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Homework Help: 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


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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


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    Staff Emeritus
    Science Advisor
    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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