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Homework Help: Kinematics Final Velocity Question

  1. Oct 13, 2009 #1
    Hi all. New to the forums. Rookie to Physics. I know this is a really simple question, but I am struggling with it. Help would be greatly appreciated.

    Q: Determine the displacement of a plane that experiences uniform acceleration from 66 m/s to 88 m/s in 12s.

    Wouldn't I use: Vf^2=Vi^2+2ad

    Assuming, this is the correct equation to use what would be the a) Initial Velocity; b) Final Velocity; c) acceleration; and d) distance/displacement?

    I am pretty well stuck. Help would be appreciated. Thanks.
  2. jcsd
  3. Oct 13, 2009 #2
    This equation is fine to use, but you will have to solve for acceleration (you have the initial and final velocities, plus the time!) Once you solve for acceleration, you can solve for the displacement over time.
  4. Oct 13, 2009 #3
    Okay, so to find acceleration, I used Vf=Vi+at

    Setting the Vf as 88 and the Vi as 68.

    Algebraically solving the equation, I found the acceleration to be 1.67. I then plugged into Vf2=Vi2+2ad

    Getting my result, 934.13 meters. Is this correct?
  5. Oct 13, 2009 #4
    Is the initial velocity 66 or 68 m/s? If it is 68 m/s, your answer is very close to mine---I'm sure just rounding differences. Just double check the problem.
  6. Oct 13, 2009 #5
    Excuse me. It's 68. Great, thank you so much for your help.
  7. Oct 13, 2009 #6
    Glad you understand it!
  8. Oct 13, 2009 #7
    One more question if you can help:

    You throw a ball downward from a window at a speed of 2.0 ms. How fast will it be moving when it hits the sidewalk 2.5m below?

    Simple enough right? So my question is.... is it the initial velocity or final velocity that is 2.0? Wouldn't it be initial because it's the final velocity you're trying to find?
  9. Oct 13, 2009 #8
    exactly! 2 m/s would be the initial velocity.
  10. Oct 13, 2009 #9
    Sweet, so technically, since that is "gravity" withheld, the acceleration would be -9.8?
  11. Oct 13, 2009 #10
    also correct! acceleration is -9.8 m/s2.

    something to think about--what is the direction of the velocity?
  12. Oct 13, 2009 #11
    What is the direction of the velocity? Well I guess it doesn't really matter as long as its constant... i/e, you could call acceleration +9.8 as long as Vi was -2.0... Is that what you mean?
    Last edited: Oct 13, 2009
  13. Oct 13, 2009 #12
    because the velocity is downward it is negative, as well as the acceleration. you can't interchange them. it is just important to note the direction of the velocity depending on the equation you decide to use!
    Last edited: Oct 13, 2009
  14. Oct 13, 2009 #13
    Oh duh. Lol. Thanks... I don't process information well at night. Haha. Thanks for the help, I really appreciate it. G'night to ya!
  15. Oct 13, 2009 #14
    very happy to help:D
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