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: Projectile Motion- 2D Kinematics

  1. Sep 12, 2010 #1
    1. The problem statement, all variables and given/known data

    A mortar elevated to 60 degrees scores a hit 15 m up a building 30 m away. With what speed was the shell fired?

    I know I have to find the launch velocity, or VL. Here is a quick sketch i did:


    2. Relevant equations

    s= vot + 1/2at2

    v = s/t

    3. The attempt at a solution

    I began by setting up a table of known factors:


    I first set up a distance equation with the x values-

    30 = (VL * cos60)t {the second part of the equation presumably cancels out, because acceleration is 0}

    Then, I set up the equation using y values-

    15 = (VL * sin60)t - 4.9t2

    Next I tried solving for time using a different equation using x values:

    v = s/t

    t = 30/VL * cos60

    This being said- do I just plug what I found for t into the x-distance equation? I feel as if I'm not doing the problem correctly because then I would just be squaring the velocity in the x direction and dividing by 30 which leads me back to the original distance. I apologize in advance if the answer is staring at me from right under my nose, I'm not the smartest student. Any and all help is greatly appreciated, thank you so much in advance!
  2. jcsd
  3. Sep 13, 2010 #2


    User Avatar
    Homework Helper

    15 = (VL * sin60)t - 4.9t^2

    Substitute the value of t = 30/(VL*cos60)

    15 = VLtan60 - 4.9*30^2/(VLcos60)^2

    Now solve for VL.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook