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Constant acceleration

  1. May 17, 2015 #1
    1. The problem statement, all variables and given/known data
    A speeding car has instantenous velocity of 1 m/s when a stopwatch reads 10 seconds. It has constant acceleration of 2 m/s^2.
    What is the cars speed when it reaches the stopwatch reads 12 seconds.

    2. Relevant equations
    x= (Vinitial)(DeltaT) + 1/2(a09deltaT)^2
    I think this is the right equation for constant acceleration...

    3. The attempt at a solution
    (1.0m/s)(2seconds) + 1/2(2m/s/s)(2)^2
    2 + 4
    = 6 m/s

    This is a problem in my textbook, and the answer they give is 5 m/s. Am i using the wron equation? Or what am i doing wrong?
     
  2. jcsd
  3. May 17, 2015 #2

    billy_joule

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    Science Advisor

    You are using an equation to find a distance 'x', but you need a velocity.

    Always check your units, e.g.:

    Should be
    (1 m/s)(2 seconds) + 1/2(2 m/s/s)(2 s)^2
    2 m + 4 m
    = 6 m


    That's the distance travelled between 10 and 12 seconds, not what you are looking for... You need a different equation.
     
  4. May 17, 2015 #3
    I'm stumped. Is it one of those other constant acceleration equations? Whenever I look at them, it always seems like I'm not looking at the right one because it has more variables than what I'm given in the question
     
  5. May 17, 2015 #4

    SteamKing

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    Staff Emeritus
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    Homework Helper

    Look at the problem statement.

    You are given a time, an initial velocity, and an acceleration. You are asked to find the final velocity. Is there an equation which contains only those quantities?

    ==========================================================================================

    To learn what the constant acceleration equations mean, you should write down each equation and write under it a list of each variable in that equation and what each variable stands for. You're not learning anything with your current approach, except how to guess (badly, unfortunately).
     
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