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Question about movement in isolated system

  1. Jul 14, 2015 #1
    I'm new to the forums and unfortunately I have a pretty dumb question...

    So in isolated system internal forces cancel each other out every single time,like pushing on a door of your car does not create any motion of your car at all.

    You basically can't pull yourself up by the bootstraps.

    I also know that momentum which is mass*velocity is conserves in an isolated system so if there is no velocity,there is no momentum,there is no conservation.

    But here is the question:

    Why does the rocket move,gun recoils if forces in such systems (as you know they are isolated,at least it says so) cancel each other out,are not they isolated JUST like the car-door system,hair-human system?

    I know that there is conservation of momentum,but are not forces supposed to make the body stay still,be in equilibrium?

    Or maybe forces do not prevent the momentum from appearing because of 1 Newton's law where it clearly states that the body can move without acceleration even though forces on it cancel each other out?

    It's just this is the main explanation for such problems such as Münchausen,pushing boat while being inside of it,pushing door of your car while being inside of it - forces cancel each other out but forces cancel each other out in rocket/gun system but they DO move!

    So basically there is no other way to explain the reason of body's movement in an isolated system but saying that in this example there is no momentum and in that example there is momentum?

    I'm really puzzled...
     
  2. jcsd
  3. Jul 14, 2015 #2

    Doc Al

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    Realize that the rocket ejects fuel and the gun ejects a bullet. If you look at the complete system of "rocket + ejected fuel" or "gun + bullet" there will be no change in the momentum of the center of mass. But if you just take the rocket or gun by itself, then the ejected material exerts forces and that subsystem is not isolated.
     
  4. Jul 14, 2015 #3

    Drakkith

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    Not true. Conservation laws always apply. For a system with zero momentum, this means that the momentum must remain zero unless an external force acts on the system.

    Conservation of momentum says that the center of mass of the system cannot change if only internal forces act on the system. As a rocket or gun expels mass, the center of mass of the system (CoM of the rocket/gun + expelled mass) does not move.
     
  5. Jul 14, 2015 #4

    HallsofIvy

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    I can't speak for what "it says" but those are NOT isolated systems because in the case of the rocket movement you are not including the rocket exhaust and in the case of the gun recoil you are not including the bullet or other projectile.

    If you did include the rocket exhaust you would find that the center of mass of the entire system, rocket and rocket exhaust, would not move. And in the case of the gun, the center of mass of gun and bullet would not move.
     
  6. Jul 14, 2015 #5
    Yes,my bad,I meant rocket+fuel and gun+bullet and I do understand that momentum conserves but I don't understand how does the velocity appears if forces are balanced,why does it move if forces are equal to zero?There is no explanation to that?

    They clearly state that car does not move when you push it's door because forces are equal and opposite,equal to zero in an isolated system but in the rocket/gun example there is movement,there is velocity.
     
  7. Jul 14, 2015 #6

    Drakkith

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    The forces aren't balanced. The force acting on the rocket from the exhaust is not balanced by anything, so the rocket accelerates. Similarly, the force acting on the exhaust from the rocket is also no balanced by anything.

    Pay careful attention to which force is acting on which object. If two forces are equal and opposite, but acting on different objects, then they DON'T cancel out.
     
  8. Jul 14, 2015 #7

    Doc Al

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    Just because two things (bullet and gun, say) exert equal and opposite forces on each other (per Newton's 3rd law) does not mean that they cannot accelerate. That depends on the situation.

    What is required for the "car + person" to start moving if you push on the door is an external force acting on it. In the gun case, the bullet is not attached to the gun so it exerts an external force on the gun. Not so in the car case, where multiple forces are involved that prevent you or the car from moving. You push on the door and on the seat (for example) and the door and the seat push on you.
     
  9. Jul 14, 2015 #8
    Are not forces in system "car" acting on different objects too?Hands act on the door and the door acts on hands yet there is no movement because how they say the system is isolated,so the forces altogether are equal to zero.
    So they consider rocket+fuel/gun+bullet systems isolated,so forces altogether are equal to zero,yet rocket moves and gun recoils.

    Here's what I've read:

    "You Can't Bully Yourself...
    Have you ever noticed that you can't push yourself? You can push a book and it accelerates, and you can push another person and they accelerate, but you can't accelerate yourself by pushing yourself.

    You can lift a book off the table, and you can lift another person off the ground, but you can't lift yourself off the ground. (A person can't literally "pull themselves up by the bootstraps" as the old saying says...)

    Because:
    Suppose that one part of an object is pushing on another part - your right hand is pushing on your left hand. Newton's Third Law tells you that both hands exert forces, and that the force on your right hand is equal and opposite to the force on your left. Previously,[/PLAIN] [Broken] you saw that the force that your right hand exerts on your left hand accelerates your left hand, and that the force your left hand exerts on your right hand accelerates your right hand - and you can see and feel that happening.

    Notice, though, that no matter how hard you push, the forces your hands exert on one another will not accelerate your body as a whole.

    Forces exerted by one part of an object on another part of the same object are called internal forces - and

    internal forces never influence the motion of an object.
    Newton's Third Law action/reaction forces between objects do not cancel - but internal forces (Newton's Third Law action/reaction forces withinan object) do cancel.

    Forces between distinct, separate objects are called external forces, and external forces DO influence the motion of objects"
     
    Last edited by a moderator: May 7, 2017
  10. Jul 14, 2015 #9

    Drakkith

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    You're missing another pair of forces though. The car acts on the door and the door acts on the car at the hinge point. These forces keep the car and door together even though you've pushed on the door. If the door wasn't connected to the car, there would indeed be motion when you pushed on the door (though the center of mass wouldn't change).
     
  11. Jul 14, 2015 #10

    Doc Al

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    Don't mix up Newton's 2nd and 3rd laws. If you want to know if something accelerates, you must use Newton's 2nd law, which involves summing all forces acting on the object/system.
    Are you sure about that?

    Do this: Put your palms together and push them against each other. They exert equal and opposite forces on each other, yet your hands don't move. Why is that?
     
    Last edited by a moderator: May 7, 2017
  12. Jul 14, 2015 #11
    I've read that from some site and oh,what they meant in palm example is that forces acting ON both of the hands are not equal and opposite.

    I still don't understand though...I thought that forces in gun+bullet system are external just like in rocket+fuel system.They say that THESE systems are isolated so forces in such systems always cancel each other out just like in example where Münghausen pulls his hair to raise himself from the swamp or whatever.
     
  13. Jul 14, 2015 #12

    Doc Al

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    If you define your system as "gun" or "bullet" (keep them as separate systems) then they exert external forces on each other. If you define your system as "gun + bullet", then there are no external forces and the center of mass of the composite system will remain at rest (if it was at rest). That doesn't mean that the separate pieces (the gun and the bullet) cannot move away from each other.
     
  14. Jul 14, 2015 #13

    Drakkith

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    To bounce off of what Doc Al said, if you consider the bullet and the gun as separate systems, then the forces acting on each system are external forces and the center of mass of each system DOES move.
     
  15. Jul 14, 2015 #14

    Nugatory

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    You have to include all the forces. Hands push on door, door pushes on hands; because the door is pushing on the hands and the hands are attached to you... what's stopping you from moving? You're pushing against the seat and the seat is pushing back on you. And the seat is attached to the car, which is attached to the door... so the car is pushing the door one direction and you're pushing the door the other direction, and the door doesn't move. Likewise, the door is pushing on you, but the seat is pushing on you in the opposite direction, so you don't move.... And it's that way for every piece of the chain.
     
  16. Jul 14, 2015 #15
    I think I understand what you mean,like i have 2 balls in the space and one pushes another,I can make 2 separate systems or a single [ball+ball] system and it won't stop their movement.

    Ok so if there is no way for me to keep 2 objects as separate systems like the example with baron Münhausen's hands and his hair - the system is his entire body (I cannot separate these systems,it just won't work) , then there is no movement,right?

    I just don't understand how is it possible to keep rocket and fuel as separate systems since it is inside of the rocket...
     
  17. Jul 14, 2015 #16

    A.T.

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    How you choose to define your system is independent of that.
     
  18. Jul 14, 2015 #17
    I know,I know but on one site there were 3 problems and in each problem we had to determine the isolation of the system,if it was not possible then the body could move,if it was then the body could not move.

    Like in the example with 2 balls the system is not isolated so there IS motion but in the example with Münhausen the system IS isolated so there is no motion.
     
  19. Jul 14, 2015 #18

    A.T.

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    Just check if there are forces on parts of the system by something that is not part of the system.
     
  20. Jul 14, 2015 #19
    Yes but I think what they mean is that if in isolated system there is no movement then there is a certain system that basically satisfies the reason body moves or not,like with Münhausen - the isolated system in this case is the body because body does not move at all when he pulls himself by his hair.
     
  21. Jul 14, 2015 #20
    Am I right,guys?
     
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