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Homework Help: Kinematics problem help

  1. Aug 31, 2011 #1
    1. The problem statement, all variables and given/known data

    A ball is thrown vertically upward from the ground with an initial velocity of 30 m/s. Assume the acceleration due to gravity is g = 10 m s-2.

    How long will it take for the ball to rise to the highest point on its trajectory? (AF:no decimals)


    i got v1 = 30m/s
    a= 10m/s2
    x1=0

    but i cant think of where to start given this information can someone point me in the right direction

    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Aug 31, 2011 #2

    SteamKing

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    What will be the velocity of the ball when it reaches the highest point of its trajectory?
     
  4. Aug 31, 2011 #3
    0 is the final velocity. is the answer 3s? i think i may have got it

    30=10t
    t=3

    anyone am i right?

    also the height it reaches using 3?

    would i use d=30*3 + 1/2(10)(3^2)
     
    Last edited: Aug 31, 2011
  5. Aug 31, 2011 #4
    anyone?
     
  6. Aug 31, 2011 #5

    Doc Al

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    Right!

    Almost. Careful with signs.
     
  7. Aug 31, 2011 #6
    would it be d= 30*3 + 1/2(-10)(3^2)?

    giving me 45?
     
  8. Aug 31, 2011 #7

    Doc Al

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    Yep!

    You can also solve it using the concept of average speed. What's the average speed of the ball as it rises?
     
  9. Aug 31, 2011 #8
    average speed 15 m/s?

    also i need help with this part... On the way up, at what height is the speed of the ball equal to half its speed of projection?

    i get that it wants me to find at what distance v = 15 but im having trouble munipulating the equations to solve this
     
  10. Aug 31, 2011 #9

    Doc Al

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    Right. So you can find the distance using average speed X time.

    Solve it exactly as you solved the first part. First figure out the time.
     
  11. Aug 31, 2011 #10
    okay thank you ill try and edit this

    t=1.5

    d=(15)(1.5)+(1/2)(-10)(1.5^2)= 11.25?

    or (30)(1.5)+(1/2)(-10)(1.5^2)= 33.75?

    i think its the second one
     
    Last edited: Aug 31, 2011
  12. Aug 31, 2011 #11

    Doc Al

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    Right. The initial velocity is 30 m/s, not 15. (That's the final velocity.)

    Verify your answer using the average velocity method.
     
  13. Aug 31, 2011 #12
    thank you so much for your help doc!
     
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