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    niki's Avatar
    niki Posts: 1, Reputation: 1
    New Member

    Nov 20, 2003, 01:19 AM
    Problem dilemma
    I need help with these problems

    1. a 1kg brick fall off a 3m wall onto a man's foot. How long does the brick take to fall. What is the Kenetic Energy of the brick relative to his foot.
    MathMaven53's Avatar
    MathMaven53 Posts: 20, Reputation: 2
    New Member

    Jan 8, 2004, 10:51 AM
    Re: Problem dilemma
    It's been years since I have studied Physics but I'll
    work with the equations and you can substitute values into them.

    Let s be distance brick falls,m the mass of the brick, g be acceleration due to gravity, t be time of fall, v be velocity of brick.

    s = (1/2)g t^2

    Solve for t

    t = sqrt((2s)/g)

    v = gt

    = sqrt(2 g s)

    Kinetic Energy is given by

    (1/2) m v^2

    = m g s

    g is 32 ft/sec^2 in English system of measurement
    but you are using metric. I forget what g is in the
    metric system but it should be listed in your physics

    In summary: To find the time it takes for the brick to fall, use the equation

    t = sqrt((2s)/g)

    The brick's Kinetic Energy is given by m g s
    Polokid16's Avatar
    Polokid16 Posts: 11, Reputation: -1
    New Member

    Dec 8, 2007, 09:28 PM
    Using SI (or english system)

    "_" - subscript

    E_p = mgh = (1kg)(9.8m/s^2)(3m) = 29.4J

    For the time:

    d = v_i * t +(0.5)at^2

    since v_i or v_initial = 0m/s

    d = (0.5)at^2

    rearrange to solve for t:

    sqrt( d / (0.5)g) = sqrt( 3m / (0.5)(9.8m/s^2)) = 0.6s
    itsme_vipsdude's Avatar
    itsme_vipsdude Posts: 29, Reputation: 0
    New Member

    Dec 15, 2007, 03:26 AM
    look as the body falls from rest therefore its initial kinetic energy is zero... therefore it must hve only the potential energy... potential energy
    thus t the lowermost point kinetic energy is equal to the potential energy lost... tus k.e=294J
    Polokid16's Avatar
    Polokid16 Posts: 11, Reputation: -1
    New Member

    Dec 15, 2007, 06:20 AM
    The wall is 3m... but yes E_k = E_p

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