The 42nd Period | STEM Education Hub

A spring scale can measure both frictions in one pull, if you watch it carefully at the right two moments: the instant before the block breaks loose, and the steady reading once it's sliding.

🎯 Goals

Measure the force required to start a block sliding (static friction) and the force to keep it sliding at constant velocity (kinetic friction).
Calculate the coefficients of static and kinetic friction from your own data.
Use your coefficients to predict whether a new applied force will cause sliding.
Draw friction correctly on a free-body diagram, in the direction it actually opposes.

🧰 Materials

📦A block or box with a hook, and a set of add-on masses

⚖️Spring scale or force sensor

📏A flat surface (table or floor) to pull the block across

📓Physics notebook

DAY 1

Measure Static and Kinetic Friction

Section 1 of 4

Set Up and Practice

Attach the spring scale to your block's hook and place the block on a flat surface.

Practice pulling slowly a couple of times before recording any real data — the technique takes a little getting used to.

Pull with slowly increasing force, watching the scale closely.

Record the highest reading right before the block starts to slide — that's your maximum static friction for this trial.

⚠️ This moment goes by fast. It helps to have one partner pulling smoothly while the other watches the scale closely.

Once the block is sliding, keep pulling at a slow, STEADY pace (constant velocity).

Record the roughly steady scale reading while it slides at constant speed — that's your kinetic friction for this trial.

Section 2 of 4

Repeat With Added Mass

Add mass to the block and repeat both measurements at least twice more.

Changing the mass changes the normal force — this is how you'll see whether your coefficients stay consistent across different weights.

Trial	mass (kg)	weight/N (N)	max static (N)	kinetic (N)	μs	μk
Trial 1 (block only)
Trial 2 (+ added mass)
Trial 3 (+ more mass)

weight/N is your normal force — on a flat surface it equals mg. Use it to calculate μs = max static / N and μk = kinetic / N for each trial.

📓 Physics Notebook

Compare your maximum static friction reading to your kinetic friction reading in each trial. Was static consistently larger, like the Deep Dive predicted? By how much?

DAY 2

Calculate and Predict

Section 3 of 4

Find Your Coefficients

Calculate μs and μk for each trial using your recorded data.

μs = max static friction / normal force. μk = kinetic friction / normal force. Fill these into your trial table.

Average your μs values and your μk values across all three trials.

If your coefficients vary a lot between trials, that's worth noting — real surfaces aren't perfectly uniform.

Section 4 of 4

Predict a New Scenario

Using your averaged coefficients, predict whether a NEW applied force (given by your teacher) would cause your block to slide.

Show your comparison: calculate μsN for the new mass, and compare it directly to the given applied force.

Draw a free-body diagram for your block at the moment just before it slides.

Include weight, normal force, applied force, and friction — pointing the correct direction, opposing the applied force.

Question 1

Explain, using your own data, why it took more force to start your block moving than to keep it moving.

Question 2

If you had used a rougher surface, would you expect μs and μk to increase, decrease, or stay the same? Explain your reasoning.

Question 3

A classmate says, 'The friction force is always μN.' Correct their statement using what you learned about static friction's maximum.

📤 Turn In

Completed trial table with calculated μs and μk for each trial
Your prediction for the new scenario, with the comparison calculation shown
Free-body diagram of the block at the moment just before sliding
Written answers to the three Conclusion questions

← Back to Activity 2-2-1 Overview Review the Deep Dive →Next up: Lesson 2-2-2, Newton's Second Law.