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Homework Help: Physics with cal 1 probs

  1. Sep 12, 2005 #1
    If you guys can help me with this I would appreciate it.

    1. A passenger plane lands with a speed of 98m/s and accelerates at -7.0m/(s^2). After how many s does it come to rest?

    2. A train moving at 32.0 m/s brakes to a stop in 22.0 s. How far in m did it move while coming to a stop?

    3. A rock is thrown upward with a speed of 23m/s. What maximum height in m does it reach before falling back to earth?

    I need to know how to do these problems as well as the answers.

  2. jcsd
  3. Sep 12, 2005 #2


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    First you need to know some basic definitions: How is "acceleration" DEFINED?

    Do you know any formulas connecting distance and acceleration?
  4. Sep 12, 2005 #3
    avg or inst Accel?

    no, dont think so
  5. Sep 12, 2005 #4

    You have your initial velocity (v0) and your acceleration. You want to see how many seconds it will take for the plane to stop. That is, how long will it take until v=0? I think you can do the rest...


    Last edited: Sep 12, 2005
  6. Sep 12, 2005 #5
    no, i can't.

    do i need to find the inst acc and inst vel at v0?
  7. Sep 12, 2005 #6
    v0 is the speed that the train is moving at when it begins to brake.
  8. Sep 12, 2005 #7
    what about at?
  9. Sep 12, 2005 #8
    TRAIN, i thought u were heliping me with the 1st one. lol
  10. Sep 12, 2005 #9
    anyone want to give me the complete solution to the first or second problem?
  11. Sep 12, 2005 #10
    i really need help here guys.................i don't under stand what I am supposed to plug into the equation. I see them in my textbook, but I dont understand what the stand for
  12. Sep 12, 2005 #11

    Tom Mattson

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    We don't do that here. You are supposed to show how you started and where you got stuck, and then we help you, not do it for you.

    Please read the notice at the top of this Forum.

  13. Sep 12, 2005 #12

    I just want you to explain it not do it for me.
  14. Sep 12, 2005 #13

    Tom Mattson

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    Then why did you ask for a complete solution?

    Look, give it a shot and see what you come up with. You've got a short list of kinematical equations in your book (which you have read, right?). You should be able to at least try to figure out what the given information is (time, acceleration, initial velocity, etc...) and figure out what the unknown is for each part. Then you can search your list of equations for the correct one that relates what you've been given to what is being asked for. There will only be one in each instance.
  15. Sep 12, 2005 #14
    I was referring to the second one, but the same holds true for the plane when it lands...
  16. Sep 12, 2005 #15

    I dont understand WHAT they stand for
    just tell me what they stand for
  17. Sep 12, 2005 #16

    Tom Mattson

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    vf= final velocity
    vi=initial velocity
    xf=final position
    xi=initial position
  18. Sep 12, 2005 #17
    IS THE ANswer 14???? to the 1st one
  19. Sep 12, 2005 #18

    Tom Mattson

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    Yes, it is. Just don't forget that the answer includes its units.
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