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I'm LOST Car is suppose to overtake truck but at what velocity?

  1. Oct 10, 2012 #1
    I'm LOST!!!! Car is suppose to overtake truck but at what velocity?

    two cars C and D move in the same direction in parrallel lanes of a straight road. C starts from rest and begins to move with a constant acceleration. at that instat the C begins to move it is overtaken by D, which is moving with a constant speed of 6ms-1
    !) it is given that C's acceleration remains constant the instant it overtakes D. this occurs 5seconds after c begins to move. Find C's speed at the instant when it overtakes D.
    now i know i should ignore how D overtook C and i believe that i should use s=vt-1/2at^2 but i dont get a number answer its always a variable with a number please someone help me.
     
  2. jcsd
  3. Oct 10, 2012 #2
    Re: I'm LOST!!!! Car is suppose to overtake truck but at what velocity?

    Show what you have tried so far.
     
  4. Oct 10, 2012 #3
    Re: I'm LOST!!!! Car is suppose to overtake truck but at what velocity?

    well i got s=vt-1/2at^2 for C is well s=d and t=T+2 (?) thus it would be d=v(T+2) - .5(a)(T+2)^2 and for D it would be d=6(T)-.5(a)(T)^2.
    so it would be 6(T)-.5(a)(T)^2=v(T+2) - .5(a)(T+2)^2 thus i ended up with 6T-vT-2v
    +2aT+2a thats it, i'm lost and i'm sure that its wrong
     
  5. Oct 11, 2012 #4
    Re: Help PLZ my test is in 5 hours and idk how to do this!

    First of all, write this equation for each of the cars, obtaining two equations. Notice that v is the initial velocity of the cars.

    Then, you have to somehow relate the two equations. How would you do so? I have put some of the words of the problem in boldface to give you some ideas.
     
  6. Oct 11, 2012 #5

    Simon Bridge

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    Re: Help PLZ my test is in 5 hours and idk how to do this!

    I had advised sketching the v-t graph for each car and noting that when the two cars draw level the second time, their displacements are equal. Since the displacements are the areas under the graphs this amounts to finding the areas of rectangles and triangles.

    That thread has apparently vanished after comment:
    ... but there are no components to worry about - all the motion is in 1D.

    You won't get a better method description without someone actually doing the problem for you and that is against the rules.
     
    Last edited by a moderator: Oct 11, 2012
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