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Wall acceleration?

  1. Sep 26, 2011 #1
    I'm a bit of a physics noob, but I am always curious about everything in the world.

    I concluded from Newton's third law that when you exert a force on a wall it exerts a force back on you. But I want to know from where does that force actually come?

    I mean ok you as a person might have a mass and an acceleration, but the wall doesn't though, so how come it has a force that it exerts on you? Does the wall accelerate to begin with?
  2. jcsd
  3. Sep 26, 2011 #2

    Doc Al

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    Staff: Mentor

    The wall or you--or any other object--will only accelerate if there's a net force on it. When you push the wall there are other forces acting on the wall (such as the ground) which prevent it from accelerating. Similarly, when you push the wall you don't accelerate either (unless you're standing on a puddle of oil) since the ground also exerts a force on you.
  4. Sep 26, 2011 #3
    Thanks for the reply, and I think things are more clear now. The formula for Force though is F=ma. But since you said neither I or the wall accelerate, then how does it come, or better put, how would you ever calculate that kind of force.
  5. Sep 26, 2011 #4


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    To measure it directly, you could do something like put a spring scale between you and the wall. If it's some other structure, for example a beam which you know the properties of, you might measure the deflection of the beam when you push on it. (This is how a beam torque wrench works).

    Anyhow, despite not contributing to any motion, these reaction forces have important implications -- for example, you might like to know how strong your wall needs to be.
  6. Sep 26, 2011 #5

    Doc Al

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    That's a formula for net force, better written as: ΣF = ma.
    To produce an acceleration, all you need is a net force.
  7. Sep 26, 2011 #6


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    In other words, there are lots of other equations for force and that one doesn't really apply here.
  8. Sep 26, 2011 #7


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    If you push on a wall, the surface of the wall compresses a bit, similar to a sping. So does the surface of your hands, if you're pushing with your hands. The amount of compression corresponds to the amount of force (it's probably not linear like a spring though).
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