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Crushing of objects due to forces

  1. Nov 12, 2014 #1
    If an object is resting on a table and is pushed downward it has an opposite and equal normal force that counteracts the applied and gravitational forces making the net force on the object 0. If the net force on an object being pushed against a table is 0, how can it be crushed?
  2. jcsd
  3. Nov 12, 2014 #2


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    Staff: Mentor

    Typically, when people say "net force" they are talking about bulk motion (it makes the whole object move). It doesn't have anything to do with structural integrity. Nor does a net force of zero mean there is no force acting on the object -- obviously, the two forces are still there.
  4. Nov 12, 2014 #3
    Just because there is "no net force" acting on an object doesn't mean that there are no forces acting on it. It just means that the forces acting on it are in balance with one another.

  5. Nov 13, 2014 #4


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    Science Advisor

    Net force is only relevant for the acceleration of the object, not crushing of the object.
  6. Nov 13, 2014 #5
    The internal tensions (stress) are relevant for deformations. Think about a spring stretched by a force applied to one end (the other end is attached somewhere). You have the same situation. Net force is zero but you have a deformation. And you have a tension in the spring.
  7. Nov 13, 2014 #6


    Staff: Mentor

    No net force means that the center of mass will not accelerate. Each end can certainly accelerate inward In a crushing motion, that is not restricted by the second law.
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