a free-body diagram
A free-body diagram is a simple sketch that shows all the forces acting on one object, and nothing else. You draw the object as a dot or box, then draw an arrow for each push or pull, pointing the way that force acts. It is the map you make before solving any force problem. The picture: strip away the scenery and look at just the one thing and the arrows pulling on it.
In a free-body diagram you isolate a single body and represent each external force acting on it as an arrow (a vector) whose direction is the force's direction and whose length suggests its size. Typical arrows are weight pointing down, the normal force perpendicular to the surface, tension along a rope, friction along the surface, and any applied push. You then pick axes, split the arrows into components, and add them to find the net force, which feeds straight into F_net = m a.
Nearly every mechanics problem becomes manageable once you draw a correct free-body diagram, and nearly every wrong answer traces back to a missing or invented force. Include only forces acting on the body, exerted by other objects; never draw forces the body exerts on other things, and never draw made-up forces.
For a book resting on a table, the free-body diagram has exactly two arrows: weight pulling straight down and the normal force pushing straight up, equal in length. Their sum is zero, so the book stays put.
A correct free-body diagram shows every force on the object and no others.
Draw only forces acting on the chosen body. Do not draw the force the body exerts on something else (its third-law partner), and do not add 'motion' as if it were a force.