The 42nd Period | STEM Education Hub

Pick at least three different surfaces. For each one, you'll measure a coefficient of friction, commit to a prediction on paper, and only then find out if you were right.

🎯 Goals

Measure the coefficient of static friction for at least three different surfaces.
Predict the critical sliding angle for each surface before testing it.
Test each prediction on a real adjustable ramp and calculate its accuracy.
Explain any gap between predicted and measured angles with a real physical reason.

🧰 Materials

📦Block with hook, plus at least three surface materials (sandpaper, wax paper, felt, bare wood, etc.)

⚖️Spring scale or force sensor

📐Adjustable ramp with a protractor for measuring angle

🧮Calculator

📓Physics notebook

DAY 1

Measure μs for Multiple Surfaces

Section 1 of 5

Choose Your Surfaces

Pick at least three distinctly different surfaces to test.

Aim for real variety — something smooth and slick, something rough and grippy, something in between.

Use the spring-scale pull method from Lesson 2-2-1 to find the maximum static friction for each surface.

Keep the block's mass the same across all three surfaces so your comparison is fair — only the surface should change between trials.

Surface (name it)	Mass (kg)	Normal Force (N)	Max Static Friction (N)	μs

Calculate μs = f_s,max / N for each surface.

Fill in the last column of your data table with these calculated values.

DAY 2

Predict the Critical Angle

Section 2 of 5

Commit to Your Predictions

For each surface, calculate the predicted critical sliding angle using θ = tan⁻¹(μs).

Write these predictions down on paper before you touch the ramp for testing — this is the whole point of the project.

DAY 3

Test and Check Accuracy

Section 3 of 5

Test Each Surface

Set up each surface on the adjustable ramp and slowly raise it, measuring the angle where sliding begins.

Raise the ramp slowly and evenly — a jerky lift can trigger sliding early and throw off your measurement.

Section 4 of 5

Check Your Accuracy

Use the calculator below to check your prediction accuracy for each surface.

Enter your μs and your actual written prediction, then your measured angle, to see your percent difference.

Enter your measured μs for each surface to get the predicted critical angle automatically (θ = tan⁻¹μs). After you've written down your prediction and tested it on the ramp, enter your measured angle to see the accuracy.

μs

Predicted θ (°)

Measured θ (°)

% difference

—

μs

Predicted θ (°)

Measured θ (°)

% difference

—

μs

Predicted θ (°)

Measured θ (°)

% difference

—

The predicted angle auto-fills from μs as a placeholder — but write your actual prediction down on paper BEFORE testing. Typing it in here afterward doesn't count as predicting first.

Section 5 of 5

Explain Your Results

For any surface where prediction and measurement didn't closely match, suggest a real physical reason.

Surface non-uniformity, contamination (dust, oils), a ramp material different from your original test surface, or an uneven ramp lift are all fair explanations — 'human error' alone is not.

📊 What Gets Graded

Measurement — correct μs calculations for at least three surfaces
Prediction — a written, committed prediction made before testing, for every surface
Accuracy analysis — a correct percent-difference calculation and a real explanation for any gap

📤 Turn In

Coefficient-of-friction data table for every surface tested
Prediction accuracy check — predicted vs. measured critical angle, with percent difference, for every surface

← Back to Project 2-2-4 OverviewNext up: Problem 2-3-1, Loading Dock Ramp Hazard.