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Homework Help: One Dimensional Baseball Kinematics

  1. Aug 25, 2009 #1
    1. The problem statement, all variables and given/known data

    52. A baseball player catches a ball 3.1 s after throwing it vertically upward. With what speed did he thow it, and what height did it reach?

    2. Relevant equations

    def of a = t^-1 (V - Vo)

    3. The attempt at a solution
    t = (3.1 s)2^-1 = 1.55 s
    def of a = t^-1 (V - Vo)
    Vo = V - at
    when did that I got
    - (- 9.80 kg^-1 N)1.55 s
    15.2 s^-1 m

    ok in order to do this problem do you decide to make gravity negative or positive then when find the distance do I use negative or positive in order to find the velocity I had to use negative or I would have gotten a negative velocity so when I find the height i have to make the acceleration aslo equal to -g?????

    I do not understand when to make gravity negative and when not to
    one time in a problem I was asked to find the time something jumped into the air and it gave me a positive distance for how hight it jumped and asked me to find the time this particular problem which isn't this one was in the back of the book and i got the answer right but the thing is I considered acceleration to be positive... and the height was positive to wouldn't accelration have to be negative...

    I DON't understand...
  2. jcsd
  3. Aug 25, 2009 #2
    Yup...generally, gravitational acceleration is taken as negative. Sometimes, depending on how you do the problem, you ignore the negative/positive signs and you get it right...but that is usually because you compensate for it in the equations without noticing.

    When doing kinematics, just know that if a vector points in the negative direction, you take it as negative and if it points in the positive direction, its positive (this applies to acceleration, velocity, displacement...). In the problem you gave, Up is usually positive and Down is usually negative.
  4. Aug 25, 2009 #3
    so when i find the distance to i take gravity to be positive or negative???
  5. Aug 25, 2009 #4
    You did well taking it negative.
  6. Aug 25, 2009 #5
    even when i find the distance use negative???
    in this problem finding how high it goes use negative????
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