Starting from Kepler's laws we can not only derive Newton's Force(F) balance equation but also the Energy(E) conservation equation. We can derive that, E=K+P, where K=Kinetic Energy and P=Potential Energy=-GMm/|r| and E=Constant. Note that, F=m(d^2r/dt^2)=mass*acceleration and F_g=-(GMm/|r|^3)r=Newton's Law of Gravity. Also we get dP/dt=-F_g.(dr/dt), a vector dot product. Here r is the position vector and |r| indicates its magnitude. Thus, we get dE/dt=[F-F_g].(dr/dt), dot product. This is true even when E is not Constant. If E=Constant then, dE/dt=[F-F_g].(dr/dt)=0. This in general means, F-F_g is perpendicular to dr/dt. Not always m(d^2r/dt^2)-F_g=0 as Newton's Universal law of Gravity is stated. Hence Newton's equation encompass only a small subset of all the phenomena covered by the equation dE/dt=0. The equation F=F_g or m(d^2r/dt^2)=-(GMm/|r|^3)r in that form is not even applicable for all 2-body problems in 2D. In general, F=Some component of F_g. Since F_g=-grad(P) we also get that, in general, F=m(d^2r/dt^2)=Some component of -gradient(P). Thus assuming F=-gradient(P) is not valid. Determining which component of F_g is causing the body to accelerate is non-trivial. The free-body diagrams are of limited use and the principle of least action(Lagrangian calculations) employ energy terms but in a much more complicated manner. We can achieve better results directly using the Energy conservation equation. Further we extend the analysis to include Lagrange type 3-body periodic orbit solutions with equilateral configuration and show that Lagrangian/Newtonian method gives some sporadic, apparently unstable solutions, where as the Energy method provides the entire set of stable elliptical orbit solutions including non-equilateral configurations. With Energy method we can also derive a condition which determines whether the 3-bodies end up in an orbit with 1 center of revolution(like in Lagrange type periodic orbits) or end up with 2 centers of revolution(like in the Sun-Earth-Moon system). We also note that the term Inertia coined by Galileo to explain the height conserving property of balls rolling down inclined planes has to be properly interpreted as energy. That is, Inertia = Energy. And we point at the need to replace Newton's Laws of Motion(and Gravity) by the Energy conservation principle. And principle of angular momentum conservation or angular velocity conservation and such.