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Math Help - Particle on light elastic thread answer not matching

  1. #1
    Member ssadi's Avatar
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    Particle on light elastic thread answer not matching

    My problem in (ii) find the mass of the particle. My answer comes as 0.54 kg whereas the answer is given as 1.98 kg.
    My working: EPE gained=(1.5*2-(2.4))*.5*90=27
    KE=.5*m*10^2
    thus m=.54
    Mark scheme:


    I think the mark scheme is wrong as the wire was prestreched by (2.4-1.6)m as shown in the diagram. But mark schemes are rarely wrong. Can you point out the fault in my reasoning? Thanks for reading.

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  2. #2
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    skeeter's Avatar
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    initial stretch ... 2.4-1.6 = .8 m

    final stretch ... 3 - 1.6 = 1.4 m

    \Delta U = -\Delta KE<br />

    \frac{150}{2}(1.4^2 - .8^2) = \frac{1}{2}m(10^2 - 0^2)

    150(1.4^2 - .8^2) = 100m

    m = 1.5(1.4^2 - .8^2) = 1.98 kg
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  3. #3
    Member ssadi's Avatar
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    Quote Originally Posted by skeeter View Post
    initial stretch ... 2.4-1.6 = .8 m

    final stretch ... 3 - 1.6 = 1.4 m

    \Delta U = -\Delta KE<br />

    \frac{150}{2}(1.4^2 - .8^2) = \frac{1}{2}m(10^2 - 0^2)

    150(1.4^2 - .8^2) = 100m

    m = 1.5(1.4^2 - .8^2) = 1.98 kg
    Thanks, rather silly mistake I see me do.
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