My FBD looks right but my answer is negative. Is it wrong?

Not necessarily. A negative magnitude usually means the force points opposite to your assumed direction. Keep the math consistent and interpret the sign at the end.

Should I draw one FBD for a multi-object system or several?

Both are useful. One combined FBD hides internal forces (fewer unknowns); separate FBDs expose them (needed for rope tensions between blocks). Many problems require doing both.

10 Common Free Body Diagram Mistakes (And How to Fix Them)

Most marks lost in mechanics are lost in the diagram, not the algebra. These are the ones graders see over and over.

Every arrow needs a source you can name. If nothing is applying it, it is not a force.

A "force of motion"

A body moving right has no rightward force just because it is moving. Motion needs no force to continue.

Fix — Only draw forces with a source: gravity, and things touching the body.

Friction left out

A rough surface with any tendency to slide has friction, whether or not the problem mentions it.

Fix — At every contact ask two questions: normal force? Always. Friction? Yes, if rough.

Centrifugal force

Nothing pushes outward in circular motion. The inward net force is made of real forces.

Fix — Draw only real forces, then set their inward components equal to mv²/r.

Assuming N = mg

The normal force is whatever the perpendicular equation demands. Inclines, lifts and angled pulls all break the equality.

Fix — Solve the perpendicular equilibrium equation for N every time.

Friction pointing the wrong way

It opposes sliding, which is not always backwards — a driving wheel is pushed forward by friction.

Fix — Ask which way the surfaces would slip without friction, then point it the other way.

No moment at a fixed support

A built-in end resists rotation, so it applies a moment as well as two force components.

Fix — Fixed support means three reactions: Rx, Ry and M. Always three.

Internal forces drawn

Inside the body you isolated, forces come in cancelling pairs and contribute nothing.

Fix — Draw only what crosses the boundary. To expose an internal force, cut there and draw a separate diagram.

A rope pushing

Cables and ropes pull and nothing else.

Fix — Every tension arrow points away from the body along the rope. If a solution needs a push, the assumption is wrong.

Forces at the wrong point

For a particle it makes no difference. For a rigid body, where a force acts changes its moment.

Fix — Weight at the centre of gravity, contact forces at the contact, a distributed load at its centroid.

Unlabelled arrows

Five anonymous arrows are not a diagram anyone can follow, including you tomorrow.

Fix — Label every arrow, mark the angles, and draw bigger forces visibly longer.

Before you write a single equation

Body isolated — no ramps, ropes or walls left in the drawing
Weight drawn from the centre of gravity
One force per contact, plus friction where the surface is rough
Reactions match the support type: 1, 2 or 3
Every arrow labelled, angles marked, axes chosen
Nothing on the diagram that is not a real external force

FAQ

My diagram looks right but the answer came out negative. Is it wrong?

Probably not. A negative magnitude almost always means the force points the opposite way to the arrow you drew. Keep the algebra consistent and read the sign at the end.

One diagram for a multi-object system, or several?

Both have their use. One combined diagram hides the internal forces and leaves fewer unknowns; separate diagrams expose them, which is how you find the tension between two blocks. Many problems want both.