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You throw a 20-N rock vertically into the air from ground level. You observe that when it is 15.0 m above the ground, it is traveling at 25.0 m/s upward. Use the work-energy theorem to find a) its speed just as it left the ground; b) its maximum height.

I drew a free-body diagram with an unknown force F pointing upwards, and the mg = 20 N pointing down. I also found the mass of the object by using 20 N = mg to get m = 2.04 kg.

I know that the work theorem is W(total) = K2 - K1, with K = (1/2)m(v)^2. I figured out K2 with the given 25.0 m/s and the 2.04 kg, which came out to be 637.5 J. Because I'm trying to find the initial speed, K1 looked like this = (1/2)(2.04 kg)(v1)^2. I tried to find out what W(total) was to complete this problem, but then became stuck because, in W = Fs, I didn't know what F was. I couldn't calculate F = ma in the vertical direction because I didn't have an acceleration. How do I go about figuring out W(total) from this information?

Thanks!

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# Homework Help: Work-Energy Theorem rock throw Question

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