This is the problem:(adsbygoogle = window.adsbygoogle || []).push({});

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!

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Work-Energy Theorem rock throw Question

**Physics Forums | Science Articles, Homework Help, Discussion**