You throw a 20-N rock vertically into the air from ground level. You observe that when it is a height 14.8m above the ground, it is traveling at a speed of 25.0m/s upward.

Question

A) Use the work-energy theorem to find its speed just as it left the ground. What is it?
B) Use the work-energy theorem to find its maximum height. What is it?

Answer 1

m*g = 20 N.
m = 20/g = 20/9.8 = 2.041 kg

A. Work = 0.5m*Vo^2 - 0.5m*V^2 = mg*h
1.02*Vo^2 - 1.02*25^2 = 20*14.8
1.02Vo^2 - 637.5 = 290
1.02Vo^2 = 290 + 637.5 = 927.5
Vo^2 = 909.31
Vo = 30.15 m/s.

B. h = (V^2-Vo^2)/2g =
(0-30.15^2)/-19.6 = 46.38 m.