Draw forces acting on one selected body: weight, normal contact, tension, thrust or resistance as appropriate. Add them as vectors to obtain net force. Newton’s first law gives constant velocity, including rest, when net force is zero. Propulsion can balance resistance without producing acceleration. A force arrow represents an interaction, not the direction of motion.
For constant mass use Fnet = ma, with acceleration in the net-force direction. Mass measures inertia; weight is gravitational force. Newton’s third-law partners are equal and opposite forces from one interaction acting on different bodies. They do not cancel on a one-body force diagram. Equal interaction forces can produce unequal accelerations when the masses differ.
Example 1
A 4.0kg cart experiences 18N right and 6N left. Find its acceleration.
Right is positive. Fnet =18−6=+12N; a= Fnet/m=412=+3.0m/s2, right.
Example 2
Earth pulls a person downward with 686N. Identify the third-law partner.
The person gravitationally pulls Earth upward with 686N. A floor’s upward force on the person is a different interaction, even if its magnitude happens to equal the weight.
Watch out: Both act on the book. Each has a partner acting on another body: the book pushes the table and gravitationally attracts Earth.