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Help With Kinematics !

  1. Sep 19, 2005 #1
    Studying for a test and i cannot figure out how this is done...


    A gold car moving at 12m/s passes a Green Car while the Green Car is at rest at a stoplight. The green car immediately accelerates at a rate of +1.8m/s^2 for 11 seconds. After how much time (relative to initial starting time) must the green car drive before catching up with the gold car.

    I bolded some things because the question is very confusing and i've asked around but noone could help me.


    Thanks to anyone that helps.
    Please list any steps, etc that should be used as well...

    BTW, this should be done with the 5 kinematics equations and v = d/t if needed.
     
  2. jcsd
  3. Sep 19, 2005 #2
    Also another questions:


    A pool ball leaves a 0.60 meter high table with an initial horizontal velocity of 2.4 m/s. Predict the time required for the pool ball to fall to the ground and the horizontal distance between the table's edge and the ball's landing location.


    Seems simple but how do i go about solving this?
     
  4. Sep 19, 2005 #3

    Doc Al

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    Staff: Mentor

    Write the distance as a function of time for both cars. One car moves at a constant speed, the other accelerates. (After 11 seconds, assume the green car maintains a constant speed.) The cars pass when they have traveled the same distance at a given time.

    Again, write the distance as a function of time. The vertical motion is uniformly accelerated.
     
  5. Sep 19, 2005 #4
    thanks, figured the first one out but the second one still giving me problems.
     
  6. Sep 19, 2005 #5

    Doc Al

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    Staff: Mentor

    Show your work and we can take a look.
     
  7. Sep 19, 2005 #6
    well i havent really figured out how to start it... I dont think writing the distance as a function of time works in this case. This one is a projectile so it has x and y components which confuse me. :cry:
     
  8. Sep 19, 2005 #7

    Diane_

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    Homework Helper

    Remember - if you can split the problem into wholly horizontal and wholly vertical parts, you can solve each of them separately. "Orthogonal" is the key word here.
     
  9. Sep 20, 2005 #8
    Please correct me if I am wrong but I do not believe you have to divide into x and y components for the first problem.
     
  10. Sep 20, 2005 #9

    Diane_

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    Homework Helper

    You're right, but he said he had the first one solved but was still working on the second.
     
  11. Sep 20, 2005 #10
    For the second problem, I suggest you draw a picture. It's a projectile problem (you probably know this) and relatively easy to go about solving it, once you can picture it. You know that the ball is going straight into the horizontal direction, and you know y as well as Vox. If you can solve for time, you can solve for the horizontal distance.

    Try writing out all your kinematics equations, and write out all the information you have.

    If if helps you get started on the problem, you have: Vox, Voy, a, and y. Voy, a, and y are enough to help you solve for t, and once you have t, you can solve for x (the horizontal distance between the table's edge and where the ball lands). Just remember that in this problem, you can make the assumption that Vox=Vfx (please correct me if I am wrong).
     
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