Rule 18 of 36 · Chapter III — Force & Motion
Fields carry the influence
Why this rule exists
Bodies do not act across empty space instantaneously; each charge or mass fills the surrounding space with a field, and a second body responds only to the field where it sits. This replaces spooky action at a distance with a local story: the field is real, holds energy, and propagates at finite speed. Fields are what let influence travel without any matter moving between the bodies.
In practice
To find a force, first compute the field from all other sources at the location of interest, then multiply by the test body's charge or mass. Superpose the source fields vectorially before finding the force. Thinking in fields separates the source — what makes the field — from the response — what the field does — which simplifies many-body problems.
When it doesn't apply
The field picture is classical. At small scales the electromagnetic field is quantized into photons, and forces arise from exchanging these quanta; the smooth, continuous field is then only an average over many of them.