Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: A train decelerates

  1. Apr 5, 2010 #1

    IB1

    User Avatar

    A train of mass [tex]m=1.5 \cdot 10^5 kg [/tex] is travelling at [tex]40m/s [/tex] when the brakes are applied and it decelerates steadily. The train travels a distance of [tex]250m[/tex] before coming to a halt.

    a) Calculate the deceleration of the train.
    b) Find the average braking force.





    I have tried to solve it using formulas such as:
    [tex] v^2 =v_{0} ^2 +2as [/tex] and so the others of that family, but they do not work because I need to know three variables to find the others...
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Apr 5, 2010 #2
    If you read the question carefully, you find you do have 3 variables for that equation. for [tex]v_{f}^{2} = v_{0}^{2} + 2as[/tex] you can rearrange( I won't patronise you by asking you to do it) to find [tex]a[/tex]: [tex] a = \frac{v_{f}^{2} - v_{0}^{2}}{2s}[/tex]
     
  4. Apr 5, 2010 #3
    It is important to remember in problems where something is slowing to a halt, that this implies that your final velocity is 0. this is how you know 3 variables. The same principle applies for something speeding up from rest in which case the initial velocity is 0.
     
  5. Apr 5, 2010 #4

    IB1

    User Avatar

    Thank you both for your fast replies. I understood.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook