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: Conceptual problem with projectile motion

  1. Sep 7, 2011 #1
    1. The problem statement, all variables and given/known data

    [PLAIN]http://img23.imageshack.us/img23/4554/physicshomework.png [Broken]

    2. Relevant equations

    3. The attempt at a solution

    I have the right answer because it is obvious, but I wanted to practice working this out regardless, and I have something slightly different for hmax.

    First, I found the y-component of velocity to be:

    [itex]V_{o}sin\theta_{o} - 9.8t[/itex]

    It follows that at the highest point, that this is equal to 0. So I set it equal to 0 and solved for t.

    [itex]t = \frac{V_{o}sin\theta_{o}}{9.8}[/itex]

    Integrating my y velocity gives y position:

    [itex]y(t) = (V_{o}sin\theta_{o})t - 4.9t^{2}[/itex]

    And solving for our time gives me:

    [itex]h_{max} = V_{o}sin\theta_{o}(\frac{V_{o}sin\theta_{o}}{9.8}) - 4.9(\frac{V_{o}sin\theta_{o}}{9.8})^{2}[/itex]

    Where did I go wrong?


    EDIT nevermind! They are the same, just had to do some algebra.
    Last edited by a moderator: May 5, 2017
  2. jcsd
  3. Sep 7, 2011 #2


    User Avatar
    Science Advisor
    Homework Helper

    The first thing you did wrong is writing 9.8 instead of g. If you go back and fix that and do a little algebra you'll see you already have the same answer as in the image.
  4. Sep 7, 2011 #3
    I'm not sure why you think that's wrong, I didn't forget halfway through the problem that they are the same..
  5. Sep 7, 2011 #4


    User Avatar
    Science Advisor
    Homework Helper

    It's wrong because now you have to change 9.8 back to g and 4.9 back to g/2. Why didn't you just leave it g to begin with? That's what I mean by 'wrong'. It's sort of a tactical error. You're making life complicated by leaving numbers floating around instead of 'g'.
  6. Sep 7, 2011 #5
    Not sure, I just wrote it that way without thinking. What I meant was that I didn't get to the end and say "they have g where I have 9.8, what is going on."
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook