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  4. Do rockets work due to conservation of momentum theory or imbalanced forces?
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Do rockets work due to conservation of momentum theory or imbalanced forces?

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Offline clevernever (OP)

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Do rockets work due to conservation of momentum theory or imbalanced forces?
« on: 03/02/2020 00:16:31 »
The rocket is not required to "push" or provide a force for the exhaust to overcome it's inertia. The external force to change the exhaust's inertia comes from pressure gradient force. The force that causes high pressure to move toward low pressure. Therefore, conservation is always conserved without the rocket in the picture. Pressure gradient force is a potential force like gravity. When you drop a ball from your hand from a height, the ball does not need your hand to apply a force in order to conserve momentum as gravity provides the force for the ball to overcome it's inertia.

A rocket is said to work using this example: If you fill a balloon with air and hold the neck closed, the pressure inside the balloon is slightly higher than the surrounding atmosphere. However, there is no net force on the balloon in any direction because the internal pressure on the balloon is equal in all directions. If you release the neck of the balloon, it acts like a hole, with no surface area for the internal pressure to act on. There is now an imbalanced force on the balloon, and the internal pressure on the front of the balloon is greater than the internal pressure on the back of the balloon.
 

So therefore, the conservation of momentum effect is not correct. A person standing on a skateboard throwing medicine balls to create movement is a false analogy. Why do schools and most sources teach this false analogy?
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Offline chiralSPO

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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #1 on: 03/02/2020 00:55:23 »
They are both correct.

Microscopically, the difference in pressure is effectively a difference in the number (or speed) of gas particles hitting the walls of the balloon. Each gas particle is a tiny medicine ball. And each one effectively bounces off the inside of the balloon, pushing on the whole balloon. If the balloon i sealed, then the particles will hit all parts evenly, with zero net force. As soon as a hole opens up, then there is a portion of particles that will leave through that hole, without bouncing off of the balloon wall and compensating for their last impact. (Note: this picture is more complex if the mean free path of the particles is less than the radius of the balloon, but the end . result is the same)
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Offline clevernever (OP)

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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #2 on: 03/02/2020 01:55:55 »
Quote from: chiralSPO on 03/02/2020 00:55:23
They are both correct.

Microscopically, the difference in pressure is effectively a difference in the number (or speed) of gas particles hitting the walls of the balloon. Each gas particle is a tiny medicine ball. And each one effectively bounces off the inside of the balloon, pushing on the whole balloon. If the balloon i sealed, then the particles will hit all parts evenly, with zero net force. As soon as a hole opens up, then there is a portion of particles that will leave through that hole, without bouncing off of the balloon wall and compensating for their last impact. (Note: this picture is more complex if the mean free path of the particles is less than the radius of the balloon, but the end . result is the same)

You didn't say how the conservation of momentum explanation is correct. How is a person throwing a ball while standing on a skateboard a correct analogy when the exhaust from the rocket moves out due to pressure differential?
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Offline chiralSPO

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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #3 on: 03/02/2020 02:11:13 »
The gas that exits has momentum.

So you can either think of it as the gas leaving and pushing the rocket forward. Or you can thin of the rocket throwing the gas out the back, and thereby moving forward... it's the same thing.

You can also either think of the gas as a uniform continuous fluid with mass and net velocity and heat capacity, or as a collection of particles (ideal gas or more complex models). Again, you will still have a reasonable description of how/why a rocket moves, or how a balloon moves when the end is opened.
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Offline clevernever (OP)

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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #4 on: 03/02/2020 03:13:42 »
Quote from: chiralSPO on 03/02/2020 02:11:13
The gas that exits has momentum.

So you can either think of it as the gas leaving and pushing the rocket forward. Or you can thin of the rocket throwing the gas out the back, and thereby moving forward... it's the same thing.

You can also either think of the gas as a uniform continuous fluid with mass and net velocity and heat capacity, or as a collection of particles (ideal gas or more complex models). Again, you will still have a reasonable description of how/why a rocket moves, or how a balloon moves when the end is opened.

"Conservation of momentum is a fundamental law of physics which states that the momentum of a system is constant if there are no external forces acting on the system. It is embodied in Newton's first law (the law of inertia)."

It is pressure gradient force that is the external force that causes the exhaust to overcome its inertia. The gas that exits has momentum because of pressure gradient force. Hence no force imposed on the rocket by the exiting gas. Like when you drop a ball from your hand, force of gravity causes the falling ball to gain momentum as it falls, thus no opposite force on your hand.

How do explain this? Are you saying that the exhaust movement is not caused by pressure gradient force?



* PGF.PNG (124.67 kB, 1909x796 - viewed 174 times.)
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Offline Kryptid

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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #5 on: 03/02/2020 04:34:33 »
Quote from: clevernever on 03/02/2020 03:13:42
Hence no force imposed on the rocket by the exiting gas.

The gas doesn't expand only to the rear of the rocket: it expands in all directions. The portion of the gas moving towards the inside of the rocket nozzle has momentum too. The expanding gas that is pushing against the rocket nozzle is what causes the rocket to experience a force that propels it.
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Offline clevernever (OP)

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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #6 on: 03/02/2020 14:00:25 »
Quote from: Kryptid on 03/02/2020 04:34:33
Quote from: clevernever on 03/02/2020 03:13:42
Hence no force imposed on the rocket by the exiting gas.

The gas doesn't expand only to the rear of the rocket: it expands in all directions. The portion of the gas moving towards the inside of the rocket nozzle has momentum too. The expanding gas that is pushing against the rocket nozzle is what causes the rocket to experience a force that propels it.

Yes but the imbalanced force is created by the gas pushing on the front of the rocket. It is because the exiting gas is not pushing on the rocket is what causes the imbalanced force. The opening at the rear causes less surface area on rear side than the front side in which the gas pushes on.

The momentum causing the gas to move into the nozzle is caused by pressure gradient force.

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Offline Origin

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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #7 on: 03/02/2020 14:53:30 »
Quote from: clevernever on 03/02/2020 14:00:25
Yes but the imbalanced force is created by the gas pushing on the front of the rocket. It is because the exiting gas is not pushing on the rocket is what causes the imbalanced force.  The opening at the rear causes less surface area on rear side than the front side in which the gas pushes on.
That is quite simply wrong.
There is no doubt that the pressure differential between the reaction chamber and the outside is the driving force of the rocket exhaust, but it is the reaction to the exhaust momentum that causes the rocket to move.
The principle is over 300 years old.  Do you believe that with all the rockets designed and flown over the last 100 years we still do not know the most basic aspect of rocket propulsion?  Think about how absurd that is.

I don't know where you got your bogus information, but I recommend going to reputable sites to learn about science!
« Last Edit: 03/02/2020 15:03:56 by Origin »
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Offline clevernever (OP)

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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #8 on: 03/02/2020 15:32:13 »
Quote from: Origin on 03/02/2020 14:53:30
Quote from: clevernever on 03/02/2020 14:00:25
Yes but the imbalanced force is created by the gas pushing on the front of the rocket. It is because the exiting gas is not pushing on the rocket is what causes the imbalanced force.  The opening at the rear causes less surface area on rear side than the front side in which the gas pushes on.
That is quite simply wrong.
There is no doubt that the pressure differential between the reaction chamber and the outside is the driving force of the rocket exhaust, but it is the reaction to the exhaust momentum that causes the rocket to move.
The principle is over 300 years old.  Do you believe that with all the rockets designed and flown over the last 100 years we still do not know the most basic aspect of rocket propulsion?  Think about how absurd that is.

I don't know where you got your bogus information, but I recommend going to reputable sites to learn about science!

So you are saying that the imbalanced forces explanation is incorrect? This was the explanation that I was told many years ago and there are plenty of reputable source that explain rocket propulsion in this manner.

Even NASA’s balloon vector diagram states this:

“If you fill a balloon with air and hold the neck closed, the pressure inside the balloon is slightly higher than the surrounding atmosphere. However, there is no net force on the balloon in any direction because the internal pressure on the balloon is equal in all directions. If you release the neck of the balloon, it acts like a hole, with no surface area for the internal pressure to act on. There is now an imbalanced force on the balloon, and the internal pressure on the front of the balloon is greater than the internal pressure on the back of the balloon.”

The exiting exhaust is not pushing on the balloon, it simply causes imbalanced internal pressure.
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Offline Janus

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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #9 on: 03/02/2020 16:33:31 »
Quote from: clevernever on 03/02/2020 15:32:13
Quote from: Origin on 03/02/2020 14:53:30
Quote from: clevernever on 03/02/2020 14:00:25
Yes but the imbalanced force is created by the gas pushing on the front of the rocket. It is because the exiting gas is not pushing on the rocket is what causes the imbalanced force.  The opening at the rear causes less surface area on rear side than the front side in which the gas pushes on.
That is quite simply wrong.
There is no doubt that the pressure differential between the reaction chamber and the outside is the driving force of the rocket exhaust, but it is the reaction to the exhaust momentum that causes the rocket to move.
The principle is over 300 years old.  Do you believe that with all the rockets designed and flown over the last 100 years we still do not know the most basic aspect of rocket propulsion?  Think about how absurd that is.

I don't know where you got your bogus information, but I recommend going to reputable sites to learn about science!

So you are saying that the imbalanced forces explanation is incorrect? This was the explanation that I was told many years ago and there are plenty of reputable source that explain rocket propulsion in this manner.

Even NASA’s balloon vector diagram states this:

“If you fill a balloon with air and hold the neck closed, the pressure inside the balloon is slightly higher than the surrounding atmosphere. However, there is no net force on the balloon in any direction because the internal pressure on the balloon is equal in all directions. If you release the neck of the balloon, it acts like a hole, with no surface area for the internal pressure to act on. There is now an imbalanced force on the balloon, and the internal pressure on the front of the balloon is greater than the internal pressure on the back of the balloon.”

The exiting exhaust is not pushing on the balloon, it simply causes imbalanced internal pressure.

The imbalanced internal pressure is, itself, a result of conservation of momentum.  The hot gasses in the combustion chamber are made of molecules with individual momenta.  It is the change in momentum of a molecule when it strikes the wall of of the chamber that produces the force on the wall.
For a rocket, you might imagine a person standing in a long skateboard, with a wall erected on one  end. He throws medicine balls in both directions, two at a time.  Since the pair of ball are moving in opposite directions at opposite speeds, throwing them produces no net change in his momentum.  However, one of the Ball hits a wall, bounces off and ends up moving in the same direction as the other.  The result is a change of momentum of the ball of 2mv (with m being the mass of the ball and assuming an perfectly elastic collision). The wall (and the skateboard the person is standing on) moves in the opposite direction to conserve momentum.  The end result ends up being the same as if he had just thrown both balls towards the end with no wall.

Now coat the wall with a thick rubber coating.  The compression of the rubber "softens" the impact of the ball hitting the wall, making the bounce of the ball less sudden, in turn reducing the force of the impact. But as long as the bounce is still perfectly elastic, the ball ends up still changing it momentum by 2mv, and transferring the same amount of momentum to the skateboard.

If we assume a non-elastic collision, for example the rubber absorbs some of the impact energy and converts it to heat, you can still make the impact force be equal to that for the perfectly elastic rubber, but now the ball will not change its momentum by the full 2mv, and neither will the skateboard be propelled by as much. 
In the end, it is the total end momentum of the balls that determines how fast the skateboard ends up moving.
The expanding gas in the chamber is the mechanism for the exchange of momentum, but in the end, it is the exchange and conservation of momentum that results in the rocket's motion.
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Offline Origin

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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #10 on: 03/02/2020 16:57:24 »
Quote from: clevernever on 03/02/2020 15:32:13
So you are saying that the imbalanced forces explanation is incorrect?
Yes, that is exactly what i am saying.
Quote from: clevernever on 03/02/2020 15:32:13
This was the explanation that I was told many years ago and there are plenty of reputable source that explain rocket propulsion in this manner.
Whoever told you that was wrong.  Reputable sources do not say that.
Quote from: clevernever on 03/02/2020 15:32:13
Even NASA’s balloon vector diagram states this:

“If you fill a balloon with air and hold the neck closed, the pressure inside the balloon is slightly higher than the surrounding atmosphere. However, there is no net force on the balloon in any direction because the internal pressure on the balloon is equal in all directions. If you release the neck of the balloon, it acts like a hole, with no surface area for the internal pressure to act on. There is now an imbalanced force on the balloon, and the internal pressure on the front of the balloon is greater than the internal pressure on the back of the balloon.”
That is odd, because when I look at the NASA website this is what it says:

Rocket Principles

A rocket in its simplest form is a chamber enclosing a gas under pressure. A small opening at one end of the chamber allows the gas to escape, and in doing so provides a thrust that propels the rocket in the opposite direction. A good example of this is a balloon. Air inside a balloon is compressed by the balloon's rubber walls. The air pushes back so that the inward and outward pressing forces are balanced. When the nozzle is released, air escapes through it and the balloon is propelled in the opposite direction.
The site that the above quote is from is:  https://www.grc.nasa.gov/www/k-12/rocket/TRCRocket/rocket_principles.html

Could you supply the website that you got your quote from because it was clearly a different NASA site.
« Last Edit: 03/02/2020 17:01:42 by Origin »
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Offline clevernever (OP)

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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #11 on: 03/02/2020 17:46:08 »
Quote from: Janus on 03/02/2020 16:33:31
Quote from: clevernever on 03/02/2020 15:32:13
Quote from: Origin on 03/02/2020 14:53:30
Quote from: clevernever on 03/02/2020 14:00:25
Yes but the imbalanced force is created by the gas pushing on the front of the rocket. It is because the exiting gas is not pushing on the rocket is what causes the imbalanced force.  The opening at the rear causes less surface area on rear side than the front side in which the gas pushes on.
That is quite simply wrong.
There is no doubt that the pressure differential between the reaction chamber and the outside is the driving force of the rocket exhaust, but it is the reaction to the exhaust momentum that causes the rocket to move.
The principle is over 300 years old.  Do you believe that with all the rockets designed and flown over the last 100 years we still do not know the most basic aspect of rocket propulsion?  Think about how absurd that is.

I don't know where you got your bogus information, but I recommend going to reputable sites to learn about science!

So you are saying that the imbalanced forces explanation is incorrect? This was the explanation that I was told many years ago and there are plenty of reputable source that explain rocket propulsion in this manner.

Even NASA’s balloon vector diagram states this:

“If you fill a balloon with air and hold the neck closed, the pressure inside the balloon is slightly higher than the surrounding atmosphere. However, there is no net force on the balloon in any direction because the internal pressure on the balloon is equal in all directions. If you release the neck of the balloon, it acts like a hole, with no surface area for the internal pressure to act on. There is now an imbalanced force on the balloon, and the internal pressure on the front of the balloon is greater than the internal pressure on the back of the balloon.”

The exiting exhaust is not pushing on the balloon, it simply causes imbalanced internal pressure.

The imbalanced internal pressure is, itself, a result of conservation of momentum.  The hot gasses in the combustion chamber are made of molecules with individual momenta.  It is the change in momentum of a molecule when it strikes the wall of of the chamber that produces the force on the wall.
For a rocket, you might imagine a person standing in a long skateboard, with a wall erected on one  end. He throws medicine balls in both directions, two at a time.  Since the pair of ball are moving in opposite directions at opposite speeds, throwing them produces no net change in his momentum.  However, one of the Ball hits a wall, bounces off and ends up moving in the same direction as the other.  The result is a change of momentum of the ball of 2mv (with m being the mass of the ball and assuming an perfectly elastic collision). The wall (and the skateboard the person is standing on) moves in the opposite direction to conserve momentum.  The end result ends up being the same as if he had just thrown both balls towards the end with no wall.

Now coat the wall with a thick rubber coating.  The compression of the rubber "softens" the impact of the ball hitting the wall, making the bounce of the ball less sudden, in turn reducing the force of the impact. But as long as the bounce is still perfectly elastic, the ball ends up still changing it momentum by 2mv, and transferring the same amount of momentum to the skateboard.

If we assume a non-elastic collision, for example the rubber absorbs some of the impact energy and converts it to heat, you can still make the impact force be equal to that for the perfectly elastic rubber, but now the ball will not change its momentum by the full 2mv, and neither will the skateboard be propelled by as much. 
In the end, it is the total end momentum of the balls that determines how fast the skateboard ends up moving.
The expanding gas in the chamber is the mechanism for the exchange of momentum, but in the end, it is the exchange and conservation of momentum that results in the rocket's motion.

In your example you have made the exhaust movement the balls, the person on the skateboard is the pressure gradient force and the wall is the rocket. The conservation of momentum explanation is that the rocket is the person on the skateboard. Because the imbalanced force is applied to the wall. The skateboard would not move unless the ball bounces off the wall and hits the person on the skateboard or the skateboard itself.

Your analogy needs more clarification or correction.

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Offline alancalverd

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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #12 on: 03/02/2020 18:09:03 »
Gas leaves the rocket. Gas has mass. Gas has velocity. Momentum is mass x velocity. There can be no change in total momentum so (mass x velocity)gas = (mass x velocity)rest of the rocket if you integrate over the entire flight. Instantaneous calculation is a bit more difficult because the system is accelerating and the masses aree changing whilst the fuel is burning, but von Ohain's equations describe all the phases of the flight of an ideal rocket if you have an interest in such matters. 
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Offline clevernever (OP)

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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #13 on: 04/02/2020 01:35:01 »
Quote from: alancalverd on 03/02/2020 18:09:03
Gas leaves the rocket. Gas has mass. Gas has velocity. Momentum is mass x velocity. There can be no change in total momentum so (mass x velocity)gas = (mass x velocity)rest of the rocket if you integrate over the entire flight. Instantaneous calculation is a bit more difficult because the system is accelerating and the masses aree changing whilst the fuel is burning, but von Ohain's equations describe all the phases of the flight of an ideal rocket if you have an interest in such matters.

When you drop a ball from the cusp of you had from a height, then the ball has momentum (mass * velocity). But the hand does not have an equal momentum of (mass * velocity). It is because gravity provided the ball’s momentum. Not the hand
Similarly pressure gradient force provide the exhaust’s momentum. Not the rocket.
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Offline alancalverd

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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #14 on: 04/02/2020 08:13:03 »
Oh yes it does. Ball goes down quickly because it has a small mass. Hand plus body plus planet goes up slowly because it has large mass. Same with gun recoil, astronauts in spacewalk, everything. Σ(mv) = 0, always.
« Last Edit: 04/02/2020 08:15:12 by alancalverd »
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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #15 on: 04/02/2020 13:32:48 »
Quote from: alancalverd on 04/02/2020 08:13:03
Oh yes it does. Ball goes down quickly because it has a small mass. Hand plus body plus planet goes up slowly because it has large mass. Same with gun recoil, astronauts in spacewalk, everything. Σ(mv) = 0, always.

So you are saying that a 175lbs person who picks up a 200lbs weight and drops it from a height will get lifted up from the ground?
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Offline Origin

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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #16 on: 04/02/2020 13:42:46 »
So since you did not reply where this quote came, is it safe to say it is not a quote from the NASA site and that you dishonestly made it up?
Quote from: clevernever on 03/02/2020 15:32:13
Even NASA’s balloon vector diagram states this:

“If you fill a balloon with air and hold the neck closed, the pressure inside the balloon is slightly higher than the surrounding atmosphere. However, there is no net force on the balloon in any direction because the internal pressure on the balloon is equal in all directions. If you release the neck of the balloon, it acts like a hole, with no surface area for the internal pressure to act on. There is now an imbalanced force on the balloon, and the internal pressure on the front of the balloon is greater than the internal pressure on the back of the balloon.”
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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #17 on: 04/02/2020 14:22:03 »
Quote from: clevernever on 04/02/2020 13:32:48
So you are saying that a 175lbs person who picks up a 200lbs weight and drops it from a height will get lifted up from the ground?

No. He's saying that the ground itself will be lifted up by its gravitational attraction to the 200 pound weight, and therefore lift the 175 pound person up in the process. All due to conservation of momentum.
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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #18 on: 04/02/2020 14:32:07 »
Quote from: Kryptid on 04/02/2020 14:22:03
Quote from: clevernever on 04/02/2020 13:32:48
So you are saying that a 175lbs person who picks up a 200lbs weight and drops it from a height will get lifted up from the ground?

No. He's saying that the ground itself will be lifted up by its gravitational attraction to the 200 pound weight, and therefore lift the 175 pound person up in the process. All due to conservation of momentum.

But the momentum is given to the earth, not the person. What if the person was on an airplane in the air?
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Re: Do rockets work due to conservation of momentum theory or imbalanced forces?
« Reply #19 on: 04/02/2020 14:34:19 »
Quote from: clevernever on 04/02/2020 14:32:07
But the momentum is given to the earth, not the person.

Some of the Earth's momentum is imparted to the person because they were standing on the ground.

Quote from: clevernever on 04/02/2020 14:32:07
What if the person was on an airplane in the air?

I would expect the upward movement of the Earth to cause a slight increase in air pressure, thus pushing the plane upward somewhat.
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Last post 08/01/2014 09:27:20
by evan_au
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