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Homework Help: Kinematics and dynamics

  1. Jan 13, 2008 #1
    1. The problem statement, all variables and given/known data
    I want to calculate the travelled length of a car.

    A travelling car with a acceleration from 0 m/s to 19.4 m/s in 10 sec. From there it has a speed of 19.4 m/s for 11 sec. After the 21 sec. the car hit the brakes, and deaccelerates to 0m/s in 3 sec.

    2. Relevant equations

    3. The attempt at a solution
    I find the acceleration in the to periods first

    Acceleration = 19.4/10sec = 1.9 m/s^2
    Deacceleration = -19.4/3sec. = -6-47 m/s^2

    The I want to divide the travelled length for each individual timeperiod.

    The first period (10 s) is = 1/2 * 1.9 * 10^2 = 95 meters
    Seceond period (11 s) is = 19.4*11 = 213.4 meters
    Third period (3 s) = -1/2*-6.47*3 = 9.7 meters

    Travelled distance in all = 95+213.4+9.7 = 336.1 meters.

    Is this correct?!

    And yes I'm new here, so hello to everyone. Hope this can help me be better at phys/maths. I'm starting engineering soon, so I better get this handled!
  2. jcsd
  3. Jan 13, 2008 #2
    The complete equation is [tex]s=ut+\frac{1}{2}at^2[/tex].
    For the first time period u=0 so your result looks ok, but for the third the initial velocity is u=19.4m/s which you havn't taken into consideration.
  4. Jan 13, 2008 #3
    Isn't that given in the deacceleration?

    Deacceleration = -19.4/3sec. = -6-47 m/s^2 ?

    Otherwise a hint would be cool!
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