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Homework Help: Motion at constant acceleration

  1. Mar 14, 2010 #1
    1. The problem statement, all variables and given/known data
    In coming to a stop, a car leaves skid marks 92m long on the highway. Assuming a deceleration of 7.00m/s^2 estimate the speed of the car just before braking.

    2. Relevant equations

    deceleration=7m/s^2, distance=92m

    3. The attempt at a solution
    I found there was not enough information to solve the problem, but i had a go, assuming dividing the distance by deceleration you would get time. 92/7 =13.14 seconds

    I substituted this into the equation of motion x=x0+v0t+1/2at^2
    =52.99m/s. Is this right???????

  2. jcsd
  3. Mar 14, 2010 #2


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    Homework Helper

    Why would distance divided by deceleration give you time? Why would x=x0+v0t+1/2at^2, which is an equation for displacement, give you speed?

    Are you familiar with the equation vf^2-vi^2=2ad?
  4. Mar 15, 2010 #3
    so the answer should be vi^2=2(7)(92)
    =square root 1288=35.889 m/s??
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