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Homework Help: A ball is thrown into the air + missing class

  1. May 22, 2012 #1
    1. The problem statement, all variables and given/known data
    Gravity = 10m/s/s

    The quotes are tables i just couldnt figure out how to make them.

    I dont know if velocity is suppose to change but i used 30 for each making the assumption its how hard the initial throw was.

    2. Relevant equations

    I dont know exactly what the teacher wants because i missed a class but with a couple of assumptions this is what i got

    3. The attempt at a solution

    Last edited: May 22, 2012
  2. jcsd
  3. May 22, 2012 #2


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    Note that "10m/s/s" is not "gravity"; rather, it is the acceleration [ magnitude ] due to gravity.

    Now, with that said, which one is constant and which one changes with time: acceleration or velocity?

    I'm assuming that you know the definition of these terms. If not then go back to your book/notes.
  4. May 22, 2012 #3
    i know its not gravity just trying to speed up the process, and the only thing constant is the seconds.
    thats the issue im having. In the first tabel its suppose to be blank for acceleration i will fix that.
    the only things given are for 0 seconds the velocity is 30 and the height is 0.
  5. May 22, 2012 #4
    Recall from Newton's 2nd law that an object in motion will stay in motion unless some force acts upon it.

    The force of gravity will tend to pull the ball back down to earth's surface. This force is constant! If you draw a free body diagram of the ball at any point during its trajectory, you will see that its weight is the only relevant force (due to gravity). This weight must be constant.

    Suppose you give the ball an initial velocity (you have to or it won't go anywhere). It will travel upwards, but its velocity should decrease, shouldn't it? Eventually, it reaches some maximum height and then continues downward, until it hits the ground.
  6. May 22, 2012 #5


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    If you say that the acceleration of gravity is 10m/s/s, then it is constant. Your 2nd table does not reflect that.
  7. May 22, 2012 #6
    just a question is my acceleration correct? or should acceleration just be 10m/s/s for each?
  8. May 22, 2012 #7


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    My post above yours answers that question.
  9. May 23, 2012 #8
    One small point! Newtons second law does not state "an object in motion will stay in motion unless some force acts upon it"
  10. May 23, 2012 #9
    Think the posts above me explain it well enough. All I can add is it is not m/s/s...this would mean ms/s = m.
    Acceleration is m/s^2
  11. May 23, 2012 #10
    m/s/s is not the same as ms/s.
    The best way to see this is to write it a m x s^-1 x s^-1 which is m/s^2
  12. May 23, 2012 #11


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    Now let's get on with the problem. Use the SUVAT equations of motion for constant acceleration g. The initial velocity is also given. At t = 1, find the velocity and displacement after 1 second has elapsed. Then plug in t = 2 and do the same. Etcetera.
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