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Force Problem: force direction and choice of sign for g

  1. Oct 25, 2016 #1
    1. The problem statement, all variables and given/known data


    The Force applied to an object is equal its mass times its acceleration Assume that the ball has a mass of 40g. What is the force due to gravity acting on the ball?

    2. Relevant equations
    F=ma

    3. The attempt at a solution

    F=(.04)(-9.81)

    I get a negative answer, does that mean that the force is negative? or should I change -9.81 to 9.81?
     
  2. jcsd
  3. Oct 25, 2016 #2

    PeroK

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    Gold Member

    Why do you think it matters?
     
  4. Oct 25, 2016 #3
    Because acceleration in the y dimension, is negative. It can't possibly be negative. So I think it should stay.
     
  5. Oct 25, 2016 #4

    gneill

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    Staff: Mentor

    The choice of reference frame determines the signs of vector quantities measured in that reference frame. It's up to you to choose and specify a reference frame before beginning your calculations.

    It's perfectly valid to choose "down" as being positive and "up" as negative if it is convenient for describing the problem that way. Of course, once you've made that choice you have to stick with the convention you've imposed.

    Generally g is taken to be a positive constant and you tack on the required sign to suit your choice of reference frame when you write equations involving g.
     
  6. Oct 25, 2016 #5

    CWatters

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    +1

    You choose up as positive. If you get a negative answer for the force that means it's acting downwards. Which way did you expect the force of gravity to act :-)
     
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