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    luisa_006's Avatar
    luisa_006 Posts: 1, Reputation: 1
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    #1

    Oct 21, 2007, 03:05 PM
    kinetic energy
    a 5kg rock is dropped from a height of 92m. What is the kinetic energy when the rock is 40m from the ground?

    I know that the formula is kinetic energy=1/2mv(squared)

    but I don't have the time so how would I find the speed, v?
    tickle's Avatar
    tickle Posts: 23,796, Reputation: 2674
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    #2

    Oct 21, 2007, 03:10 PM
    I don't have the time either, you will have to do it yourself in time for your homework assignment tomorrow.
    Capuchin's Avatar
    Capuchin Posts: 5,255, Reputation: 656
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    #3

    Oct 21, 2007, 03:26 PM
    You can do this in 2 ways, either realise that the loss in potential energy is identical to the gain in kinetic energy, or you can work out the end speed using a SUVAT equation and use the KE equation you have there.

    I would say the potential energy way is easier.

    Either way, you're going to need to put the work in.
    terryg752's Avatar
    terryg752 Posts: 197, Reputation: 4
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    #4

    Oct 22, 2007, 03:58 AM
    Potential energy at height h = mgh

    ( m = mass, g = force of gravity)

    Kinetic energy at height 92 = 0

    Hence loss of potential energy in falling = mg (92 - 40) = 52 mg

    Hence Kinetic energy = 52 mg

    1/2 mv^2 = 52mg

    v^2 = 104g

    Substitute value g to get the answer!
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