Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Initial velocity confusion please help!

  1. Oct 17, 2006 #1
    initial velocity confusion!! please help!

    hello all,
    i am slightly confused about initial velocity during free fall
    if a ball is being thrown in the air, and the velocity time graph shows 20m/s at 0 seconds, and -20m/s at 4 seconds, which is when the ball drops
    if i want to calculate the displacement, i know that i can use the formula d=(vo)(t) +0.5(a)(t^2)
    with vo being initial velocity
    why is it that i would have to put 0 for vo? why isn't it 20m/s?
  2. jcsd
  3. Oct 17, 2006 #2
    Which displacement are you talking about? (I'm assuming that the ball is thrown vertically)
  4. Oct 17, 2006 #3
    I'm not sure I understand your question. If a ball is thrown with initial velocity of 20 m/s, then that's your initial velocity. The V0 just means velocity at t=0. If a ball is dropped or released then its V0 is 0.
  5. Oct 17, 2006 #4
    the ball is thrown vertically, displacement being the change in distance. oksanav: why is it that If a ball is dropped or released then its V0 is 0?, but then why would the v-t graph show 20m/s at 0 seconds?
  6. Oct 17, 2006 #5
    When the ball's thrown initially, it has a velocity of 20 m/s, but as it reaches the top, its velocity goes to zero. After that, its velocity increases, now pointing down, reaching 20m/s just before hitting the ground. Depending on which part you're calculating, you're initial velocity will change.
  7. Oct 17, 2006 #6


    User Avatar
    Homework Helper

    If a ball is dropped, there usually is no initial velocity, but there can be some. But in that case, it is a throw. It's all about directions.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook