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Force Conversions - horinzontal vs vertical

  1. Sep 23, 2012 #1
    Hi guys

    Just seeking some clarification here as my prof has been very vague.

    Say we have a mass hanging off a vertical wire - the force upon that wire will be by F =MA, (m x9.81)N. I understand this.

    But say we had a wall upon which we put a static horinzontal load/force of 10kg of "push", would that in Newtons still be (10) x (9.81)N?


    Ie is the kg to N conversion still the same for horinzontal loads?

    2nd Q:
    And say we then moved the wall aganist the direction - ie if the force is in the x direction and we move the wall in the -x direction is the only force then F=MA where a is the accleration of the wall.

    Im slightly confused as if the wall was static - we would have an 'effective' accleration of 9.81m/s^2 (assuming i am correct above ) wheras say we move the wall in at 3.6m/s^2 we would actually have less force than if we only consided it static ( 9.81 > 3.6 etc)

    Much appreciated.
    Mike
     
  2. jcsd
  3. Sep 23, 2012 #2
    In the first question, the system is static, so the force on the wall is equal to the force of gravity on the mass. There is a pull on the wall from the acceleration of the mass of m*g, but the staying power of the wall (friction, if you will) is balancing this out, so the net force on the wall is zero, that's why there is no acceleration.

    In the second question, you are accelerating the wall at a. The hanging mass would also accelerate upward at a. There is a net force of Ma, where M is the mass of the entire wall + mass system.

    Your misconception is that there is no "effective" acceleration in the first part. There is a force from the hanging mass of (m * 9.81)N, but there is no net force, so there is no acceleration. In the second part, there is acceleration and therefore a net force of Ma. The same force from the weight of the mass is acting on the wall, but now there is extra force pulling the wall in the -x direction, therefore, acceleration.
     
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