AP Physics 1 · Unit 2: Forces & Translational Dynamics · Lesson 2.2
Deep Dive: Forces and Free-Body Diagrams
🔬 Deep Dive
This is your textbook for this topic. Take your time. Read it more than once.
2.2.A.1Concept
What Is a Force?
A force is a push or a pull — a vector quantity that describes an interaction between two objects or systems. The word "interaction" is doing a lot of work in that sentence: a force never exists in isolation. It always requires something doing the pushing or pulling, and something receiving it.
🔑A force exerted on an object is always due to that object's interaction with another object or system. If you can't name the second object causing the force, you haven't correctly identified the force.
2.2.A.1.iConcept⚠ Watch Out
The Self-Force Limitation
Here's a rule that sounds obvious once you hear it, but trips students up constantly: an object or system cannot exert a net force on itself. You can't push yourself forward by pushing on your own shoulder. Internal forces — even when they exist — always come in pairs that cancel.
⚠️Where this gets tricky: a person walking does push on the ground, and the ground pushes back — but that's a force between two different objects (the person and the Earth), not a self-force. The person didn't push themselves forward; the ground did.
2.2.A.2Concept
Contact Forces vs. Field Forces
Forces split into two categories based on whether the objects are actually touching. Contact forces require physical contact between surfaces. The one field force you'll use in AP Physics 1 — gravity — acts at a distance, with no touching required.
Force
Symbol
Direction
Type
Normal force
F_N
Perpendicular to the surface, away from it
Contact
Friction
F_f
Parallel to the surface, opposing relative motion
Contact
Tension
F_T
Along the rope or string, pulling
Contact
Applied force
F_app
Whatever direction it's pushed or pulled
Contact
Gravity (weight)
F_g
Straight down, toward Earth's center
Field
💡Contact forces feel mechanical, but at the atomic level they're all the result of electric forces between atoms repelling each other. When you push on a wall, you're not literally touching the wall's atoms — you're close enough that electric repulsion stops you from passing through.
2.2.B.12.2.B.22.2.B.3Concept
Building a Free-Body Diagram
A free-body diagram shows every external force acting on one chosen object — nothing more. The construction follows three simple steps:
Represent the object as a single dot at its center of mass.
Identify every external force acting on that object.
Draw each force as a straight arrow starting at the dot, pointing in the direction the force acts.
Example: a box on a flat surface
Example: a box on an inclined plane
Inclines are where free-body diagrams get harder — because the normal force and friction are no longer simply up/down and left/right. They're perpendicular and parallel to the incline's surface, not the ground.
🔑On an incline, the smart move is to tilt your coordinate axes to match the slope — one axis parallel to the surface, one perpendicular. This keeps the normal force and friction force aligned with your axes, which makes the algebra dramatically simpler later on.
ExampleGuided Example — Drawing a Free-Body Diagram
A 5 kg box sits on a horizontal table. A person pushes it to the right with 12 N. Friction opposes the motion with 4 N. Draw the free-body diagram.
Step 1 — Draw the dot
Represent the box as a single dot — its center of mass.
2.2.B.4⚠ Watch Out
AP Exam Drawing Standards
The exam graders look for specific conventions in your free-body diagrams. Get these wrong and you can lose points even if your physics reasoning is correct.
⚠️Never draw force components on a free-body diagram.Breaking a force into x and y pieces is an algebra step you do after the diagram — not something that belongs in the picture itself. Show only the real, physical forces.
When two forces point in the same direction — say, two ropes pulling the same way — draw them as separate, side-by-side arrows, never stacked directly on top of each other. Graders need to see that you recognize them as distinct forces, not one combined arrow.