Author Topic: Conservation of energy/momentum.  (Read 39400 times)

Offline 192

  • Member
  • Posts: 31
  • Liked: 21
  • Likes Given: 79
Re: Conservation of energy/momentum.
« Reply #100 on: 10/24/2017 02:50 pm »
Let your spring have mass M1, and the person have mass M2, and at the start let them be at position 0.

Then when you release the spring, the spring moves towards the hand holding it, and the body moves in the opposite direction.

If the spring is moving at speed v, then the person will be moving in the opposite direction by M1/M2 * v.

Their velocities will always be in these proportions.

So if when then the hand and spring collide and come to rest the spring is at position x, the person will have moved in the opposite direction by M1/M2 * x.

So the center of mass of the whole system will be positioned at M1 * x - M2 * (M1/M2 * x) = 0.

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2691
  • Liked: 981
  • Likes Given: 347
Re: Conservation of energy/momentum.
« Reply #101 on: 10/24/2017 02:59 pm »
Then it's a two body problem, one hand and the spring and the hand that let go.

One hand and the spring will move one way <     > the other hand will move the other way

But the centre of mass of the combined system (both hands and the spring) will stay in the same place.

Ermmmmmmmmmmmmmmmmmmmmmmmmmmm............................. no.

Ermmmmm yes. Please stop with this nonsense.

Offline chazemz

  • Full Member
  • *
  • Posts: 145
  • england
  • Liked: 5
  • Likes Given: 0
Re: Conservation of energy/momentum.
« Reply #102 on: 10/24/2017 03:41 pm »
Let your spring have mass M1, and the person have mass M2, and at the start let them be at position 0.

Then when you release the spring, the spring moves towards the hand holding it, and the body moves in the opposite direction.

If the spring is moving at speed v, then the person will be moving in the opposite direction by M1/M2 * v.

Their velocities will always be in these proportions.

So if when then the hand and spring collide and come to rest the spring is at position x, the person will have moved in the opposite direction by M1/M2 * x.

So the center of mass of the whole system will be positioned at M1 * x - M2 * (M1/M2 * x) = 0.

Also the spring forces are internal, so I am external to the spring. If I am external to the spring, the spring is external to me. When the spring is stretched, third law applies. When I let go of the spring I move a piece on the chess board, nature must respond. The spring now applies an external force to me and I move.
If it is a requirement for the jigsaw piece to fit snugly into place with the pieces already laid, then the spring applies an exterior force to me for a short period of time.
If the question is "can you move an object without any exhaust" the answer is "yes".

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2691
  • Liked: 981
  • Likes Given: 347
Re: Conservation of energy/momentum.
« Reply #103 on: 10/24/2017 03:49 pm »
When the spring is stretched, third law applies. When I let go of the spring... The spring now applies a external force to me and I move.

The spring and you are part of the same system. There are no external forces. The centre of mass of the system does not move.
« Last Edit: 10/24/2017 03:50 pm by nacnud »

Offline chazemz

  • Full Member
  • *
  • Posts: 145
  • england
  • Liked: 5
  • Likes Given: 0
Re: Conservation of energy/momentum.
« Reply #104 on: 10/24/2017 04:15 pm »
When the spring is stretched, third law applies. When I let go of the spring... The spring now applies a external force to me and I move.

The spring and you are part of the same system. There are no external forces. The centre of mass of the system does not move.

Are you saying I do not move?

Online meberbs

  • Senior Member
  • *****
  • Posts: 3096
  • Liked: 3379
  • Likes Given: 777
Re: Conservation of energy/momentum.
« Reply #105 on: 10/24/2017 04:26 pm »
When the spring is stretched, third law applies. When I let go of the spring... The spring now applies a external force to me and I move.

The spring and you are part of the same system. There are no external forces. The centre of mass of the system does not move.

Are you saying I do not move?
Read his post again, particularly the part where he modified the quote of you to be correct.

The amount you move is limited by the amount the spring moves, and moving the spring back moves you back:
So if when then the hand and spring collide and come to rest the spring is at position x, the person will have moved in the opposite direction by M1/M2 * x.

Also the spring forces are internal, so I am external to the spring. If I am external to the spring, the spring is external to me. When the spring is stretched, third law applies. When I let go of the spring I move a piece on the chess board, nature must respond. The spring now applies an external force to me and I move.
If it is a requirement for the jigsaw piece to fit snugly into place with the pieces already laid, then the spring applies an exterior force to me for a short period of time.
If the question is "can you move an object without any exhaust" the answer is "yes".
If you insist on considering the spring as external to you, then the motion of the spring is your exhaust, so no, you are not moving without exhaust.

Offline nacnud

  • Extreme Veteran
  • Senior Member
  • *****
  • Posts: 2691
  • Liked: 981
  • Likes Given: 347
Re: Conservation of energy/momentum.
« Reply #106 on: 10/24/2017 04:28 pm »
No, that the centre of mass of you plus the spring does not move.

Offline chazemz

  • Full Member
  • *
  • Posts: 145
  • england
  • Liked: 5
  • Likes Given: 0
Re: Conservation of energy/momentum.
« Reply #107 on: 10/24/2017 04:46 pm »
The amount you move is limited by the amount the spring moves, and moving the spring back moves you back:

The spring will apply equal and opposite again. If you are suggesting that I move when I move my arms then I can simply do the superman maneuver and go where I please.

If you insist on considering the spring as external to you, then the motion of the spring is your exhaust, so no, you are not moving without exhaust.

If that is all you have then I am making progress.

 No, that the centre of mass of you plus the spring does not move.

Does my position change?

Since we have established that I accelerate, which terminology do you prefer, exterior force or unbalanced force?


« Last Edit: 10/24/2017 05:02 pm by chazemz »

Offline whitelancer64

Re: Conservation of energy/momentum.
« Reply #108 on: 10/24/2017 05:13 pm »
The amount you move is limited by the amount the spring moves, and moving the spring back moves you back:

The spring will apply equal and opposite again. If you are suggesting that I move when I move my arms then I can simply do the superman maneuver and go where I please.

If you insist on considering the spring as external to you, then the motion of the spring is your exhaust, so no, you are not moving without exhaust.

If that is all you have then I am making progress.

 No, that the centre of mass of you plus the spring does not move.

Does my position change?

Since we have established that I accelerate, which terminology do you prefer, exterior force or unbalanced force?

No. You do not accelerate.

Moving your arm shifts a part of you around, but does not move your center of mass. All you are doing is redistributing your mass. Moving your arm back to its original position will return you to your original position.

Here is something you can do to understand this concept:

Take a pen and a ruler. Remove the cap from the pen so it rolls smoothly. Place the ruler on top of the pen. Imagine this is your spacecraft, with yourself and the springs inside it, and the pen is the center of mass. Shifting your body or the springs is like sliding the ruler to the left. Shifting yourself or the springs back to your original position is like sliding the ruler to the right. But no matter how you move, the spacecraft only shifts around the center of mass.
"One bit of advice: it is important to view knowledge as sort of a semantic tree -- make sure you understand the fundamental principles, ie the trunk and big branches, before you get into the leaves/details or there is nothing for them to hang on to." - Elon Musk
"There are lies, damned lies, and launch schedules." - Larry J

Offline chazemz

  • Full Member
  • *
  • Posts: 145
  • england
  • Liked: 5
  • Likes Given: 0
Re: Conservation of energy/momentum.
« Reply #109 on: 10/24/2017 05:45 pm »
The amount you move is limited by the amount the spring moves, and moving the spring back moves you back:

The spring will apply equal and opposite again. If you are suggesting that I move when I move my arms then I can simply do the superman maneuver and go where I please.

If you insist on considering the spring as external to you, then the motion of the spring is your exhaust, so no, you are not moving without exhaust.

If that is all you have then I am making progress.

 No, that the centre of mass of you plus the spring does not move.

Does my position change?

Since we have established that I accelerate, which terminology do you prefer, exterior force or unbalanced force?

No. You do not accelerate.

Moving your arm shifts a part of you around, but does not move your center of mass. All you are doing is redistributing your mass. Moving your arm back to its original position will return you to your original position.

Here is something you can do to understand this concept:

Take a pen and a ruler. Remove the cap from the pen so it rolls smoothly. Place the ruler on top of the pen. Imagine this is your spacecraft, with yourself and the springs inside it, and the pen is the center of mass. Shifting your body or the springs is like sliding the ruler to the left. Shifting yourself or the springs back to your original position is like sliding the ruler to the right. But no matter how you move, the spacecraft only shifts around the center of mass.

Thank you for the reply. I am a little confused. If I am holding the spring in my left hand after it has contracted, does this refer to moving my right hand to my left hand. If so would not the opposite happen if I move my left hand, with spring, to my empty hand. What will happen If I move both hands together at the same time?

Offline whitelancer64

Re: Conservation of energy/momentum.
« Reply #110 on: 10/24/2017 06:02 pm »
The amount you move is limited by the amount the spring moves, and moving the spring back moves you back:

The spring will apply equal and opposite again. If you are suggesting that I move when I move my arms then I can simply do the superman maneuver and go where I please.

If you insist on considering the spring as external to you, then the motion of the spring is your exhaust, so no, you are not moving without exhaust.

If that is all you have then I am making progress.

 No, that the centre of mass of you plus the spring does not move.

Does my position change?

Since we have established that I accelerate, which terminology do you prefer, exterior force or unbalanced force?

No. You do not accelerate.

Moving your arm shifts a part of you around, but does not move your center of mass. All you are doing is redistributing your mass. Moving your arm back to its original position will return you to your original position.

Here is something you can do to understand this concept:

Take a pen and a ruler. Remove the cap from the pen so it rolls smoothly. Place the ruler on top of the pen. Imagine this is your spacecraft, with yourself and the springs inside it, and the pen is the center of mass. Shifting your body or the springs is like sliding the ruler to the left. Shifting yourself or the springs back to your original position is like sliding the ruler to the right. But no matter how you move, the spacecraft only shifts around the center of mass.

Thank you for the reply. I am a little confused. If I am holding the spring in my left hand after it has contracted, does this refer to moving my right hand to my left hand. If so would not the opposite happen if I move my left hand, with spring, to my empty hand. What will happen If I move both hands together at the same time?

Either way, you are still just shifting mass around. Which arm moves, or moving both, doesn't matter.

*edit* Maybe watching videos of astronauts on the ISS would help, too. Like this one,
« Last Edit: 10/24/2017 06:03 pm by whitelancer64 »
"One bit of advice: it is important to view knowledge as sort of a semantic tree -- make sure you understand the fundamental principles, ie the trunk and big branches, before you get into the leaves/details or there is nothing for them to hang on to." - Elon Musk
"There are lies, damned lies, and launch schedules." - Larry J

Online meberbs

  • Senior Member
  • *****
  • Posts: 3096
  • Liked: 3379
  • Likes Given: 777
Re: Conservation of energy/momentum.
« Reply #111 on: 10/24/2017 07:59 pm »
Please use quotes properly.

The spring will apply equal and opposite again.
Yes, the spring applying equal and opposite again is why moving the spring back to its original position will move you back to your original position.

If you are suggesting that I move when I move my arms then I can simply do the superman maneuver and go where I please.
I have no idea how you get from me saying that you are limited by how far the spring moves to thinking the superman maneuver lets you fly. That is as opposite to what I said as possible. If you are standing on a frictionless surface (or in space) moving your arm to the right will move the rest of your body slightly to the left, and moving your arm back restores you to the original position.

Quote from: meberbs
If you insist on considering the spring as external to you, then the motion of the spring is your exhaust, so no, you are not moving without exhaust.

If that is all you have then I am making progress.
You are only making progress if you understand what I said, and your response indicates that you don't.

Quote from: nacnud
No, that the centre of mass of you plus the spring does not move.
Does my position change?
You claimed that you understand the concept of center of mass, so why don't you answer your own question: Given that the center of mass does not move, and the position of the spring has changed, has your position changed?

Since we have established that I accelerate, which terminology do you prefer, exterior force or unbalanced force?
Since you are holding onto the spring it is simpler to consider the system of you plus the spring, rather than the spring and you separately. The preferred term is therefore internal force. The same applies to the system with you and the magnets, because while you may not be physically touching the magnet the whole time, you both start and end the scenario while touching the magnet.

Offline chazemz

  • Full Member
  • *
  • Posts: 145
  • england
  • Liked: 5
  • Likes Given: 0
Re: Conservation of energy/momentum.
« Reply #112 on: 10/25/2017 01:36 pm »
Some people seem to be getting confused by the spring. We will assume that my center of mass is the middle of my chest. With no spring attached move my arms away from my chest and then return them to their original position. Now consider what will happen when I do so.
Now attach the spring to the middle of my chest and stretch the spring, letting go, and returning my arms to the end of the spring.
Now consider what will happen. Since the spring has applied an unbalanced force to me, the result must be different.

Online meberbs

  • Senior Member
  • *****
  • Posts: 3096
  • Liked: 3379
  • Likes Given: 777
Re: Conservation of energy/momentum.
« Reply #113 on: 10/25/2017 04:18 pm »
Some people seem to be getting confused by the spring.
The only person here confused by the spring is you.

We will assume that my center of mass is the middle of my chest. With no spring attached move my arms away from my chest and then return them to their original position.Now consider what will happen when I do so.
Since you manage to find a way to misunderstand everything you have been told, I supplied an attached diagram. Your arms move one way, the rest of your body moves the other. The blue line represents the position of the center of mass relative to the horizontal axis. You return to your original position by returning your arms to your sides.

Now attach the spring to the middle of my chest and stretch the spring, letting go, and returning my arms to the end of the spring.
Now consider what will happen. Since the spring has applied an unbalanced force to me, the result must be different.
Net forces between you and the spring are no different than the net forces between your arms and the rest of your body. The exact same diagram I drew applies, except with the spring you are holding effectively making your arms more massive (but I had exaggerated the relative mass of arms in the diagram anyway for clarity.)

Breaking it up this way with your arms not being part of you seems weird, but works no different than you continuing to treat a spring that is attached to you as not part of the system.

Edit: Missed the attachment the first time.
« Last Edit: 10/25/2017 04:27 pm by meberbs »

Offline chazemz

  • Full Member
  • *
  • Posts: 145
  • england
  • Liked: 5
  • Likes Given: 0
Re: Conservation of energy/momentum.
« Reply #114 on: 10/26/2017 03:12 pm »
Since you manage to find a way to misunderstand everything you have been told, I supplied an attached diagram. Your arms move one way, the rest of your body moves the other. The blue line represents the position of the center of mass relative to the horizontal axis. You return to your original position by returning your arms to your sides.


Just so I do not misunderstand you. You are saying that as I move my arms away from my body, my body moves away from my arms and when I move my arms towards my body , my body moves towards my arms. Is that correct?

Online meberbs

  • Senior Member
  • *****
  • Posts: 3096
  • Liked: 3379
  • Likes Given: 777
Re: Conservation of energy/momentum.
« Reply #115 on: 10/26/2017 03:43 pm »
Just so I do not misunderstand you. You are saying that as I move my arms away from my body, my body moves away from my arms and when I move my arms towards my body , my body moves towards my arms. Is that correct?
Correct, this is clearly necessary for your center of mass to remain in the same position, and for conservation of momentum.

Offline Bob012345

  • Full Member
  • ****
  • Posts: 735
  • Liked: 171
  • Likes Given: 278
Re: Conservation of energy/momentum.
« Reply #116 on: 10/26/2017 04:49 pm »
Just so I do not misunderstand you. You are saying that as I move my arms away from my body, my body moves away from my arms and when I move my arms towards my body , my body moves towards my arms. Is that correct?
Correct, this is clearly necessary for your center of mass to remain in the same position, and for conservation of momentum.

This is a rare case where if you cut your nose off to spite your face, you may actually get somewhere. :)

Offline Bob012345

  • Full Member
  • ****
  • Posts: 735
  • Liked: 171
  • Likes Given: 278
Re: Conservation of energy/momentum.
« Reply #117 on: 10/26/2017 06:19 pm »
Since you manage to find a way to misunderstand everything you have been told, I supplied an attached diagram. Your arms move one way, the rest of your body moves the other. The blue line represents the position of the center of mass relative to the horizontal axis. You return to your original position by returning your arms to your sides.


Just so I do not misunderstand you. You are saying that as I move my arms away from my body, my body moves away from my arms and when I move my arms towards my body , my body moves towards my arms. Is that correct?

Not exactly the same thing but a related concept is what happens when ice skaters spin up and down. In that case, the center of mass is also fixed but the Moment of Inertia changes as arms are pushed out or pulled in and the spin speed adjusts to conserve angular momentum. In this case, the center of mass is fixed but the distribution of mass changes when arms are pushed out or pulled in.

Offline Nick

  • Full Member
  • *
  • Posts: 105
  • UK
  • Liked: 21
  • Likes Given: 4
Re: Conservation of energy/momentum.
« Reply #118 on: 10/26/2017 07:10 pm »
Just so I do not misunderstand you. You are saying that as I move my arms away from my body, my body moves away from my arms and when I move my arms towards my body , my body moves towards my arms. Is that correct?
That is absolutely correct. It is somewhat counter-intuitive, because it is at variance with our everyday experience.

When you stand on the ground and raise your arms out in front of you, you normally do not detect any movement of your body. That is because friction on the soles of your feet couple you to the planet and so the system that moves back is your body plus the planet. The planet is so much more massive than your arms that the movement is infinitesimal.

If you conducted the experiment on a frictionless surface (an air table or ice rink, say) or in space, the effect would be observable if you took care. Your arms account for about 10% of an average body mass, so if you raise your arms out in front of you, you move their centre of mass forward about a foot (half an arm's length). The centre of mass of the rest of your body will move back about one-ninth of that, i.e. just over one inch. The centre of mass of the combined system (arms plus body) won't move at all.

(Similarly, your body will move down as you raise your arms - again not observed in everyday experience, because of the forces exerted by the surface of the Earth supporting you)

Offline chazemz

  • Full Member
  • *
  • Posts: 145
  • england
  • Liked: 5
  • Likes Given: 0
Re: Conservation of energy/momentum.
« Reply #119 on: 10/27/2017 09:21 am »
Just so I do not misunderstand you. You are saying that as I move my arms away from my body, my body moves away from my arms and when I move my arms towards my body , my body moves towards my arms. Is that correct?
That is absolutely correct. It is somewhat counter-intuitive, because it is at variance with our everyday experience.

When you stand on the ground and raise your arms out in front of you, you normally do not detect any movement of your body. That is because friction on the soles of your feet couple you to the planet and so the system that moves back is your body plus the planet. The planet is so much more massive than your arms that the movement is infinitesimal.

If you conducted the experiment on a frictionless surface (an air table or ice rink, say) or in space, the effect would be observable if you took care. Your arms account for about 10% of an average body mass, so if you raise your arms out in front of you, you move their centre of mass forward about a foot (half an arm's length). The centre of mass of the rest of your body will move back about one-ninth of that, i.e. just over one inch. The centre of mass of the combined system (arms plus body) won't move at all.

(Similarly, your body will move down as you raise your arms - again not observed in everyday experience, because of the forces exerted by the surface of the Earth supporting you)


 We can now move to the spring. As I pull the spring away from me and stretch it, the spring will resist and apply a force against the movement of my arms and my body.
So does my body still move backwards (and if so by how much), applying a ever greater force on the spring and vice versa or is its movement hampered by the increasing potential energy of the spring?


Tags:
 

Advertisement NovaTech
Advertisement Northrop Grumman
Advertisement
Advertisement Margaritaville Beach Resort South Padre Island
Advertisement Brady Kenniston
Advertisement NextSpaceflight
Advertisement Nathan Barker Photography
0