What is the potential problem with solar energy?

Simple Physics Problem involving Kinetic & Potential Energy.?

• Problem: a body fire vertically reaches a max height of 120m. What is the location at which the kinetic energy is 75% of the potential energy. Now I figured the way to set this up would be KE=.75PE>>> so (1/2)(m)(V^2)=(.75)(m)(g)(h)... the m's cancel and I eventually get V=42 m ... Put I don't think that is right because my notes say the answer is 68.5 m... PLEASE HELP STEP BY STEP!

• Answer:

  The total energy anywhere along the path is; E = (1/2)mv^2 + mgy You first get an expression for "E" by calculating it at max height, because the velocity is zero there; E = 0 + mg(120) Since "E" is constant everywhere along the path you can now use its value to find the location in question; mg(120) = (.75)mgy + mgy cancel mg to get; 120 = .75y + y = 1.75y Solve this for "y".