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!

Equation for initial velocity

  1. Sep 8, 2006 #1
    A ball is thrown vertically upward, which is the positive direction. A little while later it returns to its point of release. The ball is in the air for a total time of 10 s. Note: Near the earth's surface, g is approximately 9.80 m /s2.

    (a1) What is the algebraic expression for the initial velocity v0 of the ball? Express your answer in terms of the ball's displacement y, its acceleration a in the vertical direction, and the elapsed time t.

    is this correct?:

  2. jcsd
  3. Sep 8, 2006 #2


    User Avatar
    Science Advisor
    Gold Member

    "A ball is thrown vertically uwpard which is the positive direction"

    And you get -49, which since its negative, is the down direction. Will a ball thrown towards the ground last 10 seconds in the air? Only if you throw it from the roof of a tall building.
  4. Sep 8, 2006 #3
    You seem to have used the formula y=ut+0.5a(t^2), transposed it and obtained u=(y/t)-0.5at, so as long as you have your positive and negative directions right you've done it right, i think.
  5. Sep 8, 2006 #4
    one thing to think about, if the positive direction is upwards, how could the ball travel up when it has a negative initial velocity?
  6. Sep 8, 2006 #5


    User Avatar
    Homework Helper

    The vertical 'throw' equation for the y-direction is [tex]y(t)\vec{j}=v_{0}t\vec{j}-0.5gt^2\vec{j}[/tex], which is equal to y(t) = v0*t - 0.5*g*t^2. Actually, I don't see any problems with the directions; the initial velocity v0 is 'positive', and gravitiy is acting in the 'negative' direction all the time. I don't see how you got a negative velocity from your equation. v0 equals 49 [m/s].
  7. Sep 8, 2006 #6
    So my answer is correct? just instead of negative its positive?
    i got negative because, the displacement is 0,

    However when i use the equation radou gives, i get the positive 49... so i think its just the eqution i used..... Thanks guys!
  8. Sep 8, 2006 #7


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Your equation is correct, it's just that the acceleration due to gravity is [itex] a_y = -g = -9.80 m/s^2 [/itex]. (assuming the positive y axis pointing upward). You used a=+g which is the problem.

    Hope this helps
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Equation for initial velocity
  1. Initial Velocity (Replies: 2)

  2. Initial velocity (Replies: 1)