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Physics energy homework

  1. Jul 1, 2009 #1
    The composition of meters shown in Figure traffic to 72,4 km/h when the brakes of the wheels A and B were activated, causing the derailment. Determine (a) the distance traveled by the rest and (b) the force in each coupling. The coefficient of kinetic friction between the wheels and rails is 0,30.

    Answer:
    (a) 98,1 m
    (b) FAB = 2,4x10^4 N; FBC = 5,6x10^4 N


    My solution:
    (a) Kinetic energy:
    T = 1/2mv^2 = 1,83x10^7 J
    Work:
    U = (PT - FT).x
    PT (Total weight) = 89x10^4
    FT (Total force of friction) = 2,67x10^5
    U = 6,23x10^5x

    x = T / U = 29,44m Is not the correct answer
     

    Attached Files:

  2. jcsd
  3. Jul 1, 2009 #2

    Doc Al

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    Staff: Mentor

    Re: Energy

    OK.
    I don't understand this equation. You just need the work done by friction.
    Only two of the three cars produce friction.
     
  4. Jul 1, 2009 #3
    Re: Energy

    Yes. Thanks

    U = 1,87x10^5
    x = T/U = 97,86m

    How do I calculate (b)?
     
  5. Jul 1, 2009 #4

    Doc Al

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    Staff: Mentor

    Re: Energy

    Consider each car separately. Apply Newton's 2nd law. (What's the acceleration of each car?)
     
  6. Jul 1, 2009 #5
    Re: Energy

    Acceleration is the same for all cars?

    V^2 = (Vo)^2 +2aX
    a = V^2/2X = 2,066 m/s^2
     
  7. Jul 2, 2009 #6
    Re: Energy

    I not find the correct answer

    FAB = FatA + FatB + (mA+mB)a

    FatA = PA . u
    FatB = PB . u

    FAB = 3,18x10^5 N
     
  8. Jul 2, 2009 #7

    Doc Al

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    Re: Energy

    Looks OK.

    I don't understand what you are doing. Analyze each car separately.

    For example, consider car A. What horizontal forces act on it? (One force is the coupling force FB/A from car B.) Apply Newton's 2nd law to solve for that force.
     
  9. Jul 2, 2009 #8
    Re: Energy

    Yes. But I do not find the answer

    Fb/a = (2,67x10^5)/9,81 * 2,066 + 2,67x10^5 * 0,3
    Fb/a = 136330,581 N
     
  10. Jul 2, 2009 #9

    Doc Al

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    Staff: Mentor

    Re: Energy

    ΣF = Ff + Fb/a = m*a
     
  11. Jul 2, 2009 #10
    Re: Energy

    Ok. Thank you very much
     
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