PHYSICS - formula?

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Here is one of my questions from my physics study. It tells me the answer (35 J), but how do I find that answer? I would appreciate a formula or step by step calculation. Thanks! :)

A 30 kg kid on rollerskates is moving 1 m/s. His brother pushes him with a force of 5 N. What will be his kinetic energy after his brother pushes him 4.0 m? (35 J)

  • PHYSICS - formula? -

    What acceleration does the pushing impart?
    F=ma
    5=30a
    a = 1/6

    How long does it take to go 4m, starting at 1m/s?
    s = Vo t + 1/2 at^2
    4 = t + 1/2 * 1/6 t^2
    t^2 + 12t - 48 = 0
    t = 4

    What's the speed after 4 seconds?
    v = Vo+at
    = 1 + 1/6 * 4
    = 5/3

    What's the KE then?
    KE = 1/2 mv^2
    = 1/2 (30)(5/3)^2
    = 15*25/9
    = 125/3

    Hmmm. better check my math.

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