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Average force exerted on the ball

  1. Oct 12, 2016 #1
    1. The problem statement, all variables and given/known data
    upload_2016-10-12_10-43-9.png

    upload_2016-10-12_10-43-34.png
    upload_2016-10-12_10-43-55.png

    2. Relevant equations
    ∑Fav = mv2 - mv1

    3. The attempt at a solution
    We have to separate v in x dir and y dir?
     
  2. jcsd
  3. Oct 12, 2016 #2

    Doc Al

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    Why? (Note that it starts from rest.)
     
  4. Oct 12, 2016 #3
    but are part one and part two asking the same thing?
     
  5. Oct 12, 2016 #4

    Doc Al

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    No. One asks for the average total force on the ball. The other part asks for just the force exerted by the foot.
     
  6. Oct 12, 2016 #5
    For the average total force on the ball (1st part). what equation should i use?

    For the (2nd part) force exerted by the foot, the equation is F = (mv2- mv1 ) / Δt
     
  7. Oct 12, 2016 #6

    Doc Al

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    No, that will give the average total force on the ball.
     
  8. Oct 12, 2016 #7
    That's total force exerted on the ball?
     
  9. Oct 12, 2016 #8
    For part 1,

    F = 1/ Δt * ∫ F dt ?? which is (mv2- mv1 ) / Δt ??

    For part 2,

    F = (mv2- mv1 ) / Δt - mg (cuz the question said subtract the weight)
     
  10. Oct 12, 2016 #9

    Doc Al

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    Yep.
     
  11. Oct 12, 2016 #10

    Doc Al

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    Yes.

    Careful. Forces are vectors, so here you need to consider the directions of each.
     
  12. Oct 12, 2016 #11
    For part 2

    (mv2- mv1 ) / Δt (going up)

    mg (going down)

    what should i do with the 60 deg which is given?
     
  13. Oct 12, 2016 #12

    Doc Al

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    That's the direction of the average total force on the ball.
     
  14. Oct 13, 2016 #13
    Part 1, F = 45 N
    Part 2 F = 41.236 N?
     
  15. Oct 13, 2016 #14

    haruspex

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    I'm not sure whether the hint given in the question is simply wrong or is assuming the weight, being a downward force, will be expressed as a negative quantity. Ask yourself this: does the presence of gravity increase or reduce the force needed from the boot to achieve the given velocity?
    Secondly, forces are vectors. You cannot add (or subtract) forces acting in different directions by adding their magnitudes.
     
  16. Oct 13, 2016 #15
    Is #1 correct, 45 N
     
  17. Oct 13, 2016 #16
    #2 i have F = Fav (cos i + sin j) + mg (-j)

    = (Fav cos )i + ( Fav sin - mg ) j
     
  18. Oct 13, 2016 #17

    Doc Al

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    Yes.
     
  19. Oct 13, 2016 #18
    why is my #2 not correct?
     
  20. Oct 13, 2016 #19

    Doc Al

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    You made a mistake with the sign of the weight vector.
     
  21. Oct 13, 2016 #20
    Fav - mg (-j) ??
    so it is a plus?
     
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