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

Force Scavenger & FBD Sketching

🧲 Activity
1 class day  ·  Work in pairs
Activity 2-1-1: Forces and Free-Body Diagrams

Every scenario today looks ordinary — a book on a table, a box sliding across a floor. The skill is seeing past the ordinary picture to the forces hiding inside it, and putting every one of them on paper correctly.

🎯 Goals

🧰 Materials

🖼️Scenario cards or images (book on a table, box being pushed, object hanging from a string, a ball in the air)
📐Ruler or straightedge, for drawing clean arrows
📄Blank FBD templates (a dot with space around it)
📓Physics notebook
Section 1 of 4

Warm-Up — Naming the Forces

1
Look at your first scenario card without drawing anything yet.
With your partner, say out loud every force you think is acting on the object. Don't worry about direction or size yet — just build the list.
2
Check your list against the six common forces.
Gravity, normal, applied, tension, friction, air resistance. Did you miss one? Did you include a force that isn't actually acting on your object?
Section 2 of 4

Building Your First FBD

3
Draw a single dot to represent the object, in the center of your blank template.
Nothing else goes on the page except this dot and the arrows you're about to add.
4
Add gravity first, every time.
Every object near Earth's surface has weight. Draw it straight down before adding anything else — it's one less force to forget.
5
Add every remaining force from your list, with a straightedge.
Direction matters as much as presence. Normal force is always perpendicular to the surface; tension always points along the rope; friction opposes motion or attempted motion.
⚠️ Only include forces acting ON your chosen object. If your object pushes on something else, that push does not belong on this diagram.
6
Compare arrow lengths to guess relative size, and label every arrow.
A longer arrow means a bigger force. You don't need exact numbers yet — just relative sizes that make sense for the scenario.
Section 3 of 4

Scenario Practice

7
Work through all four scenarios below.
For each one: list the forces, sketch the full FBD on your template, and decide whether the object is in equilibrium.
Scenario 1. A book resting on a table.
Forces acting on the object
Equilibrium?
Scenario 2. A box being pushed across the floor at a constant speed.
Forces acting on the object
Equilibrium?
Scenario 3. A ball hanging at rest from a string.
Forces acting on the object
Equilibrium?
Scenario 4. A skydiver falling at constant (terminal) velocity.
Forces acting on the object
Equilibrium?

Sketch the full FBD for each scenario on your blank template — this table is just your force list and equilibrium call, not a substitute for the drawing.

📓 Physics Notebook
Pick one scenario where you almost missed a force. What made it easy to overlook, and what will you check for next time to catch it sooner?
Section 4 of 4

Conclusion

Question 1
Explain why a free-body diagram never includes the forces your chosen object exerts on something else.
Question 2
The skydiver in Scenario 4 is falling, but their FBD is in equilibrium. Explain how both of those things can be true at once.
Question 3
Describe a situation from your own life where friction's direction might be easy to guess wrong. What would help you get it right?
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