http://physics.ucsd.edu/students/courses/summer2009/session1/managed/session1/physics1a/documents/Ch5_solutions.pdf [Broken] Please go to the website and scroll down to question 75a, why is g=9.8ms^2 is used in the solution instead of g=-9.8ms^2? Isn't it defined by the question that downward is negative? ( x1=-0.100m ) For part d, what is -kx in the solution? Isn't it -1/2kx^2? Thank you very much!!