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Old 30th July 2019, 11:37 PM   #41
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Originally Posted by smartcooky View Post
The highlighted is a dead giveaway of them being Flat Earthers, and is probably even money that they are YEC's as well.
Nuke Pete and Peter!!
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Old 31st July 2019, 02:07 AM   #42
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Anyone can do some pretty simple math and find a fairly significant vector pushing the opposite direction as the thrust of the balloon exhaust.

It's all based on Bernoulli's laws that a faster moving fluid has lower pressure. So the inside of that long cone has less air pressure than the outside of that long cone. (Note that this is the opposite as the balloon which has higher air pressure inside the balloon than out)

That means perpendicular to the surface of the cone is a net force pushing inwards that can be vectored directly proportional to the angle. The horizontal vector actually pushes the cart backwards against the simulated balloon jet engine. Because the cone is very long and thin it maximizes the surface area pushing the exact opposite as the thrust at the same time as minimizing it's thrust by giving the air time to gradually slow down as it mixes with the outside air.

In essence this little experiment would not happen in space because vacuum would have a lower pressure outside the cone as inside the cone!

He just proved that under certain conditions and configurations a rocket engine actually works BETTER in vacuum!
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Old 31st July 2019, 11:14 AM   #43
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Originally Posted by Horhang View Post
The people making the video are trying to prove a rocket can’t work in a vacuum.
Then they should conduct their test in a vacuum.
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Old 31st July 2019, 01:02 PM   #44
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Originally Posted by carlitos View Post
Then they should conduct their test in a vacuum.
Better yet, leave the experiment as is, but the researchers should being vacuum while running it.

And suits or masks are cheating.
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Old 31st July 2019, 03:45 PM   #45
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Originally Posted by DevilsAdvocate View Post
This thread is not that thread. This thread is about explaining what happens in this video.

I would like an answer and be able to understand that answer. I don't think a sufficient answer has been provided to explain what we see in the video.

Why doesn't the car move? Would the same thing happen in space? I don't know. I would like to hear answers.
But Zig gave you a good explanation of what was happening in the video.

Originally Posted by Ziggurat View Post
The problem is probably mostly the reduction in thrust because of the nozzle size. The gradual expansion means the air flow will expand with the cone, and will decrease in velocity, leaving the cone at much lower velocity. If it’s, say, a factor of 5 bigger diameter, that’s a factor of 25 more in area, so a factor of 25 less in exit velocity and a factor of 25 less in force. At some point, the force won’t overcome friction.

The video is a load of crap I might add, which in no way disproves the way a rocket works.

In basic terms a rocket works by accelerating a mass, the exhaust material, and throwing it out. It takes force to accelerate the mass, (F=MA), and although the mass accelerated is small the velocity attained is very high, so the acceleration and force, are high also.
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Old 1st August 2019, 03:44 PM   #46
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Originally Posted by Thor 2 View Post
But Zig gave you a good explanation of what was happening in the video.




The video is a load of crap I might add, which in no way disproves the way a rocket works.

In basic terms a rocket works by accelerating a mass, the exhaust material, and throwing it out. It takes force to accelerate the mass, (F=MA), and although the mass accelerated is small the velocity attained is very high, so the acceleration and force, are high also.
Yes, but as I explained, the cone actually has a force vector in the exact opposite direction as the thrust due to a sort of venturi effect. So that cone actually cancels the thrust from the balloon.
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Old 1st August 2019, 06:12 PM   #47
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Originally Posted by Red Baron Farms View Post
Yes, but as I explained, the cone actually has a force vector in the exact opposite direction as the thrust due to a sort of venturi effect. So that cone actually cancels the thrust from the balloon.
That's just another way of deriving the same end result, as I already explained. You can look at the balloon and cone as separate parts which almost cancel, or you can look at just the net result by examining the final exhaust velocity, without separating the balloon and cone. Either method will work.
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Old 1st August 2019, 08:40 PM   #48
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Originally Posted by Ziggurat View Post
That's just another way of deriving the same end result, as I already explained. You can look at the balloon and cone as separate parts which almost cancel, or you can look at just the net result by examining the final exhaust velocity, without separating the balloon and cone. Either method will work.
aha I found it #22.
Quote:
In more detail, the air is at lower pressure inside the cone than outside the cone, with a pressure gradient that slows down the air (Bernoulli's principle). Because of the cone's shape, lower pressure inside than outside means there's a net force pushing backwards on the cone, so that the high force at the entrance to the cone is largely cancelled by the forces acting on the cone itself.
Sorry I did not mean to repost what you already explained.
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Old 2nd August 2019, 02:56 PM   #49
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Another observation from the video I picked up, is the comment that because the ballon was blown up larger, the pressure was greatly increased. This is just nonsense.

The tension in the ballon rubber would most certainly increase as the inflation continued, but an analysis of the forces would show, that the pressure would not necessarily increase at all. In fact it may decrease, because of the larger volume under pressure, and the reduced cross sectional area of the ballon material under tension.

A dead give away when people pose as being technically astute, make absurd comment.
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Old 4th August 2019, 12:51 AM   #50
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Originally Posted by Thor 2 View Post
But Zig gave you a good explanation of what was happening in the video.
I'm still trying to understand this. I'm looking at things and learning things. I'm not even sure how to ask the right questions. It is taking some time. This is more complex than I thought.

But I would like to have an explanation of where the presumed argument of the video goes wrong. I know it is wrong. I even know to some degree what is wrong.

I just can't figure out how to easily understand or explain where the fault in the presumed logic is.
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Old 4th August 2019, 12:59 AM   #51
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Originally Posted by DevilsAdvocate View Post
I'm still trying to understand this. I'm looking at things and learning things. I'm not even sure how to ask the right questions. It is taking some time. This is more complex than I thought.

But I would like to have an explanation of where the presumed argument of the video goes wrong. I know it is wrong. I even know to some degree what is wrong.

I just can't figure out how to easily understand or explain where the fault in the presumed logic is.
Can you outline the presumed logic? I have no idea why they think that what they do proves that rockets don't work in a vacuum.

Apparently you understand at least what they are claiming, so it would be helpful if you could outline their logic as you see it.

Then, knowing what their argument is, we can show where it goes wrong.
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Old 4th August 2019, 01:43 AM   #52
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Yes. I think that is a good idea.

I think the argument is this:

Rockets in a vacuum in outer space can’t move by expelling gas because the gas doesn’t have anything to push against.

Scientists say that rockets in a vacuum in outer space do move because of Newton’s 3rd Law. For every action there is an equal and opposite reaction. The rocket pushes gas out the back. The gas going out the back creates and equal and opposite force on the rocket. The rocket propels forward.

The argument is that this doesn’t happen.

The experiment shows a balloon car that is like a rocket. It pushes gas out the back and goes forward.

The argument is that the car, like a rocket, is not propelled by Newton’s 3rd Law.

If the car is being moved forward by Newton’s 3rd Law, the cone would not matter. The same amount of gas is being pushed out by the balloon. If the car were propelled by Newton’s 3rd Law, it should have an equal and opposite reaction. The car would move forward the same whether there is a cone or not.

But it doesn’t. Therefore, the car is not being moved by Newton’s 3rd Law, just as a rocket would not be moved by Newton’s 3rd Law.

I think the most obvious answer is probably that the balloon car is not actually being moved by Newton’s 3rd Law. It probably is a tiny bit, but not significantly in any way. It is being moved by other forces. But what are those?

When the car doesn’t have a cone, what drives it forward? Newton’s 3rd Law? Air from the balloon pushing against the atmosphere/air molecules? Differentials in air pressure? Something else? A combination of these?

When the cone is applied, how does it cancel out the force that otherwise drives the car forward? If the same force is coming out of the balloon at the same rate, where is the force being applied to if it is not driving the car forward like it does when there is no cone?

Does the cone have any effect on the force of the car from Newton’s 3rd Law? Of does that force remain the same, but other forces are cancelled to the extent that the force is not sufficient to overcome gravitational and frictional force to move the car forward?
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Old 4th August 2019, 01:53 AM   #53
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Originally Posted by DevilsAdvocate View Post
When the cone is applied, how does it cancel out the force that otherwise drives the car forward? If the same force is coming out of the balloon at the same rate, where is the force being applied to if it is not driving the car forward like it does when there is no cone?
This has been explained twice above. Post #9 from Ziggurat explains it best.

So for a start you could review that and come back with questions if you don't understand it.

(Edited to note that the convergent-divergent nozzles used on most rockets depend upon a much higher velocity of gas to work)
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Old 4th August 2019, 02:01 AM   #54
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Originally Posted by DevilsAdvocate View Post
If the car is being moved forward by Newton’s 3rd Law, the cone would not matter. The same amount of gas is being pushed out by the balloon. If the car were propelled by Newton’s 3rd Law, it should have an equal and opposite reaction. The car would move forward the same whether there is a cone or not.
Just to note a specific gap in the logic. The thrust is not just proportional to the amount of gas being pushed out but also to the velocity at which it is pushed out.

Pushing out the same amount of gas much more slowly will result in a lower force forward. A small enough force forward will be insufficient to overcome static and kinetic friction.

I refer you again to Ziggurat's explanation in #9 as to how the cone slows the gas.
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Old 4th August 2019, 02:21 AM   #55
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Originally Posted by Robin View Post
This has been explained twice above. Post #9 from Ziggurat explains it best.

So for a start you could review that and come back with questions if you don't understand it.

(Edited to note that the convergent-divergent nozzles used on most rockets depend upon a much higher velocity of gas to work)
I've been reading that post many times and trying to figure things out. I have found that it is much more complex that I thought. I still don't really understand what is happening.

I asked a number of other questions.

Maybe we can start with:

When the balloon car doesn't have the cone, what makes the car go forward?
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Old 4th August 2019, 02:29 AM   #56
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When the balloon doesn't have the cone, the gas exiting out the back has higher momentum, because it moves faster.
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Old 4th August 2019, 02:34 AM   #57
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Originally Posted by Robin View Post
Pushing out the same amount of gas much more slowly will result in a lower force forward.
But the balloon is pushing out the same amount of gas at the same rate in both cases, is it not?

I understand the balloons aren't exactly the same in the experiment. But let's assume they are. I assume the result would be the same.

The balloon pushes out the same amount of gas at the same rate with our without the cone.

I understand the cone is affecting the way the gas is moved out. But it is still the same amount of gas at the same rate.

If the car is moved by Newton's 3rd Law, there is the same force, so there should be the same equal and opposite force, so the car should move the same.

But it doesn't. Something is wrong. Where is the error in that logic?
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Old 4th August 2019, 02:37 AM   #58
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Originally Posted by HansMustermann View Post
When the balloon doesn't have the cone, the gas exiting out the back has higher momentum, because it moves faster.
Why does it move slower? Where does the extra energy go? Pushing against the walls of the cone? Friction?

And if the same amount of gas leaves the balloon at the same rate, why isn't there an equal and opposite force to move the car forward? Is the force diverted into something else? Turbulence? Heat transfer?
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Old 4th August 2019, 02:58 AM   #59
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Originally Posted by DevilsAdvocate View Post
But the balloon is pushing out the same amount of gas at the same rate in both cases, is it not?

I understand the balloons aren't exactly the same in the experiment. But let's assume they are. I assume the result would be the same.

The balloon pushes out the same amount of gas at the same rate with our without the cone.

I understand the cone is affecting the way the gas is moved out. But it is still the same amount of gas at the same rate.

If the car is moved by Newton's 3rd Law, there is the same force, so there should be the same equal and opposite force, so the car should move the same.

But it doesn't. Something is wrong. Where is the error in that logic?
You need to go step by step.

Do you accept that the conical nozzle slows the gas?
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Old 4th August 2019, 03:05 AM   #60
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Originally Posted by DevilsAdvocate View Post
But the balloon is pushing out the same amount of gas at the same rate in both cases, is it not?
On what are you basing that assumption?

Are you assuming that the velocity of the gas at the end of the nozzle will be the same as the velocity of the gas at the end of the tube to which the nozzle is attached?
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Old 4th August 2019, 03:08 AM   #61
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Originally Posted by DevilsAdvocate View Post
Why does it move slower?
Did you review post #9 as I suggested?

That is why it moves slower.
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Old 4th August 2019, 03:10 AM   #62
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Originally Posted by DevilsAdvocate View Post
But the balloon is pushing out the same amount of gas at the same rate in both cases, is it not?

I understand the balloons aren't exactly the same in the experiment. But let's assume they are. I assume the result would be the same.

The balloon pushes out the same amount of gas at the same rate with our without the cone.

I understand the cone is affecting the way the gas is moved out. But it is still the same amount of gas at the same rate.

If the car is moved by Newton's 3rd Law, there is the same force, so there should be the same equal and opposite force, so the car should move the same.

But it doesn't. Something is wrong. Where is the error in that logic?
The flawed assumption is in assuming that all of the force of the escaping gas from the balloon MUST act in a single direction.

Hint: It doesn't.
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Old 4th August 2019, 08:09 AM   #63
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Originally Posted by DevilsAdvocate View Post
Maybe we can start with:

When the balloon car doesn't have the cone, what makes the car go forward?
The force that pushed out the air has an equal and opposite force, as per Newton's Third Law. That force then is pushing in the opposite direction.

OK, your turn. Below is a schematic, on top is the bottle with the balloon attached, and under it the identical structure with the nozzle.

Suppose these were fixed in position would you expect the same air velocity at B as you would at A? Or would you expect it to be slower?

For example if an anemometer were placed at A and one at B, would the read the same air velocity?
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Old 4th August 2019, 09:50 AM   #64
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Originally Posted by DevilsAdvocate View Post
But the balloon is pushing out the same amount of gas at the same rate in both cases, is it not?

I understand the balloons aren't exactly the same in the experiment. But let's assume they are. I assume the result would be the same.

The balloon pushes out the same amount of gas at the same rate with our without the cone.

I understand the cone is affecting the way the gas is moved out. But it is still the same amount of gas at the same rate.

If the car is moved by Newton's 3rd Law, there is the same force, so there should be the same equal and opposite force, so the car should move the same.

But it doesn't. Something is wrong. Where is the error in that logic?
The cone pushes the opposite direction. It's the same force working on the paper cone as the force that makes an airplane fly. You can prove this yourself quite easily.

Lay a paper on a flat. Use an airhose to blow horizontally just above the paper. Make sure the air is flowing horizontally or even slightly upwards. See how the paper lifts up?

Or watch a flag? See how in the wind it flaps to and fro from pressure differences?

Faster moving air has less pressure. But in this case that faster moving air is inside a rigid cone. The cone cant move. But there is pressure pushing on the outside of the cone trying to force the car backwards. The balloon jet pushes forwards, the cone pushes backwards. The two forces counter-act each other and the car sits still.
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Old 4th August 2019, 10:32 AM   #65
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Originally Posted by DevilsAdvocate View Post
But the balloon is pushing out the same amount of gas at the same rate in both cases, is it not?
I'm not quite sure what you mean by rate. Momentum is mass times velocity.
p=m*v

Originally Posted by DevilsAdvocate View Post
Why does it move slower?
The cone is already full of air. The air in the cone gets in the way of the air coming out of the balloon.
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Old 4th August 2019, 04:28 PM   #66
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Originally Posted by DevilsAdvocate View Post
I'm still trying to understand this. I'm looking at things and learning things. I'm not even sure how to ask the right questions. It is taking some time. This is more complex than I thought.

But I would like to have an explanation of where the presumed argument of the video goes wrong. I know it is wrong. I even know to some degree what is wrong.

I just can't figure out how to easily understand or explain where the fault in the presumed logic is.

The jet of air escaping the balloon is pushing against a static air mass which resists the flow and provides a reaction thus propelling the car.

When the cone is fitted the exit velocity of the air is greatly reduced so the resisting reaction much reduced.

The forces are very low so the rolling resistance of the vehicle is significant. The static friction would be higher than the effort provided with the cone fitted. The cone also adds mass and therefore increases rolling resistance.

There would be a slight rocket effect due to the acceleration of the air but this would be very slight because the accelerating force is low.
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Old 4th August 2019, 04:48 PM   #67
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Originally Posted by Thor 2 View Post
The jet of air escaping the balloon is pushing against a static air mass which resists the flow and provides a reaction thus propelling the car.
not true. Don't confuse the man with the woo from the original thread
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Old 4th August 2019, 05:09 PM   #68
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Originally Posted by Red Baron Farms View Post
The cone pushes the opposite direction. It's the same force working on the paper cone as the force that makes an airplane fly. You can prove this yourself quite easily.

Lay a paper on a flat. Use an airhose to blow horizontally just above the paper. Make sure the air is flowing horizontally or even slightly upwards. See how the paper lifts up?

Or watch a flag? See how in the wind it flaps to and fro from pressure differences?

Faster moving air has less pressure. But in this case that faster moving air is inside a rigid cone. The cone cant move. But there is pressure pushing on the outside of the cone trying to force the car backwards. The balloon jet pushes forwards, the cone pushes backwards. The two forces counter-act each other and the car sits still.
Yes. I will defer to your better knowledge of the subject.
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Old 4th August 2019, 05:58 PM   #69
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Originally Posted by DevilsAdvocate View Post
If the car is moved by Newton's 3rd Law, there is the same force, so there should be the same equal and opposite force, so the car should move the same.

But it doesn't. Something is wrong. Where is the error in that logic?
The logic is fine, the premise is wrong.

The premise basically says "if the car is moved by Newton's 3rd law, then alterations to the structure of the car will have no effect on the sum of forces acting on the car".

Even if we had no explanation for why it doesn't move, that premise would still be wrong. Any change in the structure will change the sum of forces acting on it.

The burden should really be on them to demonstrate the claim that the alteration to the structure of the car should leave the sum of forces acting on it unchanged or negligibly changed.
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Old 5th August 2019, 07:17 AM   #70
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Originally Posted by DevilsAdvocate View Post
The balloon pushes out the same amount of gas at the same rate with our without the cone.

I understand the cone is affecting the way the gas is moved out. But it is still the same amount of gas at the same rate.

If the car is moved by Newton's 3rd Law, there is the same force, so there should be the same equal and opposite force, so the car should move the same.

But it doesn't. Something is wrong. Where is the error in that logic?
The rate of gas leaving the balloon isn't what determines the force. The rate of momentum change to the gas (ie, the mass flow rate times the velocity) is what matters. That's what determines the force. Think about it: force is a vector quantity, but the rate of mass leaving the balloon is a scalar quantity. You need a vector quantity involved too, and you get that from the velocity of the gas. Lower velocity, lower force. Different direction of velocity, different direction of force. Or, if you like, think about the units. Mass flow rate doesn't give you the right units for force. The SI unit for mass flow is kg/s, but force is kg*m/s2. You're missing a m/s, which you get from adding in the velocity.
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Old 7th August 2019, 12:56 AM   #71
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Originally Posted by DevilsAdvocate View Post
Why does it move slower? Where does the extra energy go? Pushing against the walls of the cone? Friction?

And if the same amount of gas leaves the balloon at the same rate, why isn't there an equal and opposite force to move the car forward? Is the force diverted into something else? Turbulence? Heat transfer?
Just out of interest - are you satisfied that your questions have been answered?
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Old 7th August 2019, 04:50 PM   #72
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I like Zig's answer but to look at it a slightly different way with the same conclusion:

The air in the cone is part of the car assembly. It must be because if you move the care the air goes with it - it won't create a vacuum in that space.

As the air exits the nozzle it strikes the contained air in the cone, mixes with it, and the exit velocity of the air from the end of the cone much reduced. As the kinetic energy of a moving mass, is a the product of the mass and a square of the velocity divided by two, it can be seen that the energy of the exiting air is dramatically reduced. Hence the force is dramatically reduced.

In space, as I understand it, matter thrust from a rocket meets no resistance. This would reduce the thrust produced by the rocket to some degree, compared to that produced in an atmosphere.
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Old 7th August 2019, 07:45 PM   #73
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Originally Posted by Robin View Post
Just out of interest - are you satisfied that your questions have been answered?
No. I can see each piece clearly, but I can't seem to put the pieces together to see the whole picture of the puzzle.

I'm a bit slow in responding because I go read other information to try to get a better understanding and I like to have some time for things to sink in.

I'll still ask some more questions.

Nobody knew that rocket science could be so complicated.
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Old 7th August 2019, 08:27 PM   #74
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Originally Posted by DevilsAdvocate View Post
Nobody knew that rocket science could be so complicated.
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Old 7th August 2019, 08:39 PM   #75
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Originally Posted by Ziggurat View Post
Simple momentum transfer.

Force is the change in momentum over time. The forward momentum you add to the rocket is equal to the backward momentum you give to the exhaust. And momentum is mass times velocity. The mass flowing out the back is controlled by the balloon, that's the same in either case. What changes is the exhaust velocity. If the nozzle is small, the exhaust velocity is large. If the nozzle is big, that same amount of air can pass out the back in the same amount of time at much lower velocity. Hence, lower momentum and less force transmitted to the rocket.

Now, you might naively say that since the air enters the cone at a high velocity, it should still exert a large (relatively speaking) force on the rocket cart. And if you chop the system up into parts and look at just that part, then yeah, the force should still be large. But you still can't ignore the cone. In order for the air to enter the cone at high velocity but leave the cone at low velocity, it has to slow down. So a net force has to act on the air. The cone has to, in effect, "suck" at the air. In more detail, the air is at lower pressure inside the cone than outside the cone, with a pressure gradient that slows down the air (Bernoulli's principle). Because of the cone's shape, lower pressure inside than outside means there's a net force pushing backwards on the cone, so that the high force at the entrance to the cone is largely cancelled by the forces acting on the cone itself.

The net effect is the same as if you don't chop the system into parts: low exhaust velocity means small net force, and for the purposes of propulsion we don't care about any internal stresses we're creating within the rocket itself (though on a real rocket, you'd need to make sure any internal forces don't break anything).
Let me go back here because this seems the easiest for me to understand.

If I understand this:

1. When the car has no cone. The balloon pushes air out the back. There is a certain amount of air being moved over a certain amount of time from the balloon to the back end of the car. The movement of this mass has force. Due to Newton's 3rd Law, there must be an equal and opposite action. That means there is a force on the balloon in the opposite direction. The balloon is attached to the car. The force on the balloon/car is sufficient to overcome friction. So the car moves forward.

The air escaping the balloon does push against the air molecules at the back of the car, which does add a bit of thrust, but that is a tiny amount. The car is primarily moving because of Newton's 3rd Law.

Because the air escaping the balloon is moving faster than the air around it, under Bernoulli's principle that creates a low pressure zone. There is higher pressure pushing against that air, all around it. That would create a sort of tunnel that the air is moving through. The escaping air pushes against the atmosphere, which causes it to gradually slow down. As it slows down, there is less difference in pressure and the air can escape and merge with the atmosphere. If we colored the air in the balloon, I expect we would see a tube of air coming out the back for some distance, then where it slows down it would expand into a general cloud shape.

The pressure caused by Bernoulli's principle does not slow down the car. If we look at a cross section of the air coming out of the balloon, we would see a ring of pressure pushing in on the escaping air. Because that pressure is not pushing either forward or backward, it has no effect on the force on the car.

2. Now we add the cone. We have the same amount (mass) of air being moved out of the balloon during the same amount of time. We have the same force from that air, so we would expect the same force to be applied to the car and it would move at the same speed. But it doesn't.

Something is happening to that force where it is not being applied to pushing the car forward. It is reduced enough that the car can't overcome friction, so the car doesn't move.

It has a little weight, but not much. It would create a bit of drag when the car is moving forward because the cone has to move through the atmosphere, but not much. There is a little friction of the air moving against the cone. There might be a little bit of turbulence in the cone. But none of those things are what is really slowing down the car or preventing it from moving.

What is preventing the car from moving is the pressure on the cone. If we look at a cross section, we would see the same thing as when there is no cone (or there is a tube). We would see a ring of pressure around the escaping air. The cross section behind that would be another ring, only slight larger. For the material of the cone to move forward, it has to push against the ring of pressure in front of it. It has to expand that ring. Then the next section of cone has to push that ring out of the way. And so on...

That pressure is making it harder for the cone to move forward. The cone needs force to push through that cone-shaped high pressure zone.

For the air escaping the balloon, Newton's 3rd Law still applies. We are still getting the same equal and opposite force on the car. But now the pressure pushing around the escaping air is preventing the cone from moving forward. The cone is attached to car. So the car needs more force to move forward.

That pressure on the cone creates a force that must be overcome if the cone/car is to move forward. It creates enough resistance that there is not enough force on the car/balloon/cone overcome friction. So the car doesn't move.

Is that right?
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Old 7th August 2019, 09:28 PM   #76
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Originally Posted by Thor 2 View Post

In space, as I understand it, matter thrust from a rocket meets no resistance. This would reduce the thrust produced by the rocket to some degree, compared to that produced in an atmosphere.
Wrong again.
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Old 7th August 2019, 09:50 PM   #77
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Originally Posted by DevilsAdvocate View Post

The air escaping the balloon does push against the air molecules at the back of the car, which does add a bit of thrust, but that is a tiny amount. The car is primarily moving because of Newton's 3rd Law.
Wrong

Originally Posted by DevilsAdvocate View Post
Because the air escaping the balloon is moving faster than the air around it, under Bernoulli's principle that creates a low pressure zone. There is higher pressure pushing against that air, all around it. That would create a sort of tunnel that the air is moving through. The escaping air pushes against the atmosphere, which causes it to gradually slow down. As it slows down, there is less difference in pressure and the air can escape and merge with the atmosphere. If we colored the air in the balloon, I expect we would see a tube of air coming out the back for some distance, then where it slows down it would expand into a general cloud shape.

The pressure caused by Bernoulli's principle does not slow down the car. If we look at a cross section of the air coming out of the balloon, we would see a ring of pressure pushing in on the escaping air. Because that pressure is not pushing either forward or backward, it has no effect on the force on the car.
The turbulence does have a minimal effect on the car as drag, but at such low speeds and small surface area, it really is so small as to be negligible.

Originally Posted by DevilsAdvocate View Post
2. Now we add the cone. We have the same amount (mass) of air being moved out of the balloon during the same amount of time. We have the same force from that air, so we would expect the same force to be applied to the car and it would move at the same speed. But it doesn't.

Something is happening to that force where it is not being applied to pushing the car forward. It is reduced enough that the car can't overcome friction, so the car doesn't move.

It has a little weight, but not much. It would create a bit of drag when the car is moving forward because the cone has to move through the atmosphere, but not much. There is a little friction of the air moving against the cone. There might be a little bit of turbulence in the cone. But none of those things are what is really slowing down the car or preventing it from moving.

What is preventing the car from moving is the pressure on the cone. If we look at a cross section, we would see the same thing as when there is no cone (or there is a tube). We would see a ring of pressure around the escaping air. The cross section behind that would be another ring, only slight larger. For the material of the cone to move forward, it has to push against the ring of pressure in front of it. It has to expand that ring. Then the next section of cone has to push that ring out of the way. And so on...

That pressure is making it harder for the cone to move forward. The cone needs force to push through that cone-shaped high pressure zone.

For the air escaping the balloon, Newton's 3rd Law still applies. We are still getting the same equal and opposite force on the car. But now the pressure pushing around the escaping air is preventing the cone from moving forward. The cone is attached to car. So the car needs more force to move forward.

That pressure on the cone creates a force that must be overcome if the cone/car is to move forward. It creates enough resistance that there is not enough force on the car/balloon/cone overcome friction. So the car doesn't move.

Is that right?
No that is not right. Some sentences are sort of correct like "It creates enough resistance that there is not enough force on the car/balloon/cone overcome friction. So the car doesn't move." however your description is horribly convoluted and confused.

The pressure on the cone with the system at rest is 1 atmosphere. But it is the same inside the cone as outside.

When the balloon is allowed to vent air through the cone as a simulated rocket engine, the air inside the cone drops in pressure. The air outside the cone is still 1 atmosphere. So the force outside the cone presses inwards against the cone. But the cone has an angle. So a % of the force is actually pushing backwards against the car. The simulated rocket engine pushes forward, the cone pushes backwards.

Now you may apply your conclusion. The net force is not enough to overcome friction. So the car doesn't move.
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Old 7th August 2019, 10:21 PM   #78
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Originally Posted by DevilsAdvocate View Post
Let me go back here because this seems the easiest for me to understand.

If I understand this:

1. When the car has no cone. The balloon pushes air out the back. There is a certain amount of air being moved over a certain amount of time from the balloon to the back end of the car. The movement of this mass has force.
More specifically, the acceleration of this mass of air has force.

Quote:
Because the air escaping the balloon is moving faster than the air around it, under Bernoulli's principle that creates a low pressure zone. There is higher pressure pushing against that air, all around it. That would create a sort of tunnel that the air is moving through.
Bernoulli's principle is easy to get tripped up on (it's based upon some assumptions which don't always hold), but to first order, the pressure drop you're examining with Bernoulli's principle is NOT between the escaping air and the surrounding atmosphere, it's between the air inside the balloon and the escaping air. And the air inside the balloon is at higher pressure than the surrounding air. The air as it escapes is at atmospheric pressure, because if it were lower pressure, it couldn't escape, it would get pushed back in.

Quote:
The pressure caused by Bernoulli's principle does not slow down the car. If we look at a cross section of the air coming out of the balloon, we would see a ring of pressure pushing in on the escaping air. Because that pressure is not pushing either forward or backward, it has no effect on the force on the car.
That's true, but it doesn't constrict the flow of escaping air, because the air escapes basically at atmospheric pressure. It has dropped in pressure from what's inside the balloon.
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Old 8th August 2019, 01:41 AM   #79
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How is so much dumbth possible?

Trolls? Sincerely deluded fools?

Maybe it doesn't even matter.
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Old 8th August 2019, 07:29 AM   #80
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Originally Posted by DevilsAdvocate View Post
We have the same force from that air,...
There are a number of forces acting on the car. We have the pressure of the air inside the bottle and balloon, the pressure of air outside the car in all directions, the gravity pushing down and the static friction (before it starts) and the kinetic friction when it is moving.

Ultimately it moves forward because the sum of all those forces is in the forward direction.
Quote:
so we would expect the same force to be applied to the car and it would move at the same speed. But it doesn't.
Again I don't understand how you reached this conclusion. You add a cone to the back which is as long again as the car and at the back as wide as the car and you say that you would not expect that to change the sum of forces acting on the car.

Obviously it will. The questions are: 1. By how much does it change that sum and in what direction and 2. By how much would that force need to be reduced in order for the car not to be able to overcome static friction?

At a rough guess, how much force in the backward direction (as a percentage of the force provided by the air escaping from the balloon) would be required for the car not to move?

You reached the conclusion that adding the cone should not prevent the car from moving, so you must have made at least a rough estimate of this.
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