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Homework Help: Magnitude of Acceleration of ball

  1. Oct 12, 2007 #1
    1. The problem statement, all variables and given/known data


    Part A) The graph shows the y-component of the velocity (with up chosen as positive) of a ball thrown up with an initial speed of 27.0 m/s from the surface of Mars. What is the magnitude of the acceleration of gravity on the surface of Mars? (NOTE: the y-axis of the graph is labeled "upward speed", but it should say "y-velocity".

    Part B) Select ALL correct answers in alphabetical order, i.e., B, AC, BCD...

    A) Maximum height is reached at about t=7.3s.
    B) The ball's KE increases during the first second.
    C) The ball's PE is a constant.
    D) v and g are in the same direction after t= 7.3s.
    E) The acceleration is 0 near t= 7.3s.
    F) The acceleration is constant.

    2. Relevant equations

    3. The attempt at a solution
    I am unsure how to do part A, so if someone could walk me through it that would be great. For part b, I was thinking A was correct, B was correct, D was correct, E was correct. So my answer for part B is ABDE. Does this sound right?
  2. jcsd
  3. Oct 12, 2007 #2


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    Staff: Mentor

    No. You might have one of those correct, but I didn't check the math.

    Please review the kinematic equations of motion for a constant acceleration field (like gravitational fields):


    Acceleration is the change in velocity with respect to time. Looking at your graph, what is the value of the constant acceleration represented? Given the initial conditions, what is the time and the height of the top of the motion of the object? What is the equation for KE? Why in the world would it increase during the first second? And so on....

    Please review the link, and take another cut at the questions. Hang in there!
  4. Oct 12, 2007 #3
    So then if I pick two points on the graph, let's say (2,20) and (6,5), and plug them into the equation V_f=V_i+at, I would get 5=20+4a. Am I on the right track?

    Last edited: Oct 12, 2007
  5. Oct 12, 2007 #4
    someone help me out, please!
    Last edited: Oct 12, 2007
  6. Oct 12, 2007 #5
    So I think I got part B. I am pretty sure the answer is ADF. Does this sound right? I am still having trouble with part A though.
  7. Oct 12, 2007 #6
    I tried using V_f=V_i +at, but when I do that I get -3.69 m/s^2 and it says its wrong. Where am I messing up at?
  8. Oct 12, 2007 #7


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    Homework Helper

    Your number of 3.69 looks right... maybe they just want the positive value ?
  9. Oct 12, 2007 #8


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    Homework Helper

    Your answer for part b is wrong...

    give your reasoning about each one...
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