecular bindings. The heated gas tends to expand because you are increasing its internal energy (by heating it), this means that each molecule of gas contains more kinetic energy which means it collides more frequently with the surrounding molecules, thus forcing the average distance between particles to increase to maintain equilibrium. If you contained the gas instead of allowing it to expand freely, you would get a proportional increase in pressure.

Anyway, in a rocket exhaust, the expanded gas is allowed to go only in one direction (out the exhaust of the rocket). Therefore Newton's 3rd Law tells us the reaction force must propel the rocket in the other direction.

Not unlike a football, it's not the kick that makes the ball travel, the kick dents the ball.
The ball wanting to return to it's natural form (pressure) is the only reason it travels (it can't do so in the same spot, as the foot carries more kinetic energy -try kicking a cannon ball-).

The cannon ball travels far less than the football because of its mass. KE = 0.5 * mass * velocity^2. Therefore, if you have a cannon ball that weights 5kg versus a 0.125kg football (I looked up the weights), the cannonball is actually 40 times heavier, and so you can expect the velocity to be decreased by approx 6 times.

The ball itself travels because when you hit it with your foot you are transferring kinetic energy to the ball. Or, an equally valid way of looking at it would be that your foot exerts a force on the the ball, and a force results in an acceleration (Newton's 2nd Law). Whilst the ball is in contact with your foot, Newton's 3rd Law would be involved also, which says that the force exerted on your foot by the ball would the same as that exerted on the ball by your foot. You would find that your foot would decelerate during the impact phase with the ball.

Curiously (or perhaps not ), the loss in kinetic energy of your foot would be equal to the gain in kinetic energy of the ball.


Consider a sailing boat, the wind just 'bends' the sail.
The sail trying to return to it's original shape makes the boat move.

Unfortunately, I don't think this is the right way to think about this. The sail (and in this case I am thinking of the most basic, parachute type sails) provides a surface area across which a pressure differential can act. On the side of the wind, we have high pressure, on the opposite side of the sail, we have low pressure. Force = Pressure x Area, therefore we have a force. Once we get into more modern sail designs (like for example on a wind-surfing board), there are also other aerodynamic effects at play which enhance the pressure differential effect once the craft starts moving.

qczhao wrote:Sorry I read your initial post again and I don't see where you have addressed gravitational pull?

At any rate, I believe your other explanations are wrong as well. I'm addressing your points in line below.

Nakojo wrote: In a vacuum it's slightly different, but it's still all about friction.

There can be no friction in a vacuum because the definition of a vacuum is a space in which there are no particles. Without particles to rub past each other, there can be no friction.

As there can be no movement without particles rubbing against each other - friction. Be they internal or environmental. In an absolute vacuum, there is no rocket. The absence of particles implies absence of mass, surely.

The heated gas expands the exhaust, this tries to return to it's original shape -molecular bindings and cooling effect by it's surroundings-, thereby creating friction (between the gasses and the exhaust).
As the rocket meets no environmental friction, this is enough for thrust.
Where else are the expanding gasses and rocket going to go?

Friction is not the root cause for molecular bindings. The heated gas tends to expand because you are increasing its internal energy (by heating it), this means that each molecule of gas contains more kinetic energy which means it collides more frequently with the surrounding molecules, thus forcing the average distance between particles to increase to maintain equilibrium. If you contained the gas instead of allowing it to expand freely, you would get a proportional increase in pressure.

Anyway, in a rocket exhaust, the expanded gas is allowed to go only in one direction (out the exhaust of the rocket). Therefore Newton's 3rd Law tells us the reaction force must propel the rocket in the other direction.

I'm not disputing the 3rd law, I'm elaborating on it. Without the molecular binding (and friction) of the exhaust, the gasses would expand circularly. The exhaust 'pushing back', is what causes the propulsion, the directional force of the expanded gasses, so you like. Without that friction, there would be no acceleration (well, marginal, as it will be affected by the expanding gasses anyhow - again, molecular friction).

Not unlike a football, it's not the kick that makes the ball travel, the kick dents the ball.
The ball wanting to return to it's natural form (pressure) is the only reason it travels (it can't do so in the same spot, as the foot carries more kinetic energy -try kicking a cannon ball-).

The cannon ball travels far less than the football because of its mass. KE = 0.5 * mass * velocity^2. Therefore, if you have a cannon ball that weights 5kg versus a 0.125kg football (I looked up the weights), the cannonball is actually 40 times heavier, and so you can expect the velocity to be decreased by approx 6 times.

The ball itself travels because when you hit it with your foot you are transferring kinetic energy to the ball. Or, an equally valid way of looking at it would be that your foot exerts a force on the the ball, and a force results in an acceleration (Newton's 2nd Law). Whilst the ball is in contact with your foot, Newton's 3rd Law would be involved also, which says that the force exerted on your foot by the ball would the same as that exerted on the ball by your foot. You would find that your foot would decelerate during the impact phase with the ball.

Curiously (or perhaps not ), the loss in kinetic energy of your foot would be equal to the gain in kinetic energy of the ball.

Again, this does not contradict my statements. Without the object carrying more kinetic energy being displaced (much), the kinetic energy of the 'lesser' object will move. Action, reaction, right? But why? Friction. Without friction, there would be no transform of energy. The foot would simply travel through the ball.

Consider a sailing boat, the wind just 'bends' the sail.
The sail trying to return to it's original shape makes the boat move.

Unfortunately, I don't think this is the right way to think about this. The sail (and in this case I am thinking of the most basic, parachute type sails) provides a surface area across which a pressure differential can act. On the side of the wind, we have high pressure, on the opposite side of the sail, we have low pressure. Force = Pressure x Area, therefore we have a force. Once we get into more modern sail designs (like for example on a wind-surfing board), there are also other aerodynamic effects at play which enhance the pressure differential effect once the craft starts moving.

Same.
Only the frictional forces of the sail v those of the wind (the environmental gasses crashing into the sail) 'create' all this.
I'm talking pure basics here and have done nothing but apply these laws.

Back to OP: yes, motion is possible without friction, acceleration however is not (ignoring gravitatitional pull from other objects).

moby wrote:May be too simple a statement, but without friction, would everything not be moving?

Nakojo wrote:

moby wrote:May be too simple a statement, but without friction, would everything not be moving?

Yes, it would, but then we wouldn't really know (significant) form or mass either.

Only free particles.

Nakojo wrote:Sure, but what makes a molecule 'stick together', or an atom or how ever far down you want to go (really, same applies to gravitational or electromagnetic pull but I was trying not to over complicate matters)?

Movement, friction, mass, no matter how you look at it, once it's true, it is true.

And all the same

Nakojo wrote:

qczhao wrote:Sorry I read your initial post again and I don't see where you have addressed gravitational pull?

At any rate, I believe your other explanations are wrong as well. I'm addressing your points in line below.

Nakojo wrote: In a vacuum it's slightly different, but it's still all about friction.

There can be no friction in a vacuum because the definition of a vacuum is a space in which there are no particles. Without particles to rub past each other, there can be no friction.

As there can be no movement without particles rubbing against each other - friction. Be they internal or environmental. In an absolute vacuum, there is no rocket. The absence of particles implies absence of mass, surely.

I think things are getting confused here. When I spoke about a vacuum, I was talking about considering the rocket to be operating in a vacuum because in deep space, the general condition is that it is in a vacuum (ignoring celestial bodies, asteroids, the rocket itself etc). That's not to say the rocket itself can't exist in a vacuum. Indeed, if that were the case, we wouldn't be here, because you can think of the Earth itself as being suspended in a vacuum.

I think the misunderstanding comes from the fact that you think there can be no movement without particles rubbing against each other. This is fundamentally incorrect. Forgetting even the action of electric, magnetic and gravitational fields, which we well know can influence motion without particles even coming into contact with one another, a force provided normal to a surface does not require friction to transmit. Let me expand in my next point.

The heated gas expands the exhaust, this tries to return to it's original shape -molecular bindings and cooling effect by it's surroundings-, thereby creating friction (between the gasses and the exhaust).
As the rocket meets no environmental friction, this is enough for thrust.
Where else are the expanding gasses and rocket going to go?

Friction is not the root cause for molecular bindings. The heated gas tends to expand because you are increasing its internal energy (by heating it), this means that each molecule of gas contains more kinetic energy which means it collides more frequently with the surrounding molecules, thus forcing the average distance between particles to increase to maintain equilibrium. If you contained the gas instead of allowing it to expand freely, you would get a proportional increase in pressure.

Anyway, in a rocket exhaust, the expanded gas is allowed to go only in one direction (out the exhaust of the rocket). Therefore Newton's 3rd Law tells us the reaction force must propel the rocket in the other direction.

I'm not disputing the 3rd law, I'm elaborating on it. Without the molecular binding (and friction) of the exhaust, the gasses would expand circularly. The exhaust 'pushing back', is what causes the propulsion, the directional force of the expanded gasses, so you like. Without that friction, there would be no acceleration (well, marginal, as it will be affected by the expanding gasses anyhow - again, molecular friction).

I think you're misunderstanding that friction has nothing to do with the molecular bindings in a solid structure (e.g.the combustion chamber of a rocket). Molecular bindings arise due to the electromagnetic force. This is not the same as friction. Friction also arises from the electromagnetic force but is not in itself the electromagnetic force. [edited to be more accurate as sub-atomic bonds are due to the strong nuclear force but inter-molecular bindings are due to electromagnetic force]

I don't disagree with the fact because the gas is re-directed due to the exhaust, then you have Newton's 3rd Law applying. I'm saying that this can happen even in a frictionless environment. The fact that particle collisions take place can happen without friction. e.g. if you dropped a ball coated with a frictionless material off a building and it landed on a frictionless surface it would still rebound according to conservation mechanics (energy/momentum). In fact, all physics questions at A-Level and even into University predicate the assumption that surfaces are frictionless and air resistance is ignored unless otherwise specified and the differences to real world results are in most cases negligible.

Not unlike a football, it's not the kick that makes the ball travel, the kick dents the ball.
The ball wanting to return to it's natural form (pressure) is the only reason it travels (it can't do so in the same spot, as the foot carries more kinetic energy -try kicking a cannon ball-).

The cannon ball travels far less than the football because of its mass. KE = 0.5 * mass * velocity^2. Therefore, if you have a cannon ball that weights 5kg versus a 0.125kg football (I looked up the weights), the cannonball is actually 40 times heavier, and so you can expect the velocity to be decreased by approx 6 times.

The ball itself travels because when you hit it with your foot you are transferring kinetic energy to the ball. Or, an equally valid way of looking at it would be that your foot exerts a force on the the ball, and a force results in an acceleration (Newton's 2nd Law). Whilst the ball is in contact with your foot, Newton's 3rd Law would be involved also, which says that the force exerted on your foot by the ball would the same as that exerted on the ball by your foot. You would find that your foot would decelerate during the impact phase with the ball.

Curiously (or perhaps not ), the loss in kinetic energy of your foot would be equal to the gain in kinetic energy of the ball.

Again, this does not contradict my statements. Without the object carrying more kinetic energy being displaced (much), the kinetic energy of the 'lesser' object will move. Action, reaction, right? But why? Friction. Without friction, there would be no transform of energy. The foot would simply travel through the ball.

So, just to illustrate my point again; collisions still take place without friction because friction is not the root force that holds molecules together. The collision case between my foot and the two different types of ball would still yield pretty much identical results regardless of whether friction was present or not. There are many ways to test my claim if you still doubt this.

Consider a sailing boat, the wind just 'bends' the sail.
The sail trying to return to it's original shape makes the boat move.

Unfortunately, I don't think this is the right way to think about this. The sail (and in this case I am thinking of the most basic, parachute type sails) provides a surface area across which a pressure differential can act. On the side of the wind, we have high pressure, on the opposite side of the sail, we have low pressure. Force = Pressure x Area, therefore we have a force. Once we get into more modern sail designs (like for example on a wind-surfing board), there are also other aerodynamic effects at play which enhance the pressure differential effect once the craft starts moving.

Same.
Only the frictional forces of the sail v those of the wind (the environmental gasses crashing into the sail) 'create' all this.
I'm talking pure basics here and have done nothing but apply these laws.

Unfortunately although you've applied the laws correctly (I don't disagree with that) the fundamental explanation for why this is happening is wrong.

Gravitational pull was addressed -not elaborated on- in my post's conclusion:

Back to OP: yes, motion is possible without friction, acceleration however is not (ignoring gravitatitional pull from other objects).

Sorry - I didn't see this

Odd how nobody disputes the F1 example, while the principle is exactly the same.
Or how people declare statements incorrect while they are in no way contrary to the arguments brought forward, supposedly discrediting said statements.

Quoting and applying laws is one thing, understanding them is quite something else, it appears.

I'll make it as simple as I can: movement (or rather acceleration) is the result of object-particles crashing into environmental-particles. Friction. All other arguments brought forward are merely an elaboration on this principle.
And, as I've tried to explain in both this post and the one previous, collision mechanics really for the most part have nothing to do with friction. Molecular bindings have nothing to do with friction. I think I agree with your applicable of Newton's Laws but I don't think your fundamental explanation for why Newton's 3rd Law occurs is correct in any way at all!

Nakojo wrote:Sure, level A physics assume zero friction and a vacuum.
Only not to complicate matters, but the example you provided should be sufficient.

If a ball bounces off a surface, we like to consider this collision without any 'loss' of energy.
However, is it not the simplest truth to acknowledge without resistance (friction) the ball would simply pass through the surface? Or like I stated earlier in the example of kicking a ball, without friction, the foot would pass through the ball. It's quite simple really.
Were this not the case, I'd imagine we'd be driving (amongst other things) 'perpertua mobile'.

As for quantum friction, well, leading physicist don't quite agree on this one, so to categorically state that there is no such thing is somewhat presumptuous. Admittedly my assumption that there is, could equally be seen as presumptuous though

Assuming there is, all my statements stand without need of any amends.

If there isn't, I'll gladly eat humble pie and admit your superiority on the matter

Nakojo wrote:However, is it not the simplest truth to acknowledge without resistance (friction) the ball would simply pass through the surface?

Nakojo wrote:Edit: just occurred to me now, rehashing the 'sail argument'.
High pressure, low pressure etc, right?
Okay, we all know the temperature rises (linear) as the pressure increases.
But why, if not because of more molecules being packed into the same volume, thereby colliding with one another more frequently. These collisions cause molecular friction, with heat as its by-product.
Makes sense, right?
Why else (other than "it's the law") would the temperature increase?
Merely trying to explain in the simplest way I can why I support the notion of molecular and even quantum friction.

qczhao wrote:

Nakojo wrote:Edit: just occurred to me now, rehashing the 'sail argument'.
High pressure, low pressure etc, right?
Okay, we all know the temperature rises (linear) as the pressure increases.
But why, if not because of more molecules being packed into the same volume, thereby colliding with one another more frequently. These collisions cause molecular friction, with heat as its by-product.
Makes sense, right?
Why else (other than "it's the law") would the temperature increase?
Merely trying to explain in the simplest way I can why I support the notion of molecular and even quantum friction.

This can get very complicated very quickly because you're asking about thermodynamics, and this is something I haven't studied for a while so I'm a bit rusty.

Anyway, as far as I understand, temperature is inherently a measure of how much potential energy a system has. Higher temperature systems have more energy (but we can't always extract this energy to do useful things).

Heat is not a by product of molecular collisions. Rather, in order to increase the number of molecules packed into the same volume (or, more easy to visualise, decrease the volume with the same number of molecules present), we have to put energy into the system. For example, imagine a sealed room with one wall that can be pushed inwards to compress the air in the room. In order to compress the air, we have to put energy into the system (in this example defined as the air in the room). Therefore the total energy of the system increases (because energy conservation says you can't destroy or create energy, only transfer it).

When the total energy of the system increases, it heats up, because temperature is the measure of the internal potential energy that a system has, and by transferring energy to it, you've increased this potential energy. The way this increased potential energy is expressed in this particular system is that each molecule has an increased kinetic energy and collisions become more violent.

The pressure increases because if each molecule is colliding more violently (and also there are more collisions as the volume is reduced), then it stands to reason that the average force exerted on the container walls would increase.

Nakojo wrote:

qczhao wrote:

Nakojo wrote:Edit: just occurred to me now, rehashing the 'sail argument'.
High pressure, low pressure etc, right?
Okay, we all know the temperature rises (linear) as the pressure increases.
But why, if not because of more molecules being packed into the same volume, thereby colliding with one another more frequently. These collisions cause molecular friction, with heat as its by-product.
Makes sense, right?
Why else (other than "it's the law") would the temperature increase?
Merely trying to explain in the simplest way I can why I support the notion of molecular and even quantum friction.

This can get very complicated very quickly because you're asking about thermodynamics, and this is something I haven't studied for a while so I'm a bit rusty.

Anyway, as far as I understand, temperature is inherently a measure of how much potential energy a system has. Higher temperature systems have more energy (but we can't always extract this energy to do useful things).

Heat is not a by product of molecular collisions. Rather, in order to increase the number of molecules packed into the same volume (or, more easy to visualise, decrease the volume with the same number of molecules present), we have to put energy into the system. For example, imagine a sealed room with one wall that can be pushed inwards to compress the air in the room. In order to compress the air, we have to put energy into the system (in this example defined as the air in the room). Therefore the total energy of the system increases (because energy conservation says you can't destroy or create energy, only transfer it).

When the total energy of the system increases, it heats up, because temperature is the measure of the internal potential energy that a system has, and by transferring energy to it, you've increased this potential energy. The way this increased potential energy is expressed in this particular system is that each molecule has an increased kinetic energy and collisions become more violent.

The pressure increases because if each molecule is colliding more violently (and also there are more collisions as the volume is reduced), then it stands to reason that the average force exerted on the container walls would increase.

I'm sorry, don't take this as being disrespectful or avoiding detailed debate, I do like simplifying matters though.

How does one 'add' energy?
Of course, when the total energy within said system increases, all you say is absolutely true.
The elaboration on 'more violent collisions' is as far as I can assess totally construent with my arguments.

The method of adding energy isn't really important to be honest. But if we take for example our sealed room with a wall that can move inwards. You can push that wall inwards against the pressure the gas is exerting by using a man. Pushing that wall inwards requires a force which moves over a distance hence you have work done on the system. Therefore energy is transferred (from the man's body) to the system (compressed box of air).

I fail to see energy 'increase', other than back to square one, magnetic 'pull' or quantum friction.
In either case, it must 'draw' the additional energy from it's environment.

So the point here is that energy is not generated because molecules are "rubbing" past one another. Indeed, it's not possible to generate energy by doing this (rubbing your hands heats them up because you're converting the kinetic motion of your hands into heat energy, that's why you get tired eventually!). The energy that gets added to the system comes from outside the system itself (in this case the man pushing the wall inwards).

Dismissing increase of energy, I still need some convincing.

The key thing here is that temperature is defined (at least in some respects) as the average translational kinetic energy of the molecules (of gas). So, if you increase the kinetic energy of the gas by adding more energy (by pushing that wall inwards - remember, when you push that wall inwards you are doing work against the pressure that the gas is exerting - therefore you are increasing the energy of the gas), by definition the temperature will rise. You can't really dismiss the increase of energy in this case as it is interlinked.

And indeed, we don't seem to be disagreeing much, rather than debating assumptions and their applications.

Nakojo wrote:Great read, thanks.

It's admittedly been close to 30 years since I studied physics (at Imperial College fwiw) and haven't worked in the field since.

My interest (and subsequent approach) is more philosophical these days, as you may have noticed, and I have found this discussion both educational and challenging.

Thank you for your patience.

I'll still beg to differ though, if only on the principle of added energy, as it again does not oppose my arguments.
Quite the contrary.

You rightly state that the method of adding energy (within a system) is irrelevant yet then continue to assume a state of increased energy without addressing the (source of) increase any further, whereas I imho did, or at least tried to sway the discussion that way.

flyboy10 wrote:Well rocket motion doesn't depend on friction - does it?

Nakojo wrote:How does one 'add' energy?
Of course, when the total energy within said system increases, all you say is absolutely true.
The elaboration on 'more violent collisions' is as far as I can assess totally construent with my arguments.

I fail to see energy 'increase', other than back to square one, magnetic 'pull' or quantum friction.
In either case, it must 'draw' the additional energy from it's environment.

Dismissing increase of energy, I still need some convincing.

And indeed, we don't seem to be disagreeing much, rather than debating assumptions and their applications.

Schumacher forever#1 wrote:Rockets get their motion before they leave a magnetic field; there is nothing in space to push the rocket.

Blinky McSquinty wrote: What was before the Big Bang? Take two aspirin before you read this..

Most of us think of space as having the three basic dimensions with time as the fourth just marching on. But before the big bang there was no space, no time, nothing, not even three dimensions. Nothing. Not even time.

James14 wrote:

Blinky McSquinty wrote: What was before the Big Bang? Take two aspirin before you read this..

Most of us think of space as having the three basic dimensions with time as the fourth just marching on. But before the big bang there was no space, no time, nothing, not even three dimensions. Nothing. Not even time.

Forgive my ignorance here because I am not that well educated.
But surely the material that combusted that caused the Big Bang needed space to exist and maybe time to be created beforehand?
Space must have existed before the Big Bang. Granted it must have been empty space. Otherwise the Big Bang would have been an implosion rather than an explosion.

Where are those aspirin? I need them now.

James14 wrote:

Blinky McSquinty wrote: What was before the Big Bang? Take two aspirin before you read this..

Most of us think of space as having the three basic dimensions with time as the fourth just marching on. But before the big bang there was no space, no time, nothing, not even three dimensions. Nothing. Not even time.

Forgive my ignorance here because I am not that well educated.
But surely the material that combusted that caused the Big Bang needed space to exist and maybe time to be created beforehand?
Space must have existed before the Big Bang. Granted it must have been empty space. Otherwise the Big Bang would have been an implosion rather than an explosion.

Where are those aspirin? I need them now.

Blinky McSquinty wrote: Most of us think of space as having the three basic dimensions with time as the fourth just marching on. But before the big bang there was no space, no time, nothing, not even three dimensions. Nothing. Not even time.

bbobeckyj wrote:

James14 wrote:

Blinky McSquinty wrote: What was before the Big Bang? Take two aspirin before you read this..

Most of us think of space as having the three basic dimensions with time as the fourth just marching on. But before the big bang there was no space, no time, nothing, not even three dimensions. Nothing. Not even time.

Forgive my ignorance here because I am not that well educated.
But surely the material that combusted that caused the Big Bang needed space to exist and maybe time to be created beforehand?
Space must have existed before the Big Bang. Granted it must have been empty space. Otherwise the Big Bang would have been an implosion rather than an explosion.

Where are those aspirin? I need them now.

The name "The Big Bang" is a misnomer.
Please forgive me if I'm unintentionally being patronising, but there was no combustion (burning) or explosion involved in the big bang. Burning is a chemical reaction between a fuel and oxygen, but there was no oxygen immediately after the big bang, only hydrogen and helium, (burnt hydrogen is water), all of the other successive elements were created by fusion in stars.
The big bang theory doesn't explain what there was or wasn't prior to the event. In extreme layperson speak, all matter spontaneously popped into existence in a singularity, an infinitely small thing of infinite mass. Then Inflation followed, a period of sudden expansion, because no thing can occupy the same space as another thing. All particles repulsed each other until they were no longer occupying the same space, (visualize a sponge squeezed very small, and released, a sudden expansion followed by slower continued expansion), then once that was done the universe continued expanding at a slower rate.
(Cue the really clever people picking holes and explaining it a lot better than I.)

Alex53 wrote:Is there a chance that time does pre-date our universse and the big bang only describes the beginning of this particular universe we are in now? i.e. universes are born out of some kind of 'dead universe' black hole?