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B Newton's Balanced Force

  1. Apr 15, 2016 #1
    I'm having a hard time overcoming a personal misconception regarding Newton's balanced and unbalanced forces. Hopefully one of you can help.
    If I am standing on a wooden box, gravity exerts a downward force, which the box absorbs and dissipates through it’s construct. Over time the box may start slowly degrading and the energy being absorbed snaps the wood fibers releasing some energy there, and continues until the construct is no longer able to absorb all the downward force of gravity and it collapses.
    Does Newton suggest that the box returns an equal amount of energy in an upward direction to create a balanced force? Doesn’t it make more sense that the force is absorbed and released through friction energy and deterioration in the construct of the box? (Please don't focus on the box, it could be a slab of marble or concrete.) Thanks in advance for any productive comments or answers.
  2. jcsd
  3. Apr 15, 2016 #2
    Newton suggests that the box pushes back on your feet with an equal force, but in the opposite direction. Nothing about energy so far.

    What happens when the box deteriorates (I assume you mean natural deterioration, such as chemical breakdown) is that its ability to respond elastically to the compressive stresses diminishes. This means that the strain of the material caused by the stresses induced in the box by your weight begins to exceed the elastic limits of the material at local scale. Once this happens, local failures can grow and coalesce, which can cause a total failure of the box. It is not the force of your weight itself that initiates deterioration in the box, unless the force is large enough that it induces a stress larger than the mechanical strength of the box right from the start.
    Last edited: Apr 15, 2016
  4. Apr 15, 2016 #3
    If we are checking on Newton's 3rd Law "Every action(Force) has an equal and opposite reaction (force) and the two forces act on the two bodies involved in the event." then one should restrict to the domain.

    In a way the above statement does not qualify the individual construct of the agencies/bodies involved in the interaction/or its state(energy/otherwise characteristic)- actually the above statement is derived from a fact that "if however the point of contact /surface of contact gets displaced /in motion" the forces are not obeying the 3rd law.

    In equilibrium condition/static condition the two forces must be equal and opposite in character as no translation(relative) is being observed.
    otherwise one will have to move to the 2nd law- about a net force (unbalanced) acting at the contact surface.
    As an example -suppose one person is standing on a rock and his friend is standing on sand nearby- both are static with respect to an observer- the observer will conclude that both are in a condition of equilibrium and net force=zero

    this conclusion demands that the structure/surfaces on which they are standing are managing an equal and opposite reaction to the weight of the men at that instant of time(it does not say anything about the past or the future state of the system.

    It is a possibility that the man standing on the stone gets down due to some movement in the earth and the sand-man stands as it is.
    the motion of stone and man will be handled by new forces/events in the surrounding but as long as he is in contact with stone the action and reaction will be equal and opposite as the stone is not displacing man or the man is not moving in the surface.

    If the body in question is elastic/plastic/decaying/not being able to withstand the force ... all these qualification moves us away from equilibrium and can be handled by other features of force which changes the dynamical state of a system.
  5. Apr 15, 2016 #4


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    The paraphrased statement above does not correctly characterize Newton's 3rd law. In the situation at hand, there are two third-law force pairs, four forces total.

    There is the downward force of the Earth's gravity on the person and the upward force of the person's gravity on the Earth. That's one pair. The forces in that pair will be equal and opposite regardless of the resulting motion of the person.

    There is the downward force of the person's feet on the box and the upward force of the box on the person's feet. That's the other pair. The forces in that pair will be equal and opposite regardless of the resulting motion of the person or the box [or on its broken shards].
  6. Apr 15, 2016 #5
    i considered the man and the stone as one closed system and man and the sand another closed system and wanted to compare the state of the system providing action and reaction forces
    the other part that the stone and earth , man and earth , sand and earth ... was knowingly removed as not to complicate the issue.
    If i place a book on a table and the table gives way then naturally the action and reaction forces are not equal and opposite and the falling motion of book can be analyzed by the net force acting downward .
    however your point is well taken to include the earth.
  7. Apr 15, 2016 #6


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    The "action" and "reaction" forces in a third law force pair are equal and opposite always. The force of the book on the table and of the table on the book are equal and opposite regardless of whether the table is solid or is disintegrating. The force of the book on the earth and of the earth on the book are equal and opposite regardless of whether the table is solid or is disintegrating.

    The force of gravity on the book and of the table on the book are not a third law force pair.
  8. Apr 15, 2016 #7
    I think the problem I have is believing that there is an applied upward force. My argument is that the downward momentum is stopped due to the densities of the objects refusing each other passing and the downward force is just absorbed and distributed into the material(box, metal, stone) not allowing further travel in the direction of that force, and there is no upward force. I can understand if Newton was using this specifically in context to support movement from an unbalanced force and no movement with balanced forces.
    Doesn’t it make more sense to say that the downward force is dissipated at the point where the two objects meet by converting the force into energy absorbed by the lower object?
  9. Apr 15, 2016 #8


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    That understanding is incorrect. The way that objects prevent interpenetration is with forces. They repel each other. In the case of contact forces, this repulsion manifests only at very short ranges, so the repulsion is not something that fits very well with our untrained intuition.

    Newton's third law is correct. There is an upward force. Momentum is a conserved quantity.
  10. Apr 15, 2016 #9
    Last edited: Apr 15, 2016
  11. Apr 15, 2016 #10
    So the interpenetration repulsion of the lower object is the upward force Newton is referring to. I think I can work with that, thanks!
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