Quote from: nacnud on 10/21/2017 05:55 pmThen 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 wayBut the centre of mass of the combined system (both hands and the spring) will stay in the same place.Ermmmmmmmmmmmmmmmmmmmmmmmmmmm............................. no.
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 wayBut the centre of mass of the combined system (both hands and the spring) will stay in the same place.
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.
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.
Quote from: chazemz on 10/24/2017 03:41 pmWhen 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.
Quote from: nacnud on 10/24/2017 03:49 pmQuote from: chazemz on 10/24/2017 03:41 pmWhen 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?
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".
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?
Quote from: chazemz on 10/24/2017 04:46 pmThe 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.
Quote from: whitelancer64 on 10/24/2017 05:13 pmQuote from: chazemz on 10/24/2017 04:46 pmThe 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?
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.
Quote from: meberbsIf 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.
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.
Quote from: nacnud No, that the centre of mass of you plus the spring does not move.Does my position change?
No, that the centre of mass of you plus the spring does not move.
Since we have established that I accelerate, which terminology do you prefer, exterior force or unbalanced force?
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.
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?
Quote from: chazemz on 10/26/2017 03:12 pmJust 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.
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?
Quote from: chazemz on 10/26/2017 03:12 pmJust 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)