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Reaction Force for Tension

  1. Oct 7, 2011 #1
    What is the reaction force for a tension force. For example someone pulling a brick by a rope.
     
  2. jcsd
  3. Oct 7, 2011 #2

    Dale

    Staff: Mentor

    The reaction to the brick's force on the rope is the rope's force on the brick.
     
  4. Oct 7, 2011 #3

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Dalespam says that the reaction force for the brick's force on the rope is the rope's force on the brick. This is true. He was being very clever and careful not to answer your question. The question asked for the reaction force for a tension force. There are always at least two reaction forces for a tension force because a tension is not a single force: a tension is the result of two or more opposing forces acting on an object, in this case, a rope.

    Let's make your example of someone pulling a brick by a rope as simple as possible: an astronaut floating in space pulling a rope that is tied to a brick that is floating in space. And let's make the mass of the rope negligible ie. orders of magnitude less than the mass of the astronaut or brick.

    In this case, you have the net forces on the two accelerating bodies (the astronaut and the brick) and tension in the rope. The tension in the rope results in two net forces: the force on the astronaut (mass x acceleration of the astronaut) and the force on the brick (the mass x acceleration of the brick).

    It can never be the case that the reaction to a tension is a net force resulting in the acceleration of only one object. You could say that the reactions to the tension in the rope are the forces accelerating the astronaut and brick. Or, you could treat the tension as a kind of force conduit rather than a force and say that the astronaut is exerting a force on the brick and the reaction is the equal and opposite force of the brick on the astronaut. This is not completely correct since a rope cannot be massless so it is not strictly just a force conduit. But it is, in my view at least, a reasonable way to analyse the problem.

    AM
     
    Last edited: Oct 7, 2011
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