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Math Help - Finding deceleration

  1. #1
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    Finding deceleration

    I am trying to solve this problem

    "The driver of a vehicle, travelling at 50 km/h, applies the brakes 100 m
    from a crosswalk. Assuming constant braking deceleration, find the
    deceleration required to bring the vehicle to a full stop at the crosswalk"

    I do not really understand how to approach this problem because I cannot easily visualize it. In this case I am not being given any function so I do not really know what to do.

    From what I know both velocity and acceleration should be 0 at the moment the car stops but I don't know how to use this information.

    Any help will be appreciated
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  2. #2
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    Quote Originally Posted by samstark View Post
    I am trying to solve this problem

    "The driver of a vehicle, travelling at 50 km/h, applies the brakes 100 m
    from a crosswalk. Assuming constant braking deceleration, find the
    deceleration required to bring the vehicle to a full stop at the crosswalk"

    I do not really understand how to approach this problem because I cannot easily visualize it. In this case I am not being given any function so I do not really know what to do.

    From what I know both velocity and acceleration should be 0 at the moment the car stops but I don't know how to use this information.

    Any help will be appreciated
    You will need two kinematic equations to solve this. First

    v_f=v_0+at

    and

    x_f=x_0+v_0t+\frac{1}{2}at^2

    Now we know that

    x_0=0 \quad x_f=100 \quad v_0=50 \quad v_f=0

    So we have two equations and two unknowns a and t.

    can you finish from here?

    Don't forget to make sure that your units are the same.
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  3. #3
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    Quote Originally Posted by TheEmptySet View Post
    You will need two kinematic equations to solve this. First

    v_f=v_0+at

    and

    x_f=x_0+v_0t+\frac{1}{2}at^2

    Now we know that

    x_0=0 \quad x_f=100 \quad v_0=50 \quad v_f=0

    So we have two equations and two unknowns a and t.

    can you finish from here?

    Don't forget to make sure that your units are the same.
    Just to make sure that I am going on the right track. Are my two equations supposed to be like this?

    0 = 50 + at
    100 = 0 + 50t + (1/2)at^2
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  4. #4
    Behold, the power of SARDINES!
    TheEmptySet's Avatar
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    No you have mixed the units!
    Last edited by TheEmptySet; May 16th 2011 at 09:26 PM. Reason: error
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