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2679 Views 3 Replies Latest reply: Jun 29, 2009 12:00 AM by TomGutman RSS
Copper 5 posts since
Oct 22, 2000
Currently Being Moderated

Oct 30, 2000 12:00 AM

Force and tension

If two teams each consisting of five people play tug of war and each team exerts a force of 300 newtons, will this cause the rope to break if it will break at 500 newtons? (will the force of tension within the rope be just 300 newtons, or them added together to create a force of 600 newtons that will break the rope?)
  • Copper 25 posts since
    Sep 21, 2000
    Currently Being Moderated
    Oct 31, 2000 12:00 AM (in response to Audrina_Kate-disabled)
    Force and tension
    The tension is distributed everywhere the same throughout the rope, so the answer is that it will not break (at 300N).

    If this is hard to believe, the next time you are in class, ask your teacher if you can borrow three 20-N scales. Connect two of them together and ask a volunteer to pull his at 10 newtons. You'll see that yours is also 10 N. Now connect the third scale between the two so that you have three scales in a row. What do you think will happen to yours and the middle one when your friend pulls his again at 10 N?
    • Copper 2 posts since
      Jun 28, 2009
      Currently Being Moderated
      Jun 29, 2009 12:00 AM (in response to nrfauch-disabled)
      Force and tension
      Is it the third scale in the middle remain unchanged for the reading?
      • TomGutman Silver 10,537 posts since
        Oct 22, 2006
        Currently Being Moderated
        Jun 29, 2009 12:00 AM (in response to hueyy-disabled)
        Force and tension
        Forces are not on an object, but between objects (that is the gist of Newton's third law).

        If an object is not accelerating, then the forces acting on that object must add up to zero. If the object is interacting with two other objects then the forces for those interactions must be equal and opposite.

        If you have three scales conected in series, with people pulling at the two end scales, you can look at all of the forces. You have person A pulling on scale 1 with a force of 10 Newtons. By Newton's third law scale 1 is then pulling person A with a force of 10N. Scale 1 connects only with person A and scale 2 (the middle scale). By equilibrium the force between scale 1 and scale 2 must be 10N. Scale 2 connects only with scale 1 and scale 3. Since it is in equilibrium (not accelerating) the force between scale 2 and scale 3 must be the same as the force between scale 2 and scale 1, i.e., 10N (but directed in opposite directions, as seen from scale 2). Scale 3 connects to scale 2 and person B. Again, by equilibrium, the force between scale 3 and person B must again be 10N.

        Note that part of the description is redundant. If you have person A pulling at one end of an object (whether it be a simple rope or a chain of objects, such as scales) and person B pulling at the other end, if the object is not accelerating the force exerted by person A must be the same as the force exerted by person B.
        � � � � Tom Gutman

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