Preview

Friction Lab

Q: A 5kg block on wood has μ_s = 0.5. What is the maximum static friction force?

F_s = μ_s × N = 0.5 × 5 × 9.8.

Q: Why does adding weight to the block change the friction force but not the coefficient?

F_friction = μN; more weight increases N, which increases F_friction; μ stays constant.

Q: Experimentally, why is μ_s > μ_k for most material pairs?

At rest, surface asperities interlock more deeply; motion reduces contact depth.

Measure static and kinetic friction coefficients

Push a crate across the floor and something uncanny happens at first: nothing. You lean in with 10 newtons, then 20, then 40 — and the floor matches you exactly, newton for newton, an invisible hand pushing back with precisely your own strength. Static friction is not a fixed force; it is a negotiator, mirroring your push until it reaches the one deal it cannot make: its ceiling, μ_s times the block's weight pressing down. Cross that line and the spell breaks — the block lurches free, and the friction force actually drops, because sliding surfaces grip less than gripping ones. In this lab you will feel all three acts on the readouts: the mirror region where friction copies your push, the breakaway at 49.00 N where the badge flips STATIC to SLIDING and the force sags to 29.40 N, and the steady kinetic plateau. Then comes the deeper surprise most people get wrong. Your gut says grip lives in the footprint — wider contact, more friction. Double the block's mass instead and watch both the normal force and the breakaway force double in perfect lockstep, 98 to 196, 49 to 98, while the footprint never changes at all. The model that predicts every number in this lab has two variables only: the materials and how hard they are pressed together. Area is not one of them.

What you'll be able to do

Distinguish static friction (reactive: matches the applied force exactly up to f_s,max = μ_sN, then releases) from kinetic friction (constant f_k = μ_kN once sliding), and read the transition on the sim's state machine — STATIC (blue) to SLIDING (orange) badge, friction readout dropping 49.00 → 29.40 N at the default surface
Compute breakaway force and sliding acceleration from the Coulomb model on a horizontal surface: N = mg with g = 9.8 m/s² (49.00 N breakaway at the 10 kg / μ_s = 0.5 default surface), then a = (F_applied − μ_kN)/m (3.06 m/s² at 60 N applied)
Show experimentally that friction tracks the normal force — dragging Mass 10 → 20 kg doubles both dNormal (98.00 → 196.00 N) and Max Static f (49.00 → 98.00 N) in lockstep with f/N pinned at μ_s = 0.50 — and argue that contact area never enters the model, confronting the wide-tires intuition with evidence

Formulas

F_{s,max} = \mu_s N

Maximum static friction

F_k = \mu_k N

Kinetic friction force

N = mg

Normal force on a horizontal surface

Make a prediction

Set Mass to 10 kg on the shipped default surface (μ_s = 0.5, μ_k = 0.3): Max Static f reads 49.00 N. Your lab partner shrugs — 'Double the mass if you like. Friction is set by the contact area, and the block's footprint isn't changing.' You drag Mass from 10 kg to 20 kg. What does the Max Static f readout do?

No grading here — commit to a guess, then scroll down and test it yourself.

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The mirror region: with Mass at 10 kg, drag Applied Force slowly from 0 to 40 N. Keep your eyes on two rows — dFriction copies dApplied exactly (25.00 N in, 25.00 N out), the blue and green force bars grow in lockstep, dAcc never leaves 0.00 m/s², and the badge holds STATIC with the box blue. Static friction is measuring your push and matching it.
Breakaway: push Applied Force to 60 N and press Play. The instant 60 N beats the 49.00 N ceiling the badge flips SLIDING (orange), the box turns orange and rolls forward with dust kicking up — and dFriction DROPS to 29.40 N while dAcc reads 3.06 m/s² (= (60 − 29.4)/10). Catch that drop: sliding grips less than gripping.
The area test, by weight: reset force to 0 and read the pair — dNormal 98.00 N, Max Static f 49.00 N. Now drag Mass to 20 kg: the pair doubles in lockstep to 196.00 N and 98.00 N, and f_s,max/N holds at 0.50 both times. The footprint never changed. Which variable did the friction actually listen to?
Same block, new surface: hit the Ice/Ice preset — Max Static f collapses to 9.80 N and a feather push (10 N) breaks the block loose. Then hit Rubber/Concrete: the ceiling jumps to 78.40 N. Eight times the grip with the same block, same weight, same area — only the material pair moved.

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Your prediction

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Answer: It doubles to 98.00 N — friction tracks the normal force N = mg exactly (98.00 → 196.00 N on the panel), and contact area never enters the model: every readout in this lab obeys f = μN

On a horizontal surface the normal force is the weight: N = mg = 10×9.8 = 98.00 N at 10 kg, and f_s,max = μ_sN = 0.5×98.00 = 49.00 N — exactly what the panel shows. Doubling the mass doubles the pressing force: N = 20×9.8 = 196.00 N and f_s,max = 0.5×196.00 = 98.00 N, a perfect doubling you can watch on two readouts at once while the ratio f_s,max/N stays pinned at μ_s = 0.50. The Coulomb model f = μN contains two variables — the material pair and the normal force — and area is not one of them, which is why this lab deliberately has no area dial. Option B is the targeted misconception, the wide-tires instinct: documented in PER interviews where students predicted a block dragged on its wide side 'would experience greater friction' and were contradicted by observation (Corpuz 2006), and flagged by OpenStax as 'a somewhat counterintuitive notion'. Option C smuggles in a half-true microscopic story — pressing harder does grow the TRUE contact area between surface high-spots — but that is precisely the mechanism that makes friction proportional to N, not to the apparent footprint; it gives the full doubling, not a slight rise.

Quiz (0/3)

A 5kg block on wood has μ_s = 0.5. What is the maximum static friction force?

Why does adding weight to the block change the friction force but not the coefficient?

Experimentally, why is μ_s > μ_k for most material pairs?

You can now

Distinguish static friction (reactive: matches the applied force exactly up to f_s,max = μ_sN, then releases) from kinetic friction (constant f_k = μ_kN once sliding), and read the transition on the sim's state machine — STATIC (blue) to SLIDING (orange) badge, friction readout dropping 49.00 → 29.40 N at the default surface
Compute breakaway force and sliding acceleration from the Coulomb model on a horizontal surface: N = mg with g = 9.8 m/s² (49.00 N breakaway at the 10 kg / μ_s = 0.5 default surface), then a = (F_applied − μ_kN)/m (3.06 m/s² at 60 N applied)
Show experimentally that friction tracks the normal force — dragging Mass 10 → 20 kg doubles both dNormal (98.00 → 196.00 N) and Max Static f (49.00 → 98.00 N) in lockstep with f/N pinned at μ_s = 0.50 — and argue that contact area never enters the model, confronting the wide-tires intuition with evidence