Object Launched from Origin: Time to Reach the Roof

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SUMMARY

The discussion centers on calculating the time it takes for an object launched from the origin at a velocity of 20 m/s and an angle of 30 degrees to reach a roof 4 meters high. The correct approach involves using the vertical motion equations, specifically the kinematic equation for displacement. The user initially misapplied the final velocity (vf) as zero, leading to an incorrect time calculation of 1.02 seconds. The correct time to hit the roof is determined to be approximately 1.49 seconds.

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  • Review the kinematic equation for vertical motion: yf = yi + vi*t + 0.5*a*t²
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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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