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Homework Help: Distance to stoppage

  1. Jul 10, 2010 #1
    1. The problem statement, all variables and given/known data

    Train goes at 280km/h. Starts breaking with a constant acceleration of -2m/s-2. How many meters does it perform until it stops completely? Sorry if the English is poor.

    2. Relevant equations

    If I had them I probably wouldn't be asking.

    3. The attempt at a solution

    Well, 280km/h equals to about 77.78 m/s. Great, what now?:confused:
     
  2. jcsd
  3. Jul 10, 2010 #2
    If you are gonna do kinematic exercises you should know the basic equations.

    [tex]v=v_{0}+at[/tex]
    [tex]v^{2}=v^{2}_{0}+2ae[/tex]
    [tex]x=x_{0}+v_{0}t+\frac{1}{2}at^{2}[/tex]

    Try with those, one should work, and write them down for the next problems.
     
  4. Jul 10, 2010 #3
    ok. that's a start. =) write down your knowns: v initial, v final, a. Your unknown is distance.
     
  5. Jul 10, 2010 #4
    I do know the first and third, but those don't seem to help do they? What do I do with the time in there? I don't have any reference to the time. What's the 'e' in the second one?

    Thanks in advance.

    EDIT: Oh, should I discover the time at which the final v is at 0 and then substitute in the motion equation to find out the position?
     
    Last edited: Jul 10, 2010
  6. Jul 10, 2010 #5
    look at your knowns and look at the equations.

    e is distance
     
  7. Jul 10, 2010 #6
    Don't need to find time!
     
  8. Jul 10, 2010 #7
    But would it work?
    Anyway I never heard about the second equation but it did work! I don't understand but it did lol.
     
  9. Jul 10, 2010 #8
    those are the 3 main equations to know for constant acceleration
     
  10. Jul 10, 2010 #9
    I don't think I would pull this off in an exam (now I would).

    Thank you both!
     
  11. Jul 10, 2010 #10
    you're welcome =)
     
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