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Find the speed of each car

  1. Sep 1, 2012 #1

    Phys student

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    1:two cars are moving with a constant speed toward each other and the distance between them is decreasing at a rate of 3 m/s if the two cars are moving with the same speed but this time in the same direction the distance between them is decreasing at a rate 1 m/s. what is the speed of each car



    2. Relevant equations : D=VT V=vo+at ?



    3. The attempt at a solution: if they have the same speed then when they are moving toward each other each car's speed must be 1.5 m/s but when i read the second part that idea must be invalid

    Can you please give me a hint on how to start guys?
     
    Last edited: Sep 1, 2012
    Phys student, Sep 1, 2012
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  3. Sep 1, 2012 #2

    azizlwl

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    Let car A at left and B on the right.

    Please sketch the diagram for better understanding.
    Let car A at origin and car B at a distance x from origin
    y=at
    z=-bt + x, car B is going opposite direction of car A

    Distance between them is -(z-y)

    s=-t(b+a) +x
    Velocity as time derivative of position,
    v=-(a+b)
    v =-3
    b+a=3 .... (1)

    For same speed and direction
    y=at
    z=bt+x

    b-a=1 ......(2)

    We have 2 equations and 2 unknowns.
     
    Last edited: Sep 2, 2012
    azizlwl, Sep 1, 2012
  4. Sep 2, 2012 #3

    Phys student

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    azizlwl thanks for the reply but can you please explain what do you mean by y,z and s , and does the b and a in the two equations mean the velocity of each car ?
     
    Phys student, Sep 2, 2012
  5. Sep 2, 2012 #4

    azizlwl

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    Sorry for not defining the variables.
    a and b are the velocities for car A and car B respectively.
    y and z is displacement of for car A and car B respectively with respect of time.
    x is the intial position of Car B whilst car A at origin at t=0.
     
    azizlwl, Sep 2, 2012
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