Gravitational potential energy negative?

[mrserv0n]

Can kinetic energy of a system or gravitational potential energy be negative?

I am reading about the two but its very complex and can't seem to describe why.

 

[Doc Al]

Kinetic energy must be positive, but gravitational PE can certainly be negative. Gravitational PE depends on what you choose as your (arbitrary) reference point, where you define PE = 0. (Only changes in PE are physically significant.)