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Homework Help: Quick Kinematics Question

  1. Nov 11, 2008 #1
    assuming negligible air resistance, how long will it take for a sphere to fall 36.0 meters if thrown downward at 12 m/s?

    2. kinematics

    x= x0 +v0t + (1/2)at^2

    3. plugging it in..

    36= 12t + 4.9t^2
    4.9t^2+12t - 36

    doing all the math and what not with the quadratic I get


    the answer is 1.75

    this is a pretty simple question....I dont know why I'm having trouble with it...
  2. jcsd
  3. Nov 11, 2008 #2
    Ok. You'll want to use conservation of total energy first of all. Try that and you should get 1.75.
    You'll find a velocity as it hits the ground, and you know the initial velocity, so you can easily find the average velocity. Knowing the distance traveled and average velocity you know how to find the time
  4. Nov 11, 2008 #3
    that doesn't make much sense to me.

    Honestly, I'm just reviewing from my chapter 2 test for physics. It shouldn't take anything more than kinematics.
  5. Nov 11, 2008 #4
    Ok, well then use that equation. you should be getting 1.75.
    not sure how you're solving your quadratic...?
  6. Nov 11, 2008 #5

    Doc Al

    User Avatar

    Staff: Mentor

    Your kinematic equation is fine. Redo your solution. (You must have made an arithmetic error.)
  7. Nov 11, 2008 #6
    Your quadratic equation is correct, but your solution is wrong. if you write down your solution explicitly, then I can see where it went wrong.
  8. Nov 11, 2008 #7
    wow. just kidding. sorry.

    stupid, stupid mistake. instead of 4ac in the quadratic I used 2ac. I was just so convinced that I was right -.-

    thank you everyone.
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