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Homework Help: Finding Time with Acceleration Inital Velocity, Displacement

  1. Apr 17, 2015 #1
    1. The problem statement, all variables and given/known data
    Currently i am studying two dimensional collisions. In order to find the impact velocity of a vehicle so i can determine the momentum and etc. I have inital velocity=130km/h, displacement (or the braking distance)=54.34m and the deceleration = 11.9812m/s/s. I need to find the time of the braking distance in order to find the impact velocity of the vehicle. Can anyone please help me do this?

    2. Relevant equations

    3. The attempt at a solution
    I attempted using the equations of motion such as, v^2=u^2+2as and s=ut+1/2at^2 but they get me different answers and ridiculous answers that are greater than the initial velocity. I believe the impact velocity should be somewhere around 60-70km/h
  2. jcsd
  3. Apr 17, 2015 #2


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    Hello NeedHelp18. Welcome to PF !

    You should show those erroneous answers and how you got them. That is according to the guidelines for this Forum.

    Here are a couple of questions that may help you.

    Were the units you used for the various quantities compatible?

    Did you use a negative value for acceleration?
  4. Apr 19, 2015 #3
    Try to include the relevant equations, i.e kinematic equations.

    From what I can determine you are attempting to determine the impact velocity, and there are many ways that you can go about doing this using the kinematic equations for motion. To clarify; this is not a two dimensional problem, this is a one dimensional problem. In a two dimensional problem there are 2 possible dimensions of motion, and this gets a lot more complicated. In this example, there is one dimension of motion, + (or positive) and - (motion).

    Vf2 = Vi2 + 2 * A * D

    The issue that you are having is a simple one: You forgot to include the sign in the acceleration! The acceleration is a negative acceleration, not a positive one. When you use a negative value for the acceleration what do you get? Be sure to take note of the units you are using! Since displacement is in meters, you should convert the initial velocity into meters per second. 130 km/h = 130,000 m/h / 60 * 60 = 36.11 m/s. When using - units of acceleration I get Vf2 = 36.112 -1302.15. Can you figure the rest out? Keep in mind that the units you get will be in m/s, so convert back to km/h if you need to :)
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