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Finding the distance in an acceleration problem (different)

  1. Mar 10, 2010 #1
    1. The problem statement, all variables and given/known data
    You are travelling at 60km/h when all of a sudden you see a pink elephant which is 0.125km down the road. Your car can slow down at a rate of -5.0m/s^2. If it takes you 20 m of distance to react and hit the brake, how far will you be from the elephant when you stop?

    2. Relevant equations
    D = (V1+V2/2)t
    A = V2-V1/T

    3. The attempt at a solution
    To be honest, I really don't even know where to begin with this problem. I don't even know how to visualize this.

    Okay so the break is hit when it's 0.105km away. (0.125 - 20).

    Time = Velocity / Acceleration
    = 60/-0.03 (5m = 0.005km/s = 0.03km/h)
    = 200 hours?

    Just from this point I know it's wrong. I can't continue further because I'm already starting off on the wrong foot.

    I need some guidance in the RIGHT direction, please & thank you!

    Also, the answer is supposed to be 77m.
     
  2. jcsd
  3. Mar 10, 2010 #2

    ehild

    User Avatar
    Homework Helper
    Gold Member

    Take care of the units and dimensions. The acceleration is not 5 m, neither 0.005 km/s, and 0.005 km/s is not 0.03 km/h.

    The best thing first when you start to solve a problem to convert all units to Si ones: m, s, kg, and so on.

    ehild
     
  4. Mar 11, 2010 #3
    You need to use the equations for motion with constant acceleration.

    Time is the variable you do not know, so use an equation that does not include time: v2=v02+2a(x-x0)

    The final velocity (the v on the left side of the equation) will be zero (because you will be stopped). You want to solve for (x-x0) and then try to work it out from there using the distance you travel before braking and the distance you start from the elephant. Also, convert the 60km/hr to meters/second.

    I worked the problem and came up with 77m.
     
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