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Superman Please help!

  1. Oct 21, 2007 #1
    1. The problem statement, all variables and given/known data
    A.) Find the speed at which Superman (mass=79.0 kg) must fly into a train (mass = 17026 kg) traveling at 70.0 km/hr to stop it.
    B.) Running into the train at that speed would severely damage both train and passengers. Calculate the minimum time Superman must take to stop the train, if the passengers experience an average horizontal force of 0.500 their own weight.
    C.) How far does the train then travel while being slowed to a stop?

    So I put the train traveling to right, positive x-direction and superman traveling left, negative x-direction


    2. Relevant equations
    Pf-Pi = 0
    Pf = Pi

    Ms= 79kg
    Mt = 17026kg
    Vti = 70km/hr = 19.444m/s
    Vft = 0m/s (because it is coming to a stop)
    Vfs = 0m/s (because it is coming to a stop)
    Vsi = ?


    3. The attempt at a solution
    Pf = Pi
    (Ms * Vfs) + (Mt * Vtf) = (Ms * Vsi) + (Mt *Vti)
    0 = (Ms * Vsi) + (Mt *Vti)
    (-Mt * Vti) / Ms = -Vsi (Negative because he is moving to the left

    I am not getting the right answer, can someone please help me out?
     
  2. jcsd
  3. Oct 21, 2007 #2

    mgb_phys

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    First part is conservation of momentum (=mv)
    Second is just v = u + a t with a = 0.5g
     
  4. Oct 21, 2007 #3
    Isn't that what I did...for the first part?
     
  5. Oct 21, 2007 #4
    So for the first part isn't it...
    Pf = Pi
    (Ms * Vfs) + (Mt * Vtf) = (Ms * Vsi) + (Mt *Vti)
     
  6. Oct 21, 2007 #5
    I guess not because its saying incorrect.
     
  7. Oct 21, 2007 #6
    I need help with the second part...but what is u??
     
  8. Oct 21, 2007 #7
    Im losing my mind with part B and C...does anyone know how to do these?
     
  9. Oct 21, 2007 #8

    mgb_phys

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    a, speed = 70km/h x (17026/79) =15086 km/h

    b, v=u +at v=0, a=-0.5g = -4.9 m/s^2 u=19.4m/s
    t = 3.96s

    c, v^2 = u^2 + 2as as above, 19.4^2 = 2 *0.5 *g *s
    s = 19.4^2/9.8 = 38.6m
     
  10. Oct 21, 2007 #9
    Thank you!
     
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