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Homework Help: Given distance and acceleration, find time

  1. Sep 29, 2010 #1
    1. The problem statement, all variables and given/known data
    A car is moving at 50.0 m/s and brakes to a halt in 6.00 seconds.
    How long does it take the car to travel 70.0 m after the car starts decelerating?


    2. Relevant equations
    a=dv/dt
    t=??

    3. The attempt at a solution
    I'm pretty sure I know how to find the acceleration but I'm confused as to how to set up an equation to find the time.
     
  2. jcsd
  3. Sep 29, 2010 #2
    Simple. Use a kinematics equation that has all the variables you need in it. Think about it. You know initial velocity, the distance it travels, and the acceleration. You need to find time. Which formula has all of those variables in it? Let me know if you haven't been given those formulae, and I can give you hints on how to solve it the long way.
    The acceleration is simple, remember the basic equation:

    [tex]a=\frac{V_{Final} - V_{Initial}}{\Delta Time}[/tex]
     
  4. Sep 29, 2010 #3
    For the acceleration I got -8.33 m/s2

    Then for the time, do I use the equation:
    x=1/2*a*t2+Vo*t

    When I tried using that equation I was left with two 't' variables and couldn't get it down to one.
     
  5. Sep 29, 2010 #4
    Correct formula. However take a look at this, you have d=(stuff)t2+(stuff)t. What kind of function does that look like?
     
  6. Sep 29, 2010 #5
    It looks like a quadratic function? I don't know how to get it to t='stuff'
     
  7. Sep 29, 2010 #6
    You don't. You wanna get t correct? Well there happens to be this thing called the quadratic formula that will solve for your quadratic variable (t in this case), where the equation is equal to zero.
     
  8. Sep 29, 2010 #7
    Oh I see! Thank you very much!
     
  9. Sep 30, 2010 #8
    You're welcome. Let me know how that works out. You should start with:

    (stuff)t2+(stuff)t-d=0 and then plug that in.
     
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