1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

    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


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    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


    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
  6. Aug 31, 2011 #5

    Doc Al

    User Avatar

    Staff: Mentor


    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

    User Avatar

    Staff: Mentor


    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

    User Avatar

    Staff: Mentor

    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


    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

    User Avatar

    Staff: Mentor

    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!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook