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Physics Concepts

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A father racing his son has one-third the kinetic energy of the son, who has two-fifths the mass of the father. The father speeds up by 1.5 m/s and then has the same kinetic energy as the son.

What is the Father's initial speed?

What is the Son's initial speed?

  • Physics Concepts -

    Let the father's mass be M sos that the son's mass must be 2M/5. Let the father's initial speed be V, and the son's v.
    (1/2)MV^2 = (1/3)(2M/5)v^2
    which leads to
    V^2 = (4/15)v^2

    (1/2)M(V +1.5)^2 = (2M/5) v^2
    V^2 + 3V + 2.25 = (4/5) v^2

    3V^2 = (4/5)v^2
    Now you can eliminate v.

    2V^2 -3V -2.25 = 0
    That can be solved for a positive V using the quadratic equation. Chack my math.

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