Posted by **Paula** on Friday, February 15, 2013 at 10:32pm.

A rock is thrown upward with a velocity of 15 meters per second from the top of a 26 meter high cliff ad i misses the cliff on the way back down. when will the rock be 5 meters from the water below?

- algebra -
**Reiny**, Friday, February 15, 2013 at 11:11pm
height = -4.9t^2 + 15t + 26

When the rock is 5 metres above the water,

5 = -4.9t^2 + 15t + 26

4.9t^2 - 15t - 21 = 0

t = (15 ± √636.6)/9.8

= 4.1052 seconds or a negative

After appr 4.1 seconds, the rock is 5 metres above the water.

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