Finding y in a projectile motion problem

  1. 1. The problem statement, all variables and given/known data
    A golfer hits a shot to a green that is elevated 4.0 m above the point where the ball is struck. The ball leaves the club at a speed of 15.0 m/s at an angle of 40.0° above the horizontal, which is the +x axis. It rises in the +y direction to its maximum height and then falls down to the green. Ignore air resistance.

    What is the algebraic expression for the y component vy of the ball's velocity just before landing on the green? Calculate for the y component vy using this equation.

    known:
    v(initial)=15.0m/s
    a_x= 0
    a_y= 9.8 m/s^2
    v_x (initial) = cos40*15 = +11.5 m/s

    2. Relevant equations

    i thought it would be v^2=v(initial)^2 + 2ay rearranged to solve for just v(y direction). therefore: square root of v(initial)^2 + 2ay.
    y being the distance in the vertical direction.

    also: for v(initial) i took the sin40*15 to get 9.6 m/s.

    3. The attempt at a solution
    when i plug this into my homework website, it tells me i am wrong. but this is the only kinematic that i can think of using only v(initial), y displacement, and acceleration. what's wrong with my equation?
     
  2. jcsd
  3. rock.freak667

    rock.freak667 6,228
    Homework Helper

    What value did you get? Did you use [itex]v_y^2=(v_0)^2_y+2ay[/itex] or [itex]v_y^2=(v_0)^2_y-2ay[/itex] ?
     
  4. i used the equation LaTeX Code: v_y^2=(v_0)^2_y-2ay and rearranged it so it would be sqtroot(v(initial)^2 + 2ay). i think i got like -8.9 m/s, but the website told me i was wrong.
     
  5. by the way i have to write it in a way solving for just v_y
     
    Last edited: May 27, 2009
  6. rock.freak667

    rock.freak667 6,228
    Homework Helper

    v=final
    u=inital

    if you use v2=u2+2ay then you are saying that acceleration is acting upwards. The only acceleration in this question is gravity which acts downwards.So, you'd need to use v2=u2-2ay
     
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