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Homework Help: Calculations with acceleration

  1. Oct 28, 2014 #1
    The problem statement, all variables and given/known data
    Initial velocity=15m/s slows down to 10m/s in 4.0s. Acceleration= -1.2m/s^2
    How far does the car travel in 4.0s
    If the car were to slow down at the same rate how much additional distance would it travel before coming to a complete stop
    2. Relevant equations

    3. The attempt at a solution
    I got the acceleration which was the first part of the question and so for the second part (calculating distance traveled in 4.0s) I plugged in the 1.25m/s^2 in the equation d=1/2at^2 and got 2.5m which was marked incorrect. For the third part I have no idea how to solve it
  2. jcsd
  3. Oct 28, 2014 #2


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    Homework Helper
    Gold Member

    Hello AymanLak,

    Welcome to Physics Forums! :)

    I think you missed a key when typing in the "-1.2m/s^2". But I see that you corrected that below ([itex] a = -1.25 \mathrm{\frac{m}{s^2}} [/itex]).

    There's two mistakes going on there.

    (1) The uniform acceleration formula of [itex] d = \frac{1}{2} a t^2 [/itex] only applies if the body starts from rest (i.e., if [itex] v_0 = 0 [/itex]). What is the full version of the formula if the object has a nonzero initial velocity?

    (2) Don't forget to square the time in the [itex] \frac{1}{2}a t^2 [/itex] term.
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