# Projectile motion at top of cliff with angle...

by aemaem0116
Tags: angle, cliff, motion, projectile
 P: 1 hi. i have this projectile problem and i cant seem to get this one question right. A ball is thrown with an initial speed of 30 m/s at an angle of 45°.The ball is thrown from a height of 11 m and lands on the ground. i know the equation i have to use is: Yf = Yo + Vot - (1/2)gt^2 i know my Vo = 30sin45 = 21.213 m/s g = -9.81 i know the given Y is 11 m but im really confused about what should be my Yf and Yo. Most importantly, how can I manipulate the equation to find t?? what i thought it could be is like this...but i know its not right: 0 = 11 + (21.213)t - (.5)(-9.81)t^2 -11 = 21.213t - 4.905t^2 and thats about the farthest i could get...which isnt very far
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P: 5,346
 Quote by aemaem0116 hi. i have this projectile problem and i cant seem to get this one question right. A ball is thrown with an initial speed of 30 m/s at an angle of 45°.The ball is thrown from a height of 11 m and lands on the ground. i know the equation i have to use is: Yf = Yo + Vot - (1/2)gt^2 i know my Vo = 30sin45 = 21.213 m/s g = -9.81 i know the given Y is 11 m but im really confused about what should be my Yf and Yo. Most importantly, how can I manipulate the equation to find t?? what i thought it could be is like this...but i know its not right: 0 = 11 + (21.213)t - (.5)(-9.81)t^2 -11 = 21.213t - 4.905t^2 and thats about the farthest i could get...which isnt very far
Looks about OK. (Lose the - in front of the -9.8 as you did when you rearranged.)

That gives you the time until it hits the ground.