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!

± acceleration due to gravity?

  1. Jul 16, 2008 #1
    A ball is thrown from the top of one building toward a tall building 50 m away. The initial velocity of the ball is 20 m/s, 40° ABOVE HORIZONTAL. How far above or below its original level will the ball strike the opposite wall?

    [tex]v_{ix} = 20 cos 40°[/tex] = 15.3 m/s
    [tex]v_{iy} = 20 sin 40°[/tex] = 12.9 m/s

    Using [tex]x = v_{x}t[/tex] to find time;
    50 = 15.3 t => t = 3.2 seconds.

    We use [tex]y = v_{iy}t + \frac{1}{2}a_{y}t^2[/tex] to find How far above or below its original level will the ball strike the opposite wall;

    Now I don't know wether I should take acceleration due to gravity, a = 9.8 as positive or negative for this. (What about viy = 12.0?)

    [tex]12.9 \times 3.27 + \frac{1}{2} \pm 9.8 \times 3.27[/tex] ?


  2. jcsd
  3. Jul 16, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    … be consistent …

    Hi roam! :smile:

    Either … but you must be consistent!

    Decide whether you're going to measure y upward or downward.

    If downward, then of course g is +9.8, but viy is -12.

    If upward, then g is -9.8, but viy is +12. :smile:
  4. Jul 16, 2008 #3
    Yes, thank you, Tiny tim. I had to make sure since I was skeptic about this.

    I understand how it goes now, thanks. :smile:
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Threads - acceleration gravity Date
Finding Acceleration Due To Gravity Jan 28, 2018
Apparent Weight Problem Jan 17, 2018
Vertical Circles Centripetal Acceleration Nov 30, 2017
Acceleration and Gravity with Circular Motion Nov 1, 2017