1. Not finding help here? Sign up for a free 30min 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!

Find initial speed?

  1. Jan 24, 2014 #1
    1. The problem statement, all variables and given/known data
    A ball thrown vertically from ground level is caught 3.0 s later by a person on a balcony which is 14 m above the ground. Determine the initial speed of the ball.

    t = 3.0 s, d = 14 m, Vi = ?

    2. Relevant equations
    d = Vi.t + 1/2 a.t^2


    3. The attempt at a solution
    I have no idea how to get this problem. I don't know if the equation is right or not?


    Thank You
     
  2. jcsd
  3. Jan 24, 2014 #2

    Doc Al

    User Avatar

    Staff: Mentor

    You can certainly use that equation. What would you use for the acceleration? (Careful with signs.)
     
  4. Jan 24, 2014 #3
    How do I find acceleration?
     
  5. Jan 24, 2014 #4

    Doc Al

    User Avatar

    Staff: Mentor

    It's a projectile. You should know the acceleration.
     
  6. Jan 24, 2014 #5
    Is it 3 ms-2?
     
  7. Jan 24, 2014 #6

    Doc Al

    User Avatar

    Staff: Mentor

    No. Where in the world did that come from?
     
  8. Jan 24, 2014 #7
    I coudn't think of any. I thought time and acceleration some how are related, which are not.
    Could we use gravity, which is 9.79 m/s2?
     
  9. Jan 24, 2014 #8

    Doc Al

    User Avatar

    Staff: Mentor

    The acceleration is constant.

    Of course you should use the acceleration due to gravity!
     
  10. Jan 24, 2014 #9
    So if we solve using acceleration, then a = 0 right!
    How can I solve using gravity? Is there any different formula?

    Thank you
     
  11. Jan 24, 2014 #10

    Doc Al

    User Avatar

    Staff: Mentor

    No. Why would you think that?

    You have the right formula. For the acceleration, use the acceleration due to gravity.
     
  12. Jan 24, 2014 #11
    Sorry a = 9.78 m/s^2
    I've tried that didn't ge the right answer.
    14 = Vi . 3 s + [4.9 m/s^2 . 9 s^2]
    14 - 44.1 / 3 s = Vi
     
  13. Jan 24, 2014 #12

    Doc Al

    User Avatar

    Staff: Mentor

    What's the sign of the acceleration? Which way does gravity act?
     
  14. Jan 24, 2014 #13
    Gravity pull an object down towards the ground. a = 9.78 ms-2!
     
  15. Jan 24, 2014 #14

    Doc Al

    User Avatar

    Staff: Mentor

    Use a proper sign convention in your equation. Let 'up' be positive and 'down' be negative.
     
  16. Jan 24, 2014 #15
    Tha gives me the right answer. So whenever its down - and up +
    a = -9.78 ms-2
    Thank you
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Find initial speed?
  1. Find initial Speed (Replies: 7)

Loading...