4.1.7—Newton's laws

Newton’s second law is ΣF=ma for a chosen body and inertial frame. First-law equilibrium is the special case a=0; the third law pairs equal opposite forces on different bodies.

Draw one free-body diagram at a time, choose axes, and distinguish action–reaction pairs from forces that act on the same object.

A 4 kg block with resultant horizontal force 12 N has acceleration 3 m s⁻²; the reaction and weight are separate vertical forces.

The third-law partner of a normal reaction acts on the other body, so it cannot cancel the weight of the same body.