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Restoring force (negative or positive)

  1. Mar 22, 2007 #1
    1. The problem statement, all variables and given/known data
    First, I'll give a problem in context..
    In anticipation of her first game, Alesia pulls back the handle of a pinball machine a distance of 5cm. The force constant of the spring in the handle is 200N/m. How much force must Alesia exert?
    Convert 5cm to 0.05m.
    F=kx..
    k=200N/m
    x=0.05m
    200N*0.05m/m=10N

    But why is it positive..
    I have a definition for restoring force as
    "Whenever a spring is stretched from its equilibrium position and released, it will move back and forth on either side of the equilibrium position. The force that pulls it back and attempts to restore the spring to equilibrium is called the restoring force."
    "The force that acts to move the spring away from the equilibrium position is equal in magnitude to the restoring force, but opposite in direction."

    So.. In Alesia's pinball machine.. the spring is in equilibrium? The force that Alesia exerts makes the spring not equilibrium.. So shouldn't the force be negative? I realized that the force exerted after she releases is going to be the same magnitude..

    And that's just the context problem..
    Gary Stewart of Reading, Ohio set a pogo stick record in 1990 by jumping 177737 times(irrelevant??). a)If pogo stick he used had a force constant(k) of 6000N/m and compressed 0.12m on each jump, what force must Gary have exerted on the pogo stick upon each jump?
    b)What force would be exerted back up on Gary each time he went up?


    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Mar 22, 2007 #2
    Note that the magnitude of a force is always positive because it is a vector. The direction of the force is another story. With Hook'e law, if you know where the equilibrium position is, the spring force vector will point to it.
     
  4. Mar 22, 2007 #3
  5. Mar 22, 2007 #4
    In situations like these you have to think about what the they physically represent.

    Think of it this way how would you move a book with negative force?

    No matter what direction you move the book the book is always moving positively with respect to the force you apply on it.
     
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