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Math Help - [SOLVED] Newtons Second Law - Simple Mechanics

  1. #1
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    Cool [SOLVED] Newtons Second Law - Simple Mechanics

    Kindly have a look at the attached image for the related question.

    I've solved (i) to get the following values...

    a= 2.5 ms^-2
    T= 3.75 N

    I want to inquire about part (ii)

    What I want to know is that since acceleration of A was 2.5 then the acceleration of B should be -2.5 ... no?

    I ask this because, for part (ii) I've got the correct answer of 'mass' by using

    T-10m=2.5m

    3.75/12.5=m

    m=0.3 kg

    But if I use -2.5 then the answer comes out to be m=0.5 kg.

    What am I doing wrong?

    Thanks!
    Attached Thumbnails Attached Thumbnails [SOLVED] Newtons Second Law - Simple Mechanics-11.jpg  
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  2. #2
    MHF Contributor
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    West Malaysia
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    Quote Originally Posted by unstopabl3 View Post
    Kindly have a look at the attached image for the related question.

    I've solved (i) to get the following values...

    a= 2.5 ms^-2
    T= 3.75 N

    I want to inquire about part (ii)

    What I want to know is that since acceleration of A was 2.5 then the acceleration of B should be -2.5 ... no?

    I ask this because, for part (ii) I've got the correct answer of 'mass' by using

    T-10m=2.5m

    3.75/12.5=m

    m=0.3 kg

    But if I use -2.5 then the answer comes out to be m=0.5 kg.

    What am I doing wrong?

    Thanks!
    the acceleration is the same for the whole system which is + 2.5 . You only account the negative for the gravity part .
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  3. #3
    Junior Member
    Joined
    Jul 2009
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    68
    in body B, if you use acceleration -2.5 m/s^{2}, this means you are considering the upwards direction to be negative

    so you need to consider T also negative because its upwards and the weight to be positive because its downwards

    so
    -T+10m=-2.5m


    you got
    T-10m=2.5m
    considering the opposite. downwards negative and upwards positive.
    its also works
    as long as same direction vectors have the same signal
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  4. #4
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    Thanks for the quick replies guys!
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