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Homework Help: Constant Acceleration; answer not correct for some reason?

  1. Jan 27, 2014 #1
    1. The problem statement, all variables and given/known data
    A motorist suddenly notices a stalled car and slams on the brakes, negatively accelerating at 6.1m/s2 . Unfortunately this isn't enough, and a collision ensues. From the damage sustained, police estimate that the car was going 22km/h at the time of the collision. They also measure skid marks 35m long. A:How fast was the motorist going when the brakes were first applied? B: How much time elapsed from the initial braking to the collision?

    2. Relevant equations
    2 a (x - xo)= v2 - vo2

    3. The attempt at a solution
    A: 2(6.1m/s^2)(35-0m)=(55/9m/s)^2-vo^2

    B: 19.7m/s=35/t

    I may just be totally off, but I feel like I understood the problem. Masteringphysics doesn't agree. Thanks again haha.
  2. jcsd
  3. Jan 27, 2014 #2


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    you mean v = 19.7 m/s ? ... are acceleration and displacement in the same direction?
    what happened to the minus sign in front of vo^2 ?
  4. Jan 27, 2014 #3


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    Gold Member

    If you take the displacement as positive, say to the right, in which direction is the acceleration? (Check your signage). Then once you get the correct value for the initial speed, you need to calculate the average speed to find the time in part b.
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