Free Body Diagrams
Applying Newton's Second Law to a real situation
usually requires unambiguous identification of the "body"
in question, so that the forces acting on it can be clearly marked
and not confused with the reaction forces on other bodies.
For example, consider the following crude drawing of you
standing still on the Earth: the symbol that looks like a belt buckle
is meant to indicate your center of gravity,
where your weight W = m g can be considered to act.
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