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    dks2114's Avatar
    dks2114 Posts: 32, Reputation: 2
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    #1

    Oct 8, 2007, 05:20 PM
    Work & Force & Power
    Bob can throw a 500g rock with a speed of 30 m/s. He moves his hand forward 1.0 m while doing so. How much work does Bob do on the rock? How much force, assumed to be constant, does bob apply to the rock? What is Bob's maw power output as he throws the rock?

    W=225000J
    F= 225000N because 225000= F (1) right?
    Is Max power calculated with the formula for regular power? (P = Fv or W/t)?
    P = 6750000 W

    IS this correct?
    ebaines's Avatar
    ebaines Posts: 12,131, Reputation: 1307
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    #2

    Oct 9, 2007, 09:09 AM
    Check your units- it's a 500g rock, not a 50 Kg one. Your work and force results are therefore off by a factor of 1000. Hint: always consider whether your answers make practical sense - here you have someone throwing a small rock and using enough force to lift a 22,900 kg weight, and enough power to light up a small town.

    For the power question I would use F*v. The W/t would work if you assume a constant velocity as his hand moves forward, but here you know that's not right, since the force his hand applies to the rock is constant (hence the rock must accelerate through the 1m distance to reach 30m/s).
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