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Homework Help: Force pictures

  1. Oct 29, 2008 #1
    1. The problem statement, all variables and given/known data
    Draw an Fn picture and find Ffs.
    Fa= 0

    2. Relevant equations
    Fll= mgsinθ
    Fnet= sum of all forces parallel to ramp
    Fnet= ma

    3. The attempt at a solution
    I have the picture drawn but I don't know how to find friction.
  2. jcsd
  3. Oct 29, 2008 #2
    I assume Ffs is the static frictional force. Therefore, even though you haven't stated otherwise I'll assume that the system is in static equilibrium (ie. the sum of all forces = 0).

    Using this information, show us your summation of all the forces.
  4. Oct 29, 2008 #3
    Yes, Ffs was meant to represent static friction. I was unaware that static friction meant that all forces equaled zero.

    Fnet=0 so Fll-Ffs

    Last edited: Oct 29, 2008
  5. Oct 29, 2008 #4
    If you are given a value for static friction, it does not mean that the sum of forces is zero, I just assumed that considering there is not enough information given (that I can see at least) to solve this question provided the mass has overcome the force of static friction.

    Your calculation is ok. Just be aware that most physics questions take acceleration due to gravity to be 9.8 or 9.81 ms-2, however if you've been using 10ms-2 consistently in your class, then it's best not to change it now.
    Last edited: Oct 29, 2008
  6. Oct 29, 2008 #5
    Oh, my teacher did not say that the forces were all equal to zero, just that the applied force was equal to zero.
    I don't know if this helps, but this is a lab and we were increasing the angle of a ramp until our mass (a piece of wood with a few weights attached) moved down the ramp. Yeah, our teacher likes to round things.
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