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Quick question about a train/car problem

  1. Aug 27, 2009 #1
    I got a few problems like this one. The only thing I can't figure out is where to put the km. Here's what I got. A car traveling 95km/hr is 100m behind a truck traveling 75km/h. How long will it take the car to reach the truck. My 2 formulas are:

    X=Xo+Vo+(1/2)at2
    Car: X=95t2
    Truck: X=75t2

    Put both formulas equal to each other and solve for t. But where do I plug in the 100m that the car is behind? Thanks.
     
  2. jcsd
  3. Aug 27, 2009 #2

    tiny-tim

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    Welcome to PF!

    Hi imagiro1! Welcome to PF! :smile:
    Nooo :cry:

    i] it's X=Xo+Vot+(1/2)at2

    ii] a (the acceleration) is obviously zero, isn't it?
    iii] that would be the Xo (a different one for each vehicle, of course) :wink:
     
  4. Aug 27, 2009 #3
    I forgot that t, I swear I got it on paper.

    I ended up with Xc=.110+95t and Xt=75t, which gave me the right answer, but negative. I tried it again with Xc=95t and Xt=.110-75t and it gave me the 20 sec which is the answer.

    If I change it to -.110 would that be the same thing as saying the car is .110km behind the truck?
     
  5. Aug 27, 2009 #4

    tiny-tim

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    The car is 100m behind the truck …

    so if the truck is at position 0 at time 0 (consistent with Xt = 75t), then the car is at position -100 at time 0, so Xc = … ? :smile:
     
  6. Aug 27, 2009 #5
    -.110+95t. Awesome. Thanks for the help. I'm sure I'll be back later.
     
  7. Aug 27, 2009 #6
    Imagiro, all tiny tim said is perfect but for further progresses I may suggest to try to see deeper into the methematics behind physics. I mean x=x0+v0t+1/2at^2 is not just a recipt where you input some data and it gives you another, certainly it can do so, but it is just its most superficial use. Cinematics, at the level that you are working (about the one the problem is), can be fully understood by learning and catching the basic calculus (functions, derivatives and integrals, in general).

    Then, once you regard space and time as variables in a "calculus" way, you'll have no problem in solving problems like this and a lot more complicated that you never expected, even trying, before.

    That's my advice, HOPE it's usefull.

    Good night and good science :biggrin:
     
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