3.1.5—Each of Newton’s laws of motion

First law defines inertial motion when resultant force is zero; second law gives ΣF=ma; third law pairs equal opposite forces acting on different bodies.

Apply one law to one chosen body at a time. Identify the frame, draw forces and distinguish acceleration from velocity.

A stationary book has weight and normal reaction balanced; the Earth–book gravitational interaction has a third-law partner acting on Earth.

Third-law forces do not cancel in one free-body diagram because they act on different objects.