Bearings & Friction · STEM Subject Lab

01Learning objectives

Describe rolling resistance as a small force that slows the car for the whole length of the lane.

Compare steel and hybrid ceramic bearings using friction, hardness and mass, not marketing.

Explain why isopropyl alcohol (IPA) dissolves light oils and evaporates without leaving a film, and why any film left behind raises friction.

Use a spin-down test as fair-test evidence and read a graph of angular speed against time.

02From the Nightmare car

After regionals we timed how long each wheel kept spinning after a flick. Flick the wheel, start the stopwatch, stop it when the wheel stops. With the stainless steel bearings a wheel ran for about ten seconds. The hybrid ceramic set from Boca Bearings ran for longer on the same axle in the same room. We also wiped every raceway with 99% IPA before assembly, so oil from our fingers and dust from 3D printing were not sitting in the path of the balls. Both changes do the same job at the finish line. Less energy is lost between the launch and the timing gate.

Fig. 1 A hybrid ceramic bearing has steel rings and ceramic balls. A ceramic ball is harder, lighter and smoother than a steel one, so it squashes less where it touches the ring. On the graph, the longer line means a smaller friction torque.

03Core idea

Rolling resistance is not like a brake. It is a small force at each wheel that turns some of the launch energy into heat and tiny squashes in the material instead of speed. It looks small, but it acts for the whole 20 m, so it adds up.

F_rr ≈ C_rr · N N is the load on the axle in newtons. C_rr is the rolling resistance coefficient and has no unit.

You can think of C_rr as the price of each newton of load. It goes up when the raceway is dirty, damaged, or made from a pairing of materials that rubs more. Our car is light, so N is small. That leaves C_rr as the one thing we can actually improve.

Reading a spin-down test

Flick a wheel and time it until it stops. If the friction torque τ_f stays roughly the same as the wheel slows, the angular speed drops in a straight line and the wheel stops after

t_stop = I · ω₀ / τ_f I is the moment of inertia of the wheel in kg·m². ω₀ is the starting angular speed in rad/s.

Same wheel, same flick, and a longer spin-down means a smaller τ_f. That is the fair test. Keep I and ω₀ the same and change only the bearing.

Why we clean with IPA

Like dissolves like. Isopropyl alcohol, (CH₃)₂CHOH, has a polar OH group and a non-polar carbon chain, so it dissolves the light oils from skin and the waxy dust from 3D printing. It boils at 82.6 °C and evaporates in seconds at room temperature, so nothing is left behind. Oil-based cleaners leave a layer that the balls have to shear through on every turn.

04Spin-down simulator

Pick a bearing and the state of its raceway, set how hard you flick it, then release. The wheel slows down under a friction torque and the graph records its angular speed against time. Change one thing at a time. That is what makes it a fair test.

Bearing

Raceway

Flick speed ω₀

The wheel is 8 g with a 13 mm radius, so I = ½mr² ≈ 6.8×10⁻⁷ kg·m². The friction torque has a constant part plus a small part that grows with speed. The numbers are realistic, not measured.

—sTime to stop

—µN·mFriction torque

—turnsTurns before stopping

Log your own spin-down

If you have a real wheel, enter three timed trials for each bearing. Compare the means, never a single run.

Bearing	Trial 1 (s)	Trial 2 (s)	Trial 3 (s)	Mean (s)
A, steel				—
B, ceramic				—

05Worked example

Given: one wheel carries N = 0.40 N. Estimate C_rr = 0.020 for the steel set and 0.012 for the ceramic set.

Find: the rolling resistance on each wheel, and the difference over four wheels.

Steel: F_rr = 0.020 × 0.40 = 0.0080 N per wheel, so 0.032 N for four wheels

Ceramic: F_rr = 0.012 × 0.40 = 0.0048 N per wheel, so 0.0192 N for four wheels

Difference: the ceramic set has 0.0128 N less force holding the car back.

Spin-down: I = 6.8×10⁻⁷ kg·m², ω₀ = 150 rad/s, and the wheel stops after 17 s.

τ_f = I·ω₀ / t = 6.8×10⁻⁷ × 150 / 17 = 6.0×10⁻⁶ N·m

These numbers are tiny because the car is light. They still matter. The CO₂ cartridge gives a fixed push, so every bit of friction takes speed off the car for the whole 20 m.

06Student questions Teacher mode is off. Answers are hidden.

Q1 · Explain[3]

Why does cleaning a bearing with IPA change the friction, even though the bearing material stays the same?

Answer key

Any three of: oil and dust sit between the ball and the ring; the balls have to push through that film on every turn, which raises the friction torque (a bigger C_rr); IPA dissolves light oils (like dissolves like) and lifts the dust; it then evaporates completely, so no new film is left behind.

Q2 · Calculate[3]

N = 0.55 N and C_rr = 0.015. Find F_rr on one wheel, then the total for four identical wheels.

Answer key

F_rr = 0.015 × 0.55 = 0.00825 N (one wheel)

Four wheels: 4 × 0.00825 = 0.033 N

One mark for the formula, one for the single-wheel value, one for the total with the unit.

Q3 · Decide[4]

Team A keeps its steel bearings but spends an hour cleaning them with IPA. Team B swaps to ceramic bearings but leaves dust in the races. Who is more likely to run faster? What evidence would you collect to check?

Answer key

Probably Team A. In the simulator, steel with IPA outlasts ceramic with dust. The dusty raceway multiplies the friction by 2.4, which is more than the material saves (9.0 against 6.0 µN·m). Evidence: repeated spin-down times, at least three per bearing, same wheel and same flick, then race times on the same lane. Give credit for a reasoned case for Team B if the student argues the dust is very light. Two marks for the decision with a reason, two for evidence with a fair-test control.

Q4 · Spin-down[3]

A wheel with I = 6.8×10⁻⁷ kg·m² is flicked to ω₀ = 150 rad/s and stops after 12 s. Work out the friction torque. Is this bearing better or worse than the one in the worked example?

Answer key

τ_f = I·ω₀/t = 6.8×10⁻⁷ × 150 / 12 = 8.5×10⁻⁶ N·m

Worse. The friction torque is bigger than 6.0×10⁻⁶ N·m, which is why it stopped sooner.

07Printable worksheet The answer key prints as well when teacher mode is on

Team Nightmare · STEM Subject Lab

NM-LAB-01 · Bearings & Friction

Name ______________________ Class __________ Date __________

Physics · Chemistry
40 minutes
Partner: Boca Bearings

Remember

F_rr ≈ C_rr · N

t_stop = I·ω₀ / τ_f

IPA (isopropyl alcohol) dissolves light oils and then evaporates. It should not leave a greasy film on the raceway.

From the Nightmare car

Regionals: stainless steel bearings. After that: Boca hybrid ceramic bearings, wiped with IPA before assembly. The partner is named because the part on the car is theirs.

rolling resistancenormal forcetorquedissolveevaporatefair test

[2]1.In one sentence, what does a lower C_rr mean for the car once the CO₂ cartridge has fired?

[3]2.N = 0.48 N and C_rr = 0.014. Work out F_rr for one wheel and for four wheels. Show the formula, the numbers, and the answer with a unit.

[3]3.Explain the chemistry reason we clean with IPA before fitting the Boca bearings. Use the words dissolve and evaporate.

[4]4.Spin-down log. Time each bearing three times from the same flick. Record the times, then find the mean.

Bearing	Trial 1 (s)	Trial 2 (s)	Trial 3 (s)	Mean (s)
A
B

Which bearing has the smaller friction torque? How do you know?

[2]5.Name two things that must be kept the same for Q4 to be a fair test.

Team Nightmare · Answer key

NM-LAB-01 · Bearings & Friction, teacher copy

Accept any equivalent working. The marks in brackets match the student sheet.

Teacher copy
Do not hand out

Diagram Vertical arrow: W (weight), pointing down. Accept N (normal force) if the direction is stated. Horizontal arrow: F_rr, rolling resistance, pointing backwards against the motion.

1. [2] Less of the launch energy is lost in the wheels and bearings (1), so the car keeps more speed to the finish and records a faster time (1).

2. [3] F_rr = C_rr × N = 0.014 × 0.48 = 0.00672 N (one wheel)4 × 0.00672 = 0.0269 N (four wheels) One mark each for the formula, the single-wheel value, and the total with the unit N.

3. [3] IPA dissolves oil, grease and dust on the raceway (1). It evaporates quickly and completely (1). So nothing is left to add friction and the balls roll on clean steel (1).

4. [4] Table completed with three trials each and correct means (2). The bearing with the longer mean spin-down has the smaller friction torque, because t_stop = Iω₀/τ_f and I and ω₀ were kept the same (2).

5. [2] Any two of: the same wheel (same I), the same flick speed ω₀, the same axle, the same temperature, the same person timing, the same rule for when to stop the watch.