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Difference between frame of reference and coordinate system?

  1. Apr 24, 2015 #1
    1. The problem statement, all variables and given/known data
    Our teacher said we can NEVER do an F=ma problem from an accelerating, or noninertial frame. (He said there are ways to do it, but we can not do it in his class), and I'm confused becuase often times he makes the "system" or makes a "free-body diagram" around an accelerating object. Today he showed as an easy problem, but I thought it uses an accelerating frame. Here is the problem:

    There is an elevator and there is a bathroom scale with a block with mass 3kg that sits on top of the scale on the bottom of the elevator. The scale reads 40N. There is another scale (a fish scale) that is attached from the ceiling to the block and that reads 20N. Find the magnitude and direction of the elevators acceleration.
    2. Relevant equations
    F=ma

    3. The attempt at a solution

    F=ma
    choose positive y in coordinate system to go downward
    -FT+Fg-FN=ma
    -FT+mg-FN=ma
    a=(mg-FT-FN)/m
    plug in values, ((3)(10)-20-40)/3=-10m/s/s.... (we assumed gravity is 10 downward),

    b/c C.S. is going downward the elevator is accelearing upward at 10 m/s/s.

    but isnt hte mass accelerating? how can that be the system?
     
  2. jcsd
  3. Apr 24, 2015 #2

    haruspex

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    Seems to me that the frame used is inertial, not a frame tied to the accelerating mass. I can't understand why you think it is.
     
  4. Apr 24, 2015 #3
    ok so what is the frame in this situation?
     
  5. Apr 24, 2015 #4

    haruspex

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    One fixed in relation to the Earth (which is considered inertial for most everyday purposes; have to be more careful where the spin of the Earth becomes significant).
     
  6. Apr 25, 2015 #5

    ehild

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    You mean frame of reference. It is the Earth. You see the lift, the block in it, and the scales, from the ground. All accelerate together, and the acceleration of the block is determined by the sum of forces: gravity, the normal force from the bathroom scale and T tension from the fish scale.

    In the frame of reference fixed to the lift, the block would be in rest (with respect to the lift) .
     
  7. Apr 25, 2015 #6
    oh ok. got it. correct me if I am wrong: Forces acting on an object are the same in ALL frames of references but the only difference may be in the acceleration of the object right?
     
  8. Apr 25, 2015 #7

    ehild

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    The forces are equal in all inertial frame of reference.
    In non-inertial frames of reference there are additional fictitious forces, causing acceleration of the object. Such force push you back in an accelerating car.
     
  9. Apr 25, 2015 #8
    ok but excluding pseudo forces (our teacher said we cant use them), would all the forces be the same but the only difference is the acceleration right>
     
  10. Apr 25, 2015 #9

    ehild

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    The "real" forces are the same, but the acceleration is different. Use only inertial frames of reference, where Newton's Laws apply. Do not worry, what happens in a non-inertial one.
     
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