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Math Help - questions in physics help me

  1. #1
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    questions in physics help me

    Q 1 :
    Two masses 5 kg and 10 kg are suspended from ends of a string (mass less inelastic )

    passing over a smooth pulley . Calculate the acceleration of the masses

    ------------------------

    This is the first body acting by two force on is T other is gravity …
    Now we write the equation

    m1g – T = m1a

    ( I write –m1g ) because T < than m1g )

    Now the second body
    T - m2g = m2a
    ( I write - m2g because T > than gravity )

    Now I add both equations
    m1g – T = m1a
    T - m2g = m2a
    ……………………>
    ,,,m1g- m2g=m1a+m2a

    a = (m1 – m 2)/ ( m1 + m 2 ) X g

    = - 735.75

    To find T
    m1g – T = m1a
    5 X 9.81 –T = 5 (- 735.75 )
    49.05 – T = -3678.78
    -T = -3678.78 - 49.05
    T = 3727.8
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  2. #2
    Senior Member BAdhi's Avatar
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    Quote Originally Posted by r-soy View Post
    Q 1 :

    m1g – T = m1a

    ( I write –m1g ) because T < than m1g )

    Now the second body
    T - m2g = m2a
    ( I write - m2g because T > than gravity )

    Now I add both equations
    m1g – T = m1a
    T - m2g = m2a
    ……………………>
    ,,,m1g- m2g=m1a+m2a

    a = (m1 – m 2)/ ( m1 + m 2 ) X g

    = - 735.75

    To find T
    m1g – T = m1a
    5 X 9.81 –T = 5 (- 735.75 )
    49.05 – T = -3678.78
    -T = -3678.78 - 49.05
    T = 3727.8
    why do you mean by the red colored statements. Can you describe more
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  3. #3
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    Sorry I write wrong now i correct >>

    In like this question required to write equation but as techer said you must beware about (- ) and ( + ) in that

    First m1g – T = m1a
    ( I write –T ) because T less than m1g ( No m1g + T = m1a)


    Also about this :

    Now the second body
    T - m2g = m2a
    ( I write - m2g because - m2g less than T)

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  4. #4
    Senior Member BAdhi's Avatar
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    it's not because T is less than m1g, it's because T acts opposite to the force m1g
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  5. #5
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    plese expline that for m1 and m2
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  6. #6
    Senior Member BAdhi's Avatar
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    for m1, if consider forces,

    T acts upwards, and the gravitational force acts downwards so resultant force would cause the acceleration

    upwards, \sum F = T - m_1g

    apply F = ma upwards

    T - m_1g = m_1a (considered a happens upwards)

    if m1 accelerates upwards m2 should accelerate downwards then

    since the forces act much similarly as above,

    apply F=ma downwards,

    m_2g - T = m_2a

    now solve for T and a (and i think there's something wrong with you answer for a..)
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  7. #7
    Senior Member BAdhi's Avatar
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    for m1, if consider forces,

    T acts upwards, and the gravitational force acts downwards so resultant force would cause the acceleration

    upwards, \sum F = T - m_1g

    apply F = ma upwards

    T - m_1g = m_1a (considered a happens upwards)

    if m1 accelerates upwards m2 should accelerate downwards then

    since the forces act much similarly as above,

    apply F=ma downwards,

    m_2g - T = m_2a

    now solve for T and a (and i think there's something wrong with you answer for a..)
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  8. #8
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    I use this a = (m1 – m 2)/ ( m1 + m 2 ) X g

    = - 735.75 where is the mistake ?
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  9. #9
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    Quote Originally Posted by r-soy View Post
    I use this a = (m1 – m 2)/ ( m1 + m 2 ) X g

    = - 735.75 where is the mistake ?
    i think you are just confused what you are doing.

    it should be (m1 – m 2)X g / ( m1 + m 2 )

    where m1=10 and m2=5

    you should get a= 3.2666666...ms^-2

    then just that to find T.
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  10. #10
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    Quote Originally Posted by r-soy View Post
    I use this a = (m1 – m 2)/ ( m1 + m 2 ) X g

    = - 735.75 where is the mistake ?

    \displaystyle {{m_1 - m_2}\over{ m_1 + m_2}}={{10-5}\over{10+5}}= {{5}\over{15}}={{1}\over{3}}

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  11. #11
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    thanks For T is correct

    m1g – T = m1a
    5 X 9.81 –T = 5 (3.27 )
    49.05 – T = 16.35
    -T = 16.35 - 49.05
    T = 32.7
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  12. #12
    MHF Contributor Unknown008's Avatar
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    You used the wrong equation. The correct equation was given earlier:

    T - m_1g = m_1a
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