d(mv)/dt=F
d(mv)=Fdt
d(m1v1)=F1dt d(m2v2)=F2dt
d(m1v1+m2v2)=(F1+F2)dt F1+F2=0
d(m1v2+m2v2)=0
d(1/2*mv^2)=Fdr (v=dr/dt)
Fdr=-dU
d(1/2*mv^2+U)=0

U=mgy F=-dU/dy=-mg
U=1/2*kx^2 F--dU/dx=-kx
U=-K/r F=-dU/dr=-K/r^