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Math Help - Kinematics

  1. #1
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    Kinematics

    Can some one help me with 5 ii) a)?

    Here is what i've done.

    Trailer:
    fx = P-250gsin3-150=250a
    fy = R-250gcos3=0

    Car:
    fx = 980-900gsin3-600-P=900a
    fy = R-900gsin3=0

    so therefore

    980-900gsin3-600=P+900a
    -250gsin3-150=250a-P

    right? simultaneous equation then solve for P and a.
    which comes out as:

    a=-0.313 ms^-2 (3s.f.)
    P=200N

    but the answer from the mark scheme says P= 200N but a = 0.713ms^-2 which suggests the equation should be

    980+900gsin3-600=P+900a
    +250gsin3-150=250a-P

    can someone explain what i've done wrong and which one is right.

    Sorry for being quite long. THanks for your help.
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  2. #2
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    For the acceleration i got 0.713 m/s^2 also.

    ((980-750) + (mgsin(3)) = 819.826 then divide by 1150
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  3. #3
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    Quote Originally Posted by Paul46 View Post
    For the acceleration i got 0.713 m/s^2 also.

    ((980-750) + (mgsin(3)) = 819.826 then divide by 1150
    why is it plus? mgsin(3)

    since the weight is going down?
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  4. #4
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    potential energy.
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  5. #5
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    Quote Originally Posted by Paul46 View Post
    potential energy.

    can you explain that?
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  6. #6
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    You have the force plus the PE of which will be converted into KE, so the resultant acceleration will be 0.713m/s^2
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  7. #7
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    Quote Originally Posted by Paul46 View Post
    You have the force plus the PE of which will be converted into KE, so the resultant acceleration will be 0.713m/s^2
    how do i know when to use +/- ?
    how come this never came up from previous examples?
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  8. #8
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    Sorry? i'm a bit confused to what you are asking?
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  9. #9
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    Here is a diagram
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  10. #10
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    Quote Originally Posted by Paul46 View Post
    You have the force plus the PE of which will be converted into KE, so the resultant acceleration will be 0.713m/s^2
    how can you turn Gravitational PE into KE in the opposite direction?
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  11. #11
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    question you gave on 5 ii) a) said its going downhill.
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  12. #12
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    Way to go.! lol
    thanks!
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