Which Forces Go on the Diagram

Why diagrams go wrong

The usual mistake is to draw everything happening in the picture. A free-body diagram is not a picture of the situation — it is a list of forces acting on one chosen body, and nothing else belongs on it.

Pick the body first, out loud if it helps. Then ask of every arrow you are tempted to draw: is something pushing or pulling this body? If the answer is that this body is pushing something else, the arrow belongs on a different diagram.

The four that are usually there

Weight mg, always straight down, always present near the Earth.

Normal force, perpendicular to the surface in contact — that is what "normal" means here, not "usual".

Friction, parallel to that surface, opposing the sliding or the tendency to slide.

Tension, along the string, always pulling away from the body. A string can pull; it cannot push.

The normal force is not the weight

On a flat table with nothing else going on, N happens to equal mg. People then remember it as a fact rather than as a coincidence of that particular situation.

Tilt the surface by θ and the normal force becomes mg·cos θ — the surface only has to hold up the component pressing into it. Push down on the block and N grows; pull up on it and N shrinks; in a lift accelerating upwards, N exceeds mg. The normal force is whatever it has to be to stop the surfaces passing through each other.

Why action and reaction never appear together

Newton's third law pairs act on different bodies. Since a free-body diagram covers exactly one body, a third-law pair can never both be on it. If you have drawn two arrows and called them action and reaction, one of them is on the wrong diagram.

The classic version of this mistake: a block resting on a table, with N up and mg down, labelled as a third-law pair. They are not. Both act on the block, and they balance only because the block is not accelerating. The genuine partner of mg is the pull of the block on the Earth; the genuine partner of N is the push of the block on the table.

Centripetal force is a job, not a force

Nothing in nature is a centripetal force. The word names the role played by whatever real force happens to point towards the centre — tension for a ball on a string, friction for a car on a bend, gravity for a satellite.

So it never gets its own arrow. Draw the real forces, then note that their resultant is what curves the path. Adding a separate centripetal arrow counts the same force twice.

Centrifugal force does not belong on the diagram either. In an inertial frame it does not exist; what you feel in a turning car is your own inertia, not a force pushing you outwards.

Zero resultant does not mean zero speed

A body moving at constant velocity has no resultant force on it. Not a small one — none. This contradicts the everyday intuition that keeping something moving takes a continuous push, which it does only because friction is quietly cancelling that push.

The sharpest version is a ball thrown straight up, at the instant it reaches the top. Its speed is zero. Its acceleration is g, downwards, exactly as it was the whole time, and the only force on it is still mg. Zero velocity and zero force are unrelated claims, and mixing them up costs more marks than any algebra slip.