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Homework Help: Car stops

  1. Aug 31, 2008 #1
    1. The problem statement, all variables and given/known data
    The minimum stopping distance for a car traveling at a speed of 30 m/s is 60 m, including the distance traveled during the driver's reaction time of 0.50 s.

    (a) What is the minimum stopping distance for the same car traveling at a speed of 60 m/s?

    2. Relevant equations
    Vf^2= V0^2 + 2ax

    3. The attempt at a solution
    Now I have my equation set up, but I can't figure out the change in x. Instead I have (60m-x) in my equation, but I'm looking for acceleration so that I can use that in my next equation when it comes to going 60 m/s. How is one to find the other x if it is not given?

  2. jcsd
  3. Aug 31, 2008 #2
    The relevant equation should be written as:

    Vf2 = Vi2 +2a[tex]\Delta[/tex]x

    [tex]\Delta[/tex]x is the distance the car goes to stop.
  4. Aug 31, 2008 #3
    Gotcha... wow, now I feel rather dense. That makes perfect sense- I've no idea why I skipped over that. All right, I worked the equations and got 7.5 m/s/s as my acceleration. I used that in my equation involving 60 m/s (0^2= 60^2 + 2(7.5)x) and got 240 as my answer. Yet this is wrong, is there something in my calculations that is incorrect?
  5. Aug 31, 2008 #4
    Did you include the distance you travel before you hit the brakes?
  6. Aug 31, 2008 #5
    Yes, using the equation: deltax= V0*t + (.5)(a)(t^2). My numbers being: V0=60; t=.5; a=7.5. totalling to 270.94, and yet that did not work either.

    (I had tried both answers before)
  7. Aug 31, 2008 #6
    The car is not accelerating before the brakes are applied.

    The equation you should use is x = vt.
  8. Aug 31, 2008 #7
    Yikes. That's just basic... I think I'm a bit tired. Right, well then, that would mean it travel 60m*.5s leaving 120m total during that time. Adding that to my original 240 brings me to 270m total.... well. That was just depressing, as that didn't work. :/ Now something must be wrong. I've done everything wrong along the way, then correcting it, and yet 270 seems as if that is correct...
  9. Aug 31, 2008 #8
    Is the correct answer 210m?
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