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Homework Help: Some Problem Related to Position vs Time Graph

  1. Jun 24, 2011 #1

    So Heres the Question:-
    A player throws a ball upwards with an initial speed of 29.4 m s–1.
    (a) What is the direction of acceleration during the upward motion of the ball ?
    (b) What are the velocity and acceleration of the ball at the highest point of its
    motion ?
    (c) Choose the x = 0 m and t = 0 s to be the location and time of the ball at its
    highest point, vertically downward direction to be the positive direction of
    x-axis, and give the signs of position, velocity and acceleration of the ball
    during its upward, and downward motion.
    (d) To what height does the ball rise and after how long does the ball return to the
    player’s hands ? (Take g = 9.8 m s–2 and neglect air resistance).[/CENTER]

    So my problem is in Part(c)
    Answer Comes Out that in both cases(upward and downward) Acceleration is Positive..
    but how? i mean shouldn't it be Negative in downward and Positive in upward motion
    Last edited: Jun 24, 2011
  2. jcsd
  3. Jun 25, 2011 #2
    They are positive provided downward direction is the positive axis. I just told acceleration is g downwards(or -g upwards:smile:)
  4. Jun 25, 2011 #3


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    Staff Emeritus
    Science Advisor
    Homework Helper

    In general, you can choose any direction you want to be positive. However, in part (c) they specifically say "Choose ... vertically downward direction to be the positive direction"
    Last edited: Jun 25, 2011
  5. Jun 25, 2011 #4
    tnxxx... guyss... u helpd... :)
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