When a person lifts a book while standing on the earth's surface. Why is his force considered external to the book-earth system? Isn't his force internal like the case of the forces that cause the exploson of a bomb. I think the forces the person exerts on both the book and the earth are internal since they are equal but opposite therefore resulting in the position of the centre of mass of the system being maintained and if i'm right is the mechanical energy of the system constant? Am i wrong? Please correct me.
[Physics] Internal and external forces
forcesfree-body-diagramnewtonian-mechanicswork
Best Answer
It all depends on what you define the system to be. If your system only consists of the book and the earth, then any force you supply is external. If you include yourself as part of the system, then your force is internal. If your system is just you and the book, then gravity is an external force.
Forces being internal or external has nothing to do with what they physically are and everything to do with how the system is subjectively defined (see drvrm's answer to understand the reasoning one would consider in this subjective decision).