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Homework Help: Acceleration thrown upward

  1. Nov 19, 2006 #1
    I know I already have a topic for a previous question, but heres another simple one, but I think im just missing some simple equations.

    "What is the magnitude of acceleration after 1.5 seconds of a sphere thrown upward from a height of 13 meters at 20 m/s^2?"

    Choices
    a) 5.3 m/s^2
    b) 9.8 m/s^2
    c) 15 m/s^2
    d) 32 m/s^2
    e) 390 m/s^2

    I realize these are quite obvious answer choices, and the answer is most likely D, but I dont know how to get it. I only have the "four kinematics" equations to work with, but I think there are other equations I can use for this. How does the height of 13m effect my answer, if it indeed does.
     
  2. jcsd
  3. Nov 19, 2006 #2
    Use the equation [tex] y = y_{0} + v_{y0}t + \frac{1}{2}a_{y}t^{2} [/tex]
     
  4. Nov 19, 2006 #3
    ok, using that equation, doesnt it leave me with two unknowns? I have time, initial distance in y, and initial velocity in y. I dont have the total distance in y, or the acceleration, which Im trying to find. Is that right, or am I mistaken....I might need to find total distance first, and then the acceleration.
     
  5. Nov 19, 2006 #4

    What causes the object to accelerate? What is the magnitude of this acceleration? This is more of a "did you memorize this constant?" kind of question than a problem about applying kinematics equations. Lastly why do you think it's D?
     
  6. Nov 19, 2006 #5
    well, the acceleration down would be gravity (9.8 m/s^2)......wait, so the answer is B!!??? I was thinking that because the object is traveling at 20m/s^2, after 1.5 seconds, it would be slighty over 20m, and the closest answer is D. But I guess if its asking for the "magnitude" of the acceleration, it would be gravity.
     
  7. Nov 19, 2006 #6
    Yes the answer is B. And no after 1.5 seconds the object is not traveling at 20m/s^2 that is an acceleration not a velocity! Acceleration is measure in units of meteres per second squared and near the surface of the earth the value of acceleration due to gravity is nearly constant at 9.8m/s^2.
     
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