my question is, when using energy conservation when do i put the potential energy of gravity negative? if its under where it set my potential energy to 0? or am i wrong, is gravity's potential energy always positive (except at my set u=0 point) ? can someone explain this to me, having trouble getting ANYthing done because i do not understand how to set up these homework questions for example, this is a ball pushed into a barrel, compressing the spring, ball has initial velocity 0 here are the values, k = 667 N/m mass of ball = 1.5kg ball is pushed into tube .25 meters if u=0 is set at the mouth of the barrel, then the initial potential energy is mgh+.5kx^2? or is it -mgh+.5kx^2? -mgh-.5kx^2?