Object Launched from Origin: Time to Reach the Roof

[nando94]

An object is launched from the origin with a velocity of 20 m/s at an angle of 30 degrees above the x axis. The object lands on a roof that is 4 meters high. What is the time when the object hits the roof?

a) 1.11 s
b) 1.27 s
c) 1.33 s
d) 1.49 s
e) 1.67 s

I did 20 (sin(30)) = 10 m/s. Then used the equation

vf = vi + at

0 = 10 - 9.8t

t = 1.02 s

What am I doing wrong.

 

[lewando]

v_f is not 0, for starters. Consider using the fact that y_i=0 and y_f=4.