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Homework Help: A ball is thrown in the air @ 30m/s

  1. Jul 5, 2009 #1
    1. The problem statement, all variables and given/known data
    A ball is thrown straight up with an initial speed of 30m/s,
    (a) Show that the time it takes to reach the top of its trajectory will be 3 seconds.
    (b) Show that it will reach a height of 45m (neglecting air resistance).

    2. Relevant equations
    Earth's gravity = 9.8 meter/second² (Professor said to just round it to 10).
    Equation unknown.
    d=1/2(g)t² ?

    3. The attempt at a solution
    (a) Played around with Subtracting negative -9.8m/s² (is it considered negative since the ball gets thrown up? - Instead of it coming down?).

    (b) 45m = 1/2(9.8m/s²)t² - I figure I need to know "t" before I could solve this.
    Last edited: Jul 5, 2009
  2. jcsd
  3. Jul 5, 2009 #2


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    Homework Helper

    Consider what the definition is of acceleration. It is the rate of change of velocity.

    If something starts out at 30 m/s and slows 10 m/s every second, it will reach 0 m/s in how many seconds?

    Here are some formulas for you:

    These lessons might be helpful:
    http://www.glenbrook.k12.il.us/gbssci/Phys/Class/vectors/u3l2c.html [Broken]
    Last edited by a moderator: May 4, 2017
  4. Jul 5, 2009 #3
    Thanks a bunch. Will get started on the lesson.. then try the problem again.
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