You throw a 20N rock into the air from ground level and observe that, when it is

Question

You throw a 20N rock into the air from ground level and observe that, when it is 15.0 m high, it is traveling upward at 25.0 m/s. Use the work-energy principle to find (a) the rock's speed just as it left the ground and (b) the maximum height the rock will reach.

Okay, so I set up my problem like this:

W=1/2mvf2-1/2mvi2

But I wasn't sure how to find the mass.

Isn't that 20N = x*9.8? I got the mass is 2.04 kg

Then I set up the problem like this:

1/2(2.04)(25.0)21/2(2.04)vi2 and got vi=25.0 m/s as my answer. But my book says the answer is 30.3 m/s. What did I do wrong?! THANKS!!!

Explanation / Answer

a. from work energy theorem, 0.5 m(v^2 - u^2) = -mgh 0.5 x (25^2 - u^2) = -9.8 x 15 Speed = u = 30.315 m/s b. 0.5 m(0 - u^2) = -mgh 0.5 x (0 - 30.315^2) = -9.8 x h Max. height = h = 46.888 meters

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