Object Launched from Origin: Time to Reach the Roof

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The discussion revolves around calculating the time it takes for an object launched at 20 m/s at a 30-degree angle to reach a roof 4 meters high. The initial vertical velocity is calculated as 10 m/s. A user mistakenly assumes the final vertical velocity is zero when the object reaches the roof, leading to an incorrect time calculation of 1.02 seconds. The correct approach involves using the vertical displacement equation with initial height at zero and final height at four meters. The accurate time for the object to hit the roof is determined to be one of the provided options, requiring further calculations.
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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.
 
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vf is not 0, for starters. Consider using the fact that yi=0 and yf=4.
 

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