Physics · Unit 2: Dynamics · Activity 2-1-1

Deep Dive: Forces and Free-Body Diagrams

🔬 Deep Dive

This is your textbook for this topic. Take your time. Read it more than once.

2.1.1.AConcept

What Is a Force?

A force is a push or a pull — an interaction that can change an object's motion. Force has both a size and a direction, which makes it a vector, just like the displacement and velocity you worked with all through Unit 1. Forces are measured in newtons (N).

💡Every force is an interaction between two objects. If you push a box, the box also pushes back on you — but a free-body diagram only ever shows the forces acting on one chosen object, never the forces that object exerts on anything else.

2.1.1.BConcept

The Cast of Common Forces

Nearly every situation you'll analyze this year is built from some combination of these six forces.

Gravity (Weight)

Pulls every object straight down, toward the center of the Earth. F_g = mg.

Normal Force

A surface pushing back, always perpendicular to that surface. Keeps objects from passing through the floor.

Applied Force

A direct push or pull from a person or another object — a hand, a rope pull, an engine.

Tension

The pulling force transmitted through a rope, string, or cable, directed along its length.

Friction

Opposes relative motion (or attempted motion) between two touching surfaces.

Air Resistance

A drag force from air, opposing an object's motion through it. Grows with speed.

2.1.1.CMath

Building a Free-Body Diagram

A free-body diagram (FBD) isolates one object and shows every force acting on it, and nothing else — no background, no other objects, no forces the object exerts on something else.

ExampleGuided Example — FBD for a Block on a Table

A block sits at rest on a table. Build its free-body diagram step by step.

Step 1 — Isolate the object

Draw a single dot or small box representing the block. Ignore the table, the room, everything else.

2.1.1.DConcept

Net Force and Equilibrium

Once every force is drawn, you can combine them into a single net force — the vector sum of everything acting on the object.

F_net  =  ΣF  (the sum of every force vector)

When the net force is exactly zero, the object is in equilibrium — every force is perfectly balanced by another. This doesn't mean the object is standing still; it means its motion isn't changing, which includes an object moving at a constant velocity in a straight line.

Build a free-body diagram for a block on a floor. Toggle forces on and off and adjust their strength — watch the arrows update and the net force recompute live.

weight20N

20N

12N

net Fx

0.0 N

net Fy

0.0 N

equilibrium?

Yes

Try turning normal force off entirely — that's what a free-falling object's FBD looks like, with only weight remaining.

🔑Equilibrium means balanced forces, not zero motion. An object gliding at constant velocity with no friction is in exactly as much equilibrium as one sitting perfectly still.

2.1.1.EWatch Out

Common FBD Mistakes

⚠️Reaction forces that don't belong: if your object pushes on something else, that outgoing push never appears on your object's own FBD — only forces acting on it do.

⚠️Missing the normal force: any time an object touches a surface, that surface is pushing back, even if nothing else seems to be happening.

⚠️Guessing friction's direction: friction opposes relative motion (or the direction an object would slide if it started moving) — it is not simply "always backwards" relative to the diagram's orientation.

2.1.1.FConcept

Looking Ahead

You now have the one diagram every remaining lesson in Unit 2 assumes you can draw without prompting. Coming up: formalizing exactly how force, mass, and acceleration relate to each other — turning the free-body diagram from a picture into a number you can calculate.

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