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Internal Forces In a System

  1. Mar 13, 2014 #1
    My high school teacher told me that internal forces of a system do not change the Centre of Gravity of a system. I have doubt regarding this.
    If internal force is applied to the wall of a system, the CG must change right? If not, where'll be the result of applied force.
    I have some examples that I think are related to this:

    1) Consider zorbing. Here, the ball rolls if you apply a force from within. So, that counts as an internal force.

    2) Consider a child swinging in a park. Once we push the swing, it keeps swinging until it stops. Now, if the child stretches its legs and folds them during forward and backward motion, the total time of swinging is prolonged. Since there is no external force, the child's movement can be considered as an internal force. So, internal force prolongs motion.

    [Fiction Ahead]
    3)Finally, if you have seen Pirates of Caribbean At World's End, you'll know what Jack Sparrow does to get out of the other world. There, the barrels hitting inside the hull of the ship inverts it. They are internal forces too. (I know this is not totally appropriate but it came to my mind and I posted it)

    Someone please resolve my doubt.

    Prajwal

    EDIT: The teacher said the position of CG is not changed.
     
    Last edited: Mar 14, 2014
  2. jcsd
  3. Mar 13, 2014 #2

    Dale

    Staff: Mentor

    Internal forces do not change the center of mass (which is not necessarily the same as the center of gravity). This follows directly from Newtons third law.

    No, the wall pushes back just as hard as the applied force, for no net acceleration.

    It is the external forces from the ground and gravity which propel the ball.

    Again, there are external forces from gravity and the hinge which cause the acceleration.

    This is fiction.
     
  4. Mar 13, 2014 #3
    DaleSpam's answer is terrible. You need to be very careful at being clear about what "the system" is.
    I can't comment on At World's End, I don't remember it, and its fiction! WHY would you post that?? Most children know the difference between fantasy and fact by age 12.
    In #2, what is the system? Is it "ok" to remove the swing's posts? how about if we put each of the swing's leg posts onto wheels...the ground is part of this system, as you should be able to see.
    In #1, same point...the ground is part of the system.
    Picture a spaceship shaped like a bar bell...two spheres separated by a tube. Now, you walk from one ball through the tube over to the other sphere. Obviously, you have redistributed the mass of the system. But what does the external observer see? Every step you take, you push the spaceship in the opposite direction (just a very small amount). The center of mass of the system doesn't change although externally the ship appears to have shifted just a bit in the direction opposite the direction you moved (only a bit, because I assume its mass is much more than yours). An approximation of this is you in a boat. You move in the boat and the boat moves in the opposite direction (in an ideal world). So, your teacher is being a bit sloppy about what s/he is talking about in terms of the system and its surroundings. You could also consider what happens to the spaceship if its spinning around an axis perpendicular to the axis of the tube (end-over-end) as you walk towards the center tube, the spin increases, as you move away, it slows...angular momentum is conserved...I leave it to you to picture what this would look like to an external observer...
    BottomLine: Every action has an equal and opposite reaction.
    BTW, generally you would say (in English) that you have "questions" or "a question" about what your teacher said. "Doubt" is often used in India, but is generally considered an emotion, not necessarily concerned with facts. You would say (in most of the world's English):"I have questions regarding this."
     
  5. Mar 14, 2014 #4

    Dale

    Staff: Mentor

    And yours is wrong.

    The definition of the system is entirely arbitrary and is in no way required to include the ground. If you include the entire planet in your system then you have an approximately isolated system where all of the forces of interest are internal. However, there is no natural definition of a system and no requirement that they be isolated. If the system is not isolated then the external forces can accelerate the COM, while the internal forces cannot.
     
  6. Mar 14, 2014 #5
    What would you say about zorbing on level ground? You dont have a slope there for the ball to roll down due to gravity. And, the ground pushes you forward due to the force you apply inside the ball. Doesn't that imply that you are causing the motion thereby changing the position of CG?

    And, while swinging, gravity acts always, whether you perform the motion of your leg or not. While you perform the motion, the time period is prolonged. Why? The motion of legs must create a force (other than that of hinges and gravity) and since you are in motion with the swing, and are sitting on it, you become a part of the system.
    If you do not understand what I'm saying, go to a park (visualize) , sit on a swing, ask someone to push you from behind. While you are moving forward, stretch out your legs forward and while you are moving backward, fold your legs. This will prolong the motion of the swing.

    Prajwal

    PS: Added an edit to OP.
     
  7. Mar 14, 2014 #6

    A.T.

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    Gold Member

    You just said yourself that the ground applies a forward force. Without that external force the CoM cannot accelerate.

    Doesn't matter what causes what. Without external forces the CoM cannot accelerate.
     
  8. Mar 14, 2014 #7
    Yes but the force that causes the external force is your push. Ultimately, the source is you pushing right?
     
  9. Mar 14, 2014 #8

    Dale

    Staff: Mentor

    No, it is only the external forces which can cause a change of the COM. Think what would happen if you were zorbing on some level but frictionless surface. Would you travel forward? You could climb up inside the ball, and all that would happen is that the ball would slip around in place without moving you forward at all. In the end, it is that external friction force which causes the motion, not the internal forces. If you remove the external forces you get no motion.

    The motion of the legs changes the shape of the system composed of you and the swing, it does not change the COM. If you make the approximation that the seat and chain are relatively light so that the COM of the system is equal to the COM of the person then think what happens when you extend your legs. Relative to the seat your COM goes backwards, as do your hands holding the chain. Since those are internal forces to the system (rider, seat, and chain), the COM is not changed, which means that the seat goes forward and the chain backwards. Now, the tension in the chain at the hinge has a component in the forward direction, and this is an external force which can and does accelerate the COM.

    Again, think what would happen if you eliminated the force due to gravity and the hinge by setting up your swing set in orbit. It doesn't matter how you wiggle your legs, your COM wouldn't go anywhere. The motion of the COM is entirely due to the external forces and without those external forces there is no COM acceleration.

    The internal forces are important because they make the system a non-rigid body and they are involved in changing the configuration of the system in such a way as to allow the external forces to accelerate the COM. But you can change a non-rigid system's configuration all you like and still not cause any acceleration in the COM in the absence of external forces.
     
    Last edited: Mar 14, 2014
  10. Mar 14, 2014 #9

    A.T.

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    Ultimately? Ultimately the Big Bang is the source of everything.
     
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