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: Projectile motion

  1. Jun 5, 2010 #1
    1. The problem statement, all variables and given/known data

    A small ball is projected with a speed of 10 m/s at an angle of elevation of p from the top of a vertical pole whose height is 15 m from the level ground. The ball hits the ground at a point whose horizontal distance from the foot of the pole is 20 m . Find the value of p . gravity=10 m/s^2

    2. Relevant equations

    3. The attempt at a solution

    Consider horizontal motion , s=Vx t

    20=10 (cos p) t

    t=2/(cos p) --1

    Consider vertical motion,

    15=10 (sin p)(2/(cos p))+1/2 (10)(2\(cos p))^2

    15 cos^2 p -20 sin p cos p -20 =0

    3 cos 2p - 4 sin 2p=1

    5 cos (2p+53.13)=1


    therefore , p=12.67 degrees

    Am i correct ?
  2. jcsd
  3. Jun 5, 2010 #2


    User Avatar
    Homework Helper

    15=10 (sin p)(2/(cos p))-1/2 (10)(2\(cos p))^2

    Till the above step you are right except one sign. Next

    15 = 20tan(p) - 20sec^2(p)

    Put sec^2(p) = 1 + tan^2(p) and solve for tan(p)
  4. Jun 5, 2010 #3
    Thanks but isn't the ball being projected downwards which is in the path of the gravity pull ?
  5. Jun 5, 2010 #4


    User Avatar
    Homework Helper

    Projected at an angle of elevation means up against the gravitational pull.
  6. Jun 5, 2010 #5
    Be careful with your signs in your equation for the vertical position. We can use, since the acceleration due to gravity is approximately constant:

    [tex]\Delta y \equiv y_{f}-y_{0}=v_{y}t+\frac{1}{2}at^{2}[/tex]

    If we define our coordinate system so that up is positive and down, negative then we are given

    [tex]y_{0}=+15, v_{y}=+10sin(p), a=-g=-9.8[/tex]

    Since it starts out above the ground and is shot with an initial velocity up and gravity pulls it down.
  7. Jun 5, 2010 #6
    got it , thanks
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook