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Application of one dimensional force - Dynamics

  1. Nov 11, 2014 #1
    1. The problem statement, all variables and given/known data

    Superman must stop a 120-km/hr train in 150 m to keep it from hitting a stalled car on the tracks. If the trains mass is 3.6 x 10^5 kg, how much force must he exert?

    Vi = 33 m/s (120 km/h)
    Vf = 0 m/s
    Displacement (Xf - Xi) = 150 m
    M = 3.6 x 10^5 kg



    2. Relevant equations

    Vf^2 = Vi^2 + a(Xf - Xi) - used to calculate the acceleration without a time value.

    F = Ma - used to calculate the force required to stop the train with the calculated acceleration.



    3. The attempt at a solution

    First convert Km/h to m/s. (120*1000)/60^2 ~ 33 m/s

    Then find the acceleration.
    A = (Vf^2 - Vi^2)/(Xf - Xi)
    A = -(33 m/s)^2 / 150 m ~ -7.3 m/s^s

    Now calculate the force required to achieve that acceleration.

    F = Ma
    F = 3.6 x 10^5 kg* -7.3 m/s^2
    F ~ -2.6 x 10^6 N

    The answer at the back of the book is half that value. (1.3 x 10^6 N) I don't understand what I'm doing wrong.
     
  2. jcsd
  3. Nov 11, 2014 #2

    PhanthomJay

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    Oh your work is good but you forgot the 2!

    Vf^2 = Vi^2 + 2a(Xf - Xi)
     
  4. Nov 11, 2014 #3
    Oh FFS.

    Thank you.
     
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