Jack asked in Science & MathematicsPhysics · 9 years ago

what is the boy's mass?

A boy on a 2.4 kg skateboard initially at rest

tosses a(n) 8.9 kg jug of water in the forward

direction.

If the jug has a speed of 2.5 m/s relative to

the ground and the boy and skateboard move

in the opposite direction at 0.63 m/s, find the

boy’s mass.

Answer in units of kg

3 Answers

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  • 9 years ago
    Favorite Answer

    I'll show you the method, you can plug in the numbers.

    Conservation of momentum

    m1*v1i + m2*v2i + .... = m1*v1f + m2*v2f + .....

    Since everything is initially at rest, all the vi's are 0

    0 = m1*v1f + m2*v2f + ......

    Set a direction as positive... I'll choose forward as the postivie direction.

    m1 = 8.0 kg

    v1f = 2.5 m/s

    m2 = mass of the boy + skateboard

    v2f = -.63 m/s (negative because it is moving in the opposite direction

    0 = m1*v1f + m2*v2f

    Solve for m2

    m2 = -(m1*v1f) / v2f

    mass of the boy = m2 - 2.4 kg

    Plug and chug.

  • D g
    Lv 7
    9 years ago

    m1v1 + m2v2 = m1fv1f + m2fv2f

    since the objects are initially at rest

    0 = m1fv1f + m2fv2f

    let m1f be the mass of the jug

    v1f is the velocity of the jug

    v2f is the velocity of the boy and the skateboard

    m2f = -m1fv1f/v2f

    m1f = 8.9kg

    v1f = 2.5m/s

    v2f = -0.63m/s

    m2f = 8.9*2.5/0.63

    the minus signs cancel

    m2f = 35.35 kg

    this is the boy and the skateboard

    m2f = mb + msb

    35.35 = mb + 2.4

    mb = 35.35 - 2.4

    mb = 32.95 kg

    so I think the boy will be 32.95 kg ..

    i used momentum is conserved to find out the mass of the boy and skateboard then i subtracted the given mass of the skateboard from the solved value for the mass of the boy..

    hope that helps

  • 9 years ago

    "AN" is fine, "a" is NOT an option.

    (8.9 x 2.5) /.63 = 35.32kg.

    Subtract 2.4kg. for the board, = 32.92kg.

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