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Homework Help: Decelerating 1D Movement

  1. Jan 28, 2015 #1
    1. The problem statement, all variables and given/known data

    - A Dragster at the starting line accelerates at 8 m/s^2 to the finish line. If it took 4.6 s, how long is the track?
    - The Dragster deccelerated to a stop in 100m. How long did it take?

    2. Relevant equations

    x = 0 + 1/2at^2

    3. The attempt at a solution

    The first part of the questions I got x = 84.64m using the above equation.
    For the life of me, I cannot figure out how to get anything viable for other formulas, when v or a is not specified for decceleration (read: Part 2).
  2. jcsd
  3. Jan 28, 2015 #2


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    Hello casey, welcome to PF :)

    Same formula, but now the initial speed isn't zero. And x isn't the unknown, because it's a given. The unknown is a. And t of course. So you'll need another equation. Something relating v initial, v final, t and a.

    From there (with v final = 0) you'll find t. (two equations with two unknowns)

    You'll do fine.
  4. Jan 28, 2015 #3
    I think I see where you are coming from.
    Thank you for the quick reply and the welcome.

    I missed the second part of your answer. I think I got it figure out. Thank you!
  5. Jan 28, 2015 #4


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    Ah, some PF culture here: you do the work, helpers help. So jot something down and solicit comments/assistance !
    You'll need this other equation anyway (it's no big deal, you must have seen it come by already at some point in the lectures/testbook) to determine the speed when the braking starts.

    Hint: check out the formulas here (where it says three key variables)
    Last edited: Jan 28, 2015
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