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10 June, 18:00

A boy on a 1.9 kg skateboard initially at rest tosses a (n) 7.8 kg jug of water in the forward direction. if the jug has a speed of 3.2 m/s relative to the ground and the boy and skateboard move in the opposite direction at 0.65 m/s, find the boy's mass. answer in units of kg.

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  1. 10 June, 18:11
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    For this case we first think that the skateboard and the child are one body.

    We have then:

    1 = jug

    2 = skateboard + boy

    By conservation of the linear amount of movement:

    M1V1i + M2V2i = M1V1f + M2V2f

    Initial rest:

    v1i = v2i = 0

    0 = M1V1f + M2V2f

    Substituting values

    0 = (7.8) (3.2) + (M2) ( - 0.65)

    0 = 24.96 + M2 (-0.65)

    -24.96 = (-0.65) M2

    M2 = (-24.96) / ( - 0.65) = 38.4 kg

    Then, the child's mass is:

    M2 = Mskateboard + Mb

    Clearing:

    Mb = M2-Mskateboard

    Mb = 38.4 - 1.9

    Mb = 36.5 Kg

    answer:

    the boy's mass is 36.5 Kg
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