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Homework Help: Kinematics truck moving at 30m/s suddenly notices a moose 70m

  1. Aug 11, 2012 #1
    1. The problem statement, all variables and given/known data
    The driver of a truck moving at 30m/s suddenly notices a moose 70m straight ahead. If the driver's reaction time is 0.5s and the maximum deceleration is 8m/s^2, can he avoid hitting the moose without steering to one side?

    3. The attempt at a solution
    First I found out how long it would take the driver to stop.
    v = u + at
    0 = 30 -8t
    t = 3.75

    I then added the drivers reaction time of 0.5s so we land up with 4.25s

    Then I tries to fid out the distance covered by using:
    s = ut + 1/2at^2
    s = 30(4.25) + 1/2(-8)(4.25)^2
    s = 127.5 - 72.25
    s = 55.25m

    It would seem that the truck stops before 70m but the answer given in the book says no he doesn't stop in time and needs a further 1.3m.

    Please help.
  2. jcsd
  3. Aug 11, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Re: kinematics

    Instead of adding the times together, treat the two segments of motion separately. Hint: How far does he get before he steps on the brakes?
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