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Definition of force.

  1. Jul 1, 2007 #1
    Dear all ,
    I have a very simple question.
    i am pushing a wall.wall didn't move an inch.
    I exerted a force(as much as i could-see iam even sweating).But the calculations and science saying NO force.,since ,by defenition,force = mass x acceleration.
    So what shall i call the thing,i was doing with the wall?As i understand,i thought push is a sort of force.So i thought i was exerting a force to the wall.

    why this injustice is happening to me?
    could anybody explain me?
    Last edited: Jul 1, 2007
  2. jcsd
  3. Jul 1, 2007 #2

    Doc Al

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    Staff: Mentor

    Newton's 2nd law tells us that the net force on an object equals mass x acceleration. You are certainly exerting a force on the wall, but that's not the only force acting on the wall. (Assuming it is attached to the floor/ground, for one.) The net force on the wall is still zero--until you push too hard and bust through!
  4. Jul 1, 2007 #3
    and if your feet were on a small rug on a slippery floor, something else would happen

    (from personal experience)
    Last edited: Jul 1, 2007
  5. Jul 1, 2007 #4
    The definition of force is the time rate of change of linear mechanical momentum, i.e. F = dp/dt

  6. Jul 1, 2007 #5
    As someone else already mentioned, Newton's Second Law states that the Sum of F = m * a. You are exerting a force against the wall, however the anchoring system for the wall is exerting an equal and opposite force (Newton's Third Law) , which results in zero net force. Since there is zero net force, there is consequently zero acceleration.
  7. Jul 1, 2007 #6
    Net force before i push the wall was zero.Logically ,my force (when i push)should be added to this zero and net force now should be a positive Qty,in the direction of my pushing.Why and where we are missing this?.
  8. Jul 1, 2007 #7
    Then what can i call a push until the subjected object is not moved.
  9. Jul 1, 2007 #8
    Not true.

    When you are not pushing on the wall, it is true that there is zero net force on the wall.

    When you begin pushing against the wall, the mechanism that is pinning the wall exerts an equal and opposite force to your push. Until you exert enough force to cause the pinning mechanism to yield, there is zero net force, since your force is exactly negated by the pinning mechanism. After it yields, then there will be a net force, and therefore a net acceleration.
  10. Jul 1, 2007 #9
    ha... that's funny.I thought i was the first one who did this experiment........
  11. Jul 1, 2007 #10
    My question is,how can you call this a force(equal and opposite force),since there is no acceleration?.
    Can we see it in other perspective.Some what like this-residual force cause an acceleration to the object.
    or in other words,residual force = mass x acceleration.
    please note that this equation is giving only the residual force ,not the force.
    Also i think Newton's third Law has to be re stated as Every action have an equal and opposite reaction, until the subjected ,object is accelerated.
    When the object is accelerated,there is a residual force ,which means action and reaction are not same at all.
    Last edited: Jul 1, 2007
  12. Jul 2, 2007 #11

    Doc Al

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    pseudoghost already answered this, but here's my restatement. Before you pushed there was zero net force on the wall: True! And if your force were the only force added to the wall, then there would be a net force on the wall. But other forces also act on the wall to negate and resist your force. So the net force on the wall remains zero.

    Note that this has nothing to do with Newton's 3rd law, at least not directly.

    You can certainly have forces acting on an object without it being accelerated. Only if there is a net force on an object will it accelerate.

    Again, this has nothing to do with Newton's 3rd law. Realize that "action" and "reaction" force pairs act on different objects. So action/reaction pairs never directly produce equilibrium.
  13. Jul 2, 2007 #12
    Is something (anything) happening to the wall (and the floor where your feet are)?


    One thing that has been assumed so far is 'what' the wall is made of....

    (as a thought experiment)-'make' the 'wall' (and the floor) out of different materials (pudding, jello, rubber, cardboard, thin steel,100m thick concrete, sharp nails, etc.) ---you'll see on what degree the 'wall' reacts and what other 'things' must be taken into account (and your hands).
  14. Jul 2, 2007 #13


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    How is your "residual force" different from our "net force"?

    If you're thinking that somehow part of the force that you exert on the wall "disappears," note that even though the wall doesn't move, it can have effects on it as a result of your pushing, namely, you can a dent in it, or even punch a hole in it. For a simpler example of this, imagine holding a blob of putty between two fingers. Now squeeze on the blob. The net force on the blob is zero, so it stays in the same position, but the individual forces cause the blob to become distorted.

    Also, note that if you push hard enough, the wall may crack where it is attached to the floor or ceiling, or break off completely, as a result of the forces that the floor or ceiling exert against the wall to try to keep it in place.
    Last edited: Jul 2, 2007
  15. Jul 2, 2007 #14
    Re-worded: You can only push on the wall as hard as the wall pushes on you (Fnet = 0), if you exert more of a force then the wall exerts on you then you'd break that wall and acceleration would come into play. You are exerting a force, it's just that the wall cancels out your force, that's why the net force of the system will be equal to 0.
  16. Jul 2, 2007 #15
    which formula can you use to find out this force? I want to get paid for the force exerted by me.i don't care ,the wall moved/cracked/distorted or not.
  17. Jul 2, 2007 #16
    What is the defenition of the above mentioned force? what is the equation to find out that force?
  18. Jul 3, 2007 #17


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    This not the definition of force.

    When you push against the wall, you compress it a little, and it responds with an opposing force. In effect, the wall is a spring with a very high spring rate constant. In physics, the amont of force per unit area is referred to as stress, and the amount of deformation due to the force as strain.

    There are other forms of stress, bending, twisting, tension, ... Wiki articles:



    force = mass x acceleration refers to the reaction of an object when there is a net imbalance of forces.

    If forces are mis-aligned, then you can have torque, and this can result in angular acceleration if there is a net imbalance of torques.
  19. Jul 3, 2007 #18
    You all ready showed the equation, F=ma. Let's try again... The example I gave (Person pushing against a wall) we will define our system as the wall. So, when we say F=ma we are talking about every force acting on the wall, that's why people use Fnet=ma. The "Net" force, or total force of the system is equal to something depending on if it is accelerating or not. If you push on a wall and nothing happens you will know that the net force is 0 because obviously neither you or the wall is moving. You are exerting a force on the system, but the system is also exerting and equal and opposite force, which cancels the two out resulting in a net force of 0. This goes back to the force pairs, every force has an equal and opposite force -- If you take that to be true then you can only push as hard on the wall as the wall pushes on you. Mathematically:

    [tex]F_{you on wall} = -F_{wall on you}[/tex]

    Since the forces are canceling each other out.

    As for being paid well
    A) Take that up with the wall
    B) Stop pushing walls and do something a tad bit more productive.
  20. Jul 3, 2007 #19
    The force you put on the wall and the force the wall puts back on you due to Newton's third law certainly do NOT cancel each other out, they don't even act on the same objects. Yes they are equal in magnitude and opposite in direction, but because the do not act on the same object they do not cancel each other out.
  21. Jul 3, 2007 #20


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    Something happens, the wall is compressed slightly, as well as whatever is being used to push the wall.

    They do act on the same objects, the net result is a compression of the molecules in the wall and the molecules of the object (a person hands) used to press against the wall. The compression is coincident with the generation of outwards forces at the point of contact.
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