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Homework Help: Find the original speed of the car

  1. Jan 16, 2015 #1
    1. The problem statement, all variables and given/known data
    Driving along a crowded freeway, you notice that it takes a time t to go from one mile marker to the next. When you increase your speed by 7.7mi/h , the time to go one mile decreases by 15s. What was your original speed?

    v1 = ?
    d1 = 1 mile
    t1 = ?

    v2 = v1 + 7.7mi/h
    d2 = 1 mile
    t2 = t1 - 15 seconds

    2. Relevant equations
    I'm not sure... We have not covered this in class yet, and we have yet to cover one dimensional kinematics (which is next week). I've seen some equations regarding velocity initial, velocity final, etc., but I don't know how to implement them. The only equations I can come up with are these:

    [itex]s_1 = \frac{d_1} {t_1}[/itex]
    [itex]s_1 = \frac{1 mile} {t_1}[/itex]

    [itex]s_2 = \frac{d_2} {t_2} [/itex]
    [itex]s_2 = \frac{1 mile} {t_! - 15 seconds} [/itex]

    3. The attempt at a solution
    This is what I have written on my paper so far:

    [itex]\frac{1 mile} {t_1} = \frac{1 mile} {t_1 -15 seconds}[/itex]

    Edit: Umm, not sure where I went wrong with my LaTeX...?
    Last edited by a moderator: Jan 17, 2015
  2. jcsd
  3. Jan 16, 2015 #2


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    Staff: Mentor

    The combination of [sub]-tags inside TeX does not work, but I'm not sure if that is the only issue.

    Is s1 supposed to be a velocity? That would be an unusual variable name, but okay. You know s2 is not the same as s1, and you even know the differences.... include that in your equation, and you can solve it for t1 (for example).
  4. Jan 17, 2015 #3
    Excuse my ignorance, but am I only able to edit once? I can't seem to find the edit button for my original post?

    Yes, I apologize for that, s1 was supposed to be a velocity. I'm going to fully edit my post so the LaTeX will work, make it more clear (e.g. swap out s1 for v1, etc.). I also just found the introductory physics formula thread, and I realize that the formula I need is probably in there. I'm going to see what I can do with one of those formulas.
  5. Jan 17, 2015 #4


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    2017 Award

    Staff: Mentor

    Just for a fixed time (a few hours). I fixed the codes.

    You can make a new post with updates formulas.
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