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: Inital velocity

  1. Mar 29, 2010 #1
    An object is thrown vertically into the air and reaches a height of 30.0 m. Neglecting air friction, what was the object’s initial velocity?

    What would the equation be for this question?

    I know that:

    Height: 30.0m
    Gravity: 9.81m/s

    I'm not quite sure what else I need to figure this out? thank! :)
  2. jcsd
  3. Mar 29, 2010 #2
    Is this a trick question where its 0m/s?
  4. Mar 29, 2010 #3


    User Avatar
    Science Advisor
    Homework Helper

    The easiest way would be to use conservation of energy:
    E = 1/2mv^2+mgh
    Alternatively, you could use the equation of motion for the object. You know it has a constant acceleration g downwards.
  5. Mar 29, 2010 #4
    Thank you! so would it look something like this:

    E=1/2 (230kg)(0.02ms) ^2 + (230kg)(9.81m/s)(32m)

    0.092 + 77201.6 = 77201 ?
  6. Mar 29, 2010 #5
    oops i mean

    5.43 + 72201.6 = 72207.03
  7. Mar 29, 2010 #6


    User Avatar
    Science Advisor
    Homework Helper

    The power behind conservation of energy is that you can use the equation for E at any point in the objects trajectory and you will always get the same answer.
    You haven't been given the mass of the object, but the answer will be independent of the objects mass anyway.

    You have two special points in the objects trajectory which are important to your question. Which are those and what is the expression for the energy at those points?
  8. Mar 29, 2010 #7
    gravity, height and time?
  9. Mar 30, 2010 #8
    I think using Vf^2=Vi^2 + 2ad would be better..Since the highest point is 30 that means at that point v equals 0.
  10. Mar 31, 2010 #9
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook