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Homework Help: Time and max. height of stone thrown into air.

  1. Apr 14, 2010 #1
    1. The problem statement, all variables and given/known data

    "The initial speed of a stone projected vertically upwards is 21.0m/s

    a)What is the maximum height it will reach?

    b) How long does it take to reach the highest point and what is the total time which it is in the air?"

    Relative information:
    a=9.8 m/s/s
    u=21.0 m/s
    v=0 m/s
    s=?
    t=?

    2. Relevant equations

    I'm not sure, but I thought v^2=u^2+2as rearranged to s=(v^2-u^2)/2a might be the equation I need for a) at least, then for b), v=u+at rearranged to t=(v-u)/a.


    3. The attempt at a solution

    When I attempted, I got

    a) s=(v^2-u^2)/2a
    s=(0^2-u^2)/2*9.8
    =-22.5

    b) ??????

    I know this can't be right, because I don't think the distance can be negative. Could some kind person please, please help me?
     
  2. jcsd
  3. Apr 14, 2010 #2
    Acceleration acts downwards, so it is supposed to be -9.8 m/s^2 for the acceleration.
     
  4. Apr 14, 2010 #3
    oooohhhhh :) thankyou !!!
     
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