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in this sketch suppose the velocity of ball is 0 and it is out from the system and stayed in 0 velocity due to two electromagnets and these two magnets are attached with two springs and both springs are in stretched position towards up and down side . The ball is expelled from the gun system and got 0 velocity after expelling from the system by using these electromagnets and a force is also applying on these magnets and springs are in stretched position. you will say that engine will not move as net force is ZERO but if we release these spring then what will be happened with engine as after releasing these springs both will work to propel the engine ? should it move or not or can we modify the direction of this engine ? please keep aside the issue of ball retrieving. Thanksenter image description here

vikram
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  • You're asking about a perpetual motion machine. Your question details even tell you what's wrong with your question--net force is still zero, even with the springs.

    Perpetual motion does not work.

     – Justin Eiler Sep 30 '16 at 02:19
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    Why did you start a new identity for a question that will have much the same answers as your last question? Admittedly, your current question underlines you did not understand those answers, but that's no good reason to establish a new identity and re-ask (effectively) the same question, which boils down to "Can we break - 'for every action, there is an equal & opposite reaction'?" – Andrew Thompson Sep 30 '16 at 02:20
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    "please keep aside the issue of ball retrieving." But that is precisely the entire problem you are apparently trying to solve. "Assume that we don't have to expend propellant. Now, I have a way for rockets to work without expending propellant!" That's just not going to fly. – Nathan Tuggy Sep 30 '16 at 02:49
  • Please read this page. Use the equations provided to determine if your system could work. – John Bode Sep 30 '16 at 03:05
  • suppose ball is also a magnet but what happened when stretched springs will get back their rest position again after Zero net force? – vikram Sep 30 '16 at 07:19
  • I know it is hard to accept, especially when answers plead with laws and experience. But please consider that a perpetual motion machine would have been found by someone if it were so simple. Ask yourself what could be wrong with your idea, users here will quickly lose their patience with this type of questions. – Andreas Sep 30 '16 at 08:13

1 Answers1

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When you fire the ball, you use the gun to put a force on it, pushing it backwards. Newton's Third Law tells us that "for every action, there is an equal and opposite reaction". This means that the gun will experience a force pushing it forward.

If you want to stop the ball, you must apply a force to it. However, in doing so, you will also apply a force to whatever is stopping the ball. In this case, it would be the electromagnets attached to the springs. This will cause the springs to stretch out. However, this will put a force on the gun, since the springs are attached to the gun. This will pull the gun back to its original position and stop it, resulting in no net motion and no net velocity.

You cannot create an engine that does not expend fuel or energy to move. The Laws of Thermodynamics state that in order to do any kind of work, such as moving something, we will lose energy. If you think you have found a way to propel a spacecraft without ever losing propellant, you haven't. No matter what you think, it cannot be done.

Phiteros
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  • but what when these stretched springs will be released? – vikram Sep 30 '16 at 07:18
  • you are right and I understand all things but what I want to ask you you are not answering me that these stretched springs will be in stretched position for ever and cannot get back their rest position again after releasing or turning off the electricity of electromagnets and if springs will get back their rest position then there will be no force applied on this engine to move it .just give me answer only of this question .we can retrieve these magnets ball again and I have a solution for this. – vikram Sep 30 '16 at 07:35
  • @vikram-gupta11 It seems your spring are initially locked in extended position after they slowed the ball. Once you unlock them, they will return to relaxed state. This return involves a small acceleration of the spring and everything attached and then a small deceleration (unless you want the spring to pass through your engine and escape in forward direction. The forces of these two actions will again cancel. – Andreas Sep 30 '16 at 08:05
  • @vikram-gupta11 Think about it this way. If you connect two people on rollerskates by a spring, then have them push off each other, what happens? They accelerate away from each other. But as the spring extends, it slows them and brings them to a stop. Then when the spring contracts, it will bring them back to their original positions. This is the same principle here. When you are dealing with a closed system with no external forces, the center of mass of the whole system will never change. Never. – Phiteros Sep 30 '16 at 17:38
  • Dear Sir, I Took a Rubber band and try to stretched it with my hands and it was stretched .Now I took the help of an another person to catch this stretched rubber band in middle with the help of fingers .after that I released the right part of the rubber band but I didn't release the left part of the rubber band .( still in stretched position) only released the right part .when I released the right part of the rubber band then the person felt a propelling force working on pushing his fingers in left direction of stretched rubber band. – vikram Oct 01 '16 at 07:03
  • suppose a person is standing up on a roller skate applying equal force with both hands to stretch the springs and these springs are attached with this skate . the roller skate is stable but when thIs person release ONLY the right hand side sprIng then the roller skate should move or not.REMEMBER PERSON IS RELEASING ONLY RIGHT SIDE SPRING BUT LEFT SIDE SPRING IS STILL IN STRETCHED POSITION. – vikram Oct 01 '16 at 10:08
  • @vikram-gupta11 If you are doing something like that, then you are applying an outside force to the system. In this case, you cannot consider this to be a closed system like a rocket and its propellant. Also, a spring does not just pull on one side. It pulls on both sides an equal amount. So you cannot just release the right side of a spring. When it compresses back to its unstretched position, it will do so on both sides. – Phiteros Oct 01 '16 at 18:49
  • Dear Sir,we can just release the right side springs just applying force on left side spring and you can do it and i hope if you try it you will be convinced that it is working as I say.In this roller skate and person example there is no outside force as the person is also a part of the system when he is applying equal force in opposite direction BUT releasing only right side spring .This roller skate will move definitely and I also a done some experiment and this will work .There is no doubt or question that this device will not work. – vikram Oct 02 '16 at 18:26
  • @vikram-gupta11 Again, if you are doing that, you are applying an external force. Where would this force be coming from? – Phiteros Oct 02 '16 at 21:47
  • But person who is standing up on the roller skate ,is applying the force with his both hands in opposite direction but releasing only one side spring and he is also a part of this system . – vikram Oct 03 '16 at 07:00
  • I can prove it that this engine will work or the person who is applying force on the roller skate then this roller skate will move or get momentum. – vikram Oct 03 '16 at 07:02
  • I am sorry, but this system will not work. You can explain it many different ways - equal and opposite forces, conservation of energy, or conservation of momentum. They all say that you cannot get a net change in velocity on one side without changing the velocity of the other side in an opposite manner. – Phiteros Oct 03 '16 at 14:28
  • without working model no one will be convinced but it will work – vikram Oct 04 '16 at 18:04
  • You cannot have a net change in momentum without an external force. A rocket-propellant system has no external force acting on it. This is a fundamental law of physics. Nobody will believe you because it cannot be done. And you cannot make a working model because, again, it cannot be done. – Phiteros Oct 04 '16 at 18:17
  • https://worldbuilding.stackexchange.com/questions/104593/can-you-get-an-orbital-mass-driver-to-work-this-way-to-propel-in-space-without – Muze Feb 19 '18 at 20:57