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Please help me with my homework

  1. Sep 15, 2003 #1
    A car starts from rest and accelerates uniformly at 3m/s2. A second car starts from rest 6 s later and accelerates uniformly at 5 m/s2/ How long does it take the second car to overtake the first car?

    I feel like I know how I am supposed to do this but I just need some clarity on what formula I need to use and what all my variables are. I have been physiced out so I just need a lil help here!!!!! Thanks so much to any and all who wanna contribute!!!!!!!
  2. jcsd
  3. Sep 15, 2003 #2


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    Use s = ut + 1/2 *at^2, remembering to account for the "lead" the first car has acheived.

    Hmm... there does seem to be a lot of uniform acceleration questions around recently...
    Last edited: Sep 16, 2003
  4. Sep 15, 2003 #3


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    Slight typo: it should be s= (1/2)at2+ ut (u is the initial speed) instead of "2at2" (i.e. 1/2 instead of 2)

    Taking the time the first car starts as t= 0, the distance the first car has gone is s1= (3/2)t2 (a= 3 and u=0 {"starts from rest"})

    Car 2 also starts from rest but has a= 5 and starts 6 seconds later (so it's "time" is t- 6 rather than t).
    s2= (5/2)(t- 6)2.

    The second car will "overtake" the first car when they have both gone the same distance: solve (3/2)t2= (5/2)(t- 6)2.
  5. Sep 15, 2003 #4
    but wait, I still am a lil confused. I guess a lot confused. that is the formula I guessed but where do I plug everything in?
  6. Sep 15, 2003 #5
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