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Homework Help: A question on units

  1. Oct 13, 2007 #1
    I *think* this is right, but if there is something obviously wrong please tell me where

    Ive got a mass of weight 'y' dropped onto a surface that can support 10y
    there is no wind resistance
    the mass starts from stationary and accelerates under gravity (assumed to be 9.8 m/s/s)
    the distance is 3m

    constant acceleration from stationary for s distance is v^2=2as. v^2=2(9.8)3=7.6 m/s
    The kinetic energy of a particle of mass m moving at velocity v is 1/2*m*v^2 K=1/2(y)(7.6^2)=29.4y

    so the particle would excede the resistance by 19.4y

    with respects to the different units involved is this correct, or can i not take the resistance of the surface (measured in neutons) away from the kinetic energy of the particle (measured in joules) to get an answer because of the different units?
  2. jcsd
  3. Oct 13, 2007 #2


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    Staff Emeritus
    Science Advisor

    The relationship between mass m and weight y is m = y/g.

    Energy/work is equated to force * distance, so 1 Joule (J) = 1 Newton * 1 m = 1 N-m.

    Force accelerates/decelerates a mass.

    It would seem one needs to determine the deflection of the surface or the time needed to decelerate the mass y/g in order to determine the force.

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