Posted by **Hayli** on Monday, February 20, 2012 at 10:22am.

Isaac pushes rock, of a mass of 20kg, if from a cliff and it falls into the lake below. The cliff was 3m above the water. (Assume no air resistance)

a. How much time did it take the rock to hit the water?

b. What was its velocity when it was half of the distance to the water?

- Physics -
**drwls**, Monday, February 20, 2012 at 10:54am
The mass of the rock will not affect the answer.

a) Solve 3 = (g/2) t^2 to get t.

b) Solve 1.5 = (g/2) t^2 to get the time to fall half way,

t = sqrt(3/g)

and they use

V = g * t for the velocity V at that time, sqrt(3g). You can also use 3nergy conservation directly to get

the same answer.

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