Projectile Motion of thrown rock

[Momentum09]

1. You are standing on the edge of a ravine that is 11.5m wide. You notice a cave on the opposite wall whose ceiling is 6.4m below your feet. The cave is 4.7m deep, and has a vertical back wall. You decide to kick a rock across the ravine into the cave.
1. What initial horizontal velocity must you give the rock so that the rock barely misses the overhang?
2. How far down the back wall of the cave will the projectile hit?



2. delta y = v0yt - 1/2 gt^2
delta x = vox t


3. First I used y = -4.9t^2 to solve for t. After that, I tried to plug the t into the delta x equation to find the initial velocity. But for delta x, I don't know which distance I should use...11.5 or (11.5+4.7)?

Thanks!

 

[PhanthomJay]

1. You are standing on the edge of a ravine that is 11.5m wide. You notice a cave on the opposite wall whose ceiling is 6.4m below your feet. The cave is 4.7m deep, and has a vertical back wall. You decide to kick a rock across the ravine into the cave.
1. What initial horizontal velocity must you give the rock so that the rock barely misses the overhang?
2. How far down the back wall of the cave will the projectile hit?



2. delta y = v0yt - 1/2 gt^2
delta x = vox t


3. First I used y = -4.9t^2 to solve for t. After that, I tried to plug the t into the delta x equation to find the initial velocity. But for delta x, I don't know which distance I should use...11.5 or (11.5+4.7)?

Thanks!

When you solved for t, the rock had descended 6.4m, just missing the overhang. What's its horizontal distance at that instant?