IP Physics Plasticine: 7 Experiments that Turn Soft Clay into Hard-Won Marks

TL;DR
Plasticine's super-power is that it can be reshaped in seconds, sticks to things on impact and sinks slowly in syrup. Those three traits unlock at least seven physics ideas—from density checks to perfectly inelastic collisions—that frequently appear in IP weighted assessments and H2 practical papers. This article lays out the exact mini-labs, common slip-ups and a 5-day micro-practice plan so you can turn a $2 block of clay into solid grades.

Exam-Scope Disclaimer

Stokes' Law is not listed as required knowledge in the current IP Physics or H2 Physics syllabuses. Examiners may, however, supply unfamiliar relationships (including Stokes'drag) in an open-ended design or data-handling question and ask you to derive or apply them using provided graphs or first-principles ideas. Memorising the Stokes'Law equation is therefore optional — keep sub-section 2.2 as an enrichment task, or skip it entirely if you prefer to focus strictly on examinable content.

1 Why every H2 lab issues a lump of plasticine

• Malleable & re-usable - one block can be rolled into spheres, flattened into pucks or packed onto carts in seconds.
• Safe & classroom-friendly - no shards, no bounce, no toxic dust.
• Density \(\pu{\approx 1.4 g.cm-3}\)—heavy enough to sink in water yet light enough to reach terminal velocity in a 1 m-tall syrup column.
• Sticky on impact - guarantees a perfectly inelastic collision every time, a requirement in many momentum tasks.

IP exam setters exploit those virtues because they let students focus on data handling and uncertainty, not tricky apparatus.

2 Seven physics ideas a clay lump can prove

2.1 Density without Archimedes

Roll five different-mass spheres, measure mass (digital balance) and diameter (vernier calliper). A log-log mass-vs-diameter³ plot gives the density as the gradient. Students spot linearisation and propagate percentage uncertainties in one go.

2.2 Stokes' law in a jam jar

Drop the spheres through glycerine or honey, time the last 10 cm fall. Plot radius² against terminal velocity; gradient yields the fluid viscosity via
\(v_\text{t}=\frac{2}{9}\frac{(\rho_\text{sphere}-\rho_\text{fluid})g}{\eta}r^{2}\).
A single kitchen jar turns into an H2-level viscometer.

2.3 Perfectly inelastic collisions on a bench-top

Load a rolling dynamics cart with a motion sensor, fire a plasticine bullet from a spring launcher. The bullet sticks; momentum is conserved but kinetic energy drops—students calculate the lost KE and discuss energy pathways.

2.4 Centre of mass & counter-intuitive balance

Shift tiny lumps along a ruler until it balances on a pencil; plot distance-of-lump vs centre-of-mass position to verify the lever rule. A neat visual for stability questions.

2.5 Elastic vs plastic deformation

Clamp a strip of plasticine and hang masses; strain rises but never returns—an instant demo of plastic region beyond Hooke's law. Compare to a steel spring on the same rig.

2.6 Rolling friction & energy dissipation

Roll an iron ball into a plasticine target on a smooth track; measure the embed depth to estimate the work done by rolling friction. AAPT's experiment shows how multiple concepts intertwine.

2.7 Pressure imprint mapping

Press a loaded test-tube onto a plasticine pad; the contact area reveals pressure distribution—great for qualitative questions on pressure = force/area.

3 Sample WA-style design-question (write-up template)

Prompt
“Design an experiment using plasticine to determine the dynamic viscosity of cooking oil.”

Variable legend

• Independent variable (IV): sphere radius — the value you deliberately change.
• Dependent variable (DV): terminal velocity — the outcome you measure.
• Control variable (CV): oil temperature — kept constant so it does not skew results.

Setup

Teachers can mark against H2 practical rubrics: MMO, PDO, ACE.

Rationale & theory

At low Reynolds number (\(Re<1\)) the net force on a falling sphere equals weight minus buoyancy minus Stokes drag, giving

\[ \eta = \frac{2 r^2 (\rho_\text{sphere}-\rho_\text{oil})g}{9v_\text{t}} \]

Stokes' law is valid only when the flow is laminar and the sphere radius is small compared with the tube diameter.

Equipment list (suggested minima)

• \(500 \space \pu{mL}\) graduated measuring cylinder (\(\geq \pu{3 cm}\) inner diameter).
• \(3-5\) plasticine spheres, radii \(\pu{3 mm - 7 mm}\), measured with a vernier calliper.
• Electronic balance \(\pu{\pm 0.01 g}\).
• Cooking oil (\(\approx \pu{0.065 Pa s}\) at \(\pu{25 ^\circ C}\)).
• Digital thermometer \(\pu{\pm 0.1 ^\circ C}\) clipped midway down the column.
• Stopwatch capable of \(\pu{0.01 s}\) or phone camera \(120\) fps for video timing.

Step-by-step method (justified)

1. Calibrate & condition: Warm the oil bath to \(\pu{25 \pm 0.5 ^\circ C}\) and stir gently to ensure uniform temperature (controls viscosity).
2. Measure sphere mass & diameter three times each; compute mean radius and density to \(\pu{\pm 1 \%}\).
3. Release protocol: Hold the sphere with tweezers at centreline to avoid wall effects, then let go without imparting spin.
4. Timing window: Start timer 5 cm below the oil surface (allows acceleration phase to finish) and stop 25 cm lower. Distance is marked with masking tape for consistency.
5. Repeat for each radius thrice; discard trials where the sphere touches the wall or creates visible wake (possible \(Re>1\)).
6. Clean-up: Retrieve spheres with perforated spoon; wash glassware with detergent, dry, store. Follow glassware safety to prevent breakage.

Uncertainty & error analysis

• Random: reaction-time error (~0.2 s) divides by long timing distance, so percent uncertainty \(\pu{\approx 0.2 s / 4 s \cong 5 \%}\). Reduce by video analysis.
• Systematic: ignoring buoyancy term overestimates \(\eta\) by \(\approx 10 \%\). Always subtract \(\rho_\text{oil}\) from \(\rho_\text{sphere}\).
• Wall effect: keep cylinder diameter ≥ 10 x sphere diameter to keep correction \(< 1 \%\).
• Temperature drift: viscosity of vegetable oils changes \(\approx 2-3 \% \space \pu{K-1}\); record bath temperature every two trials.

Apply propagation of uncertainty formally (GUM) when reporting \(\eta\).

Typical data & sample calculation

For a \(\pu{5.00 mm}\) radius sphere (mass = \(\pu{0.52 g}\)) falling \(\pu{0.25 m}\) in \(\pu{3.60 s}\) at \(\pu{25 ^\circ C}\):

1. \( v_\text{t} = 0.25/3.60 = 0.0694 \space \pu{m.s-1} \)
2. \( \rho_\text{sphere} = 0.52 / \left[ \frac{4}{3}\pi (2.5\times10^{-3})^3 \right]=1380 \space \pu{kg.m-3} \)
3. \( \eta = \frac{2(2.5 \times 10^{-3})^2 (1380-920) \space 9.81}{9 \times 0.0694}=6.9 \times 10^{-2} \space \pu{Pa.s} \)

Literature viscosity for canola oil at \(\pu{25 ^\circ C ≈ 0.067 Pa.s}\) — within \(4 \%\) of accepted value.

Validity checks & improvement ideas

• Check linearity: plot \(v\) against \(r^2\); \(R^2>0.99\) confirms Stokes regime.
• Lower Re further: use smaller spheres or colder oil if curvature appears.
• Automate timing with Light-Gate + Data-logger to cut human reaction error to < \(1 \%\).
• Compare liquids: run the same spheres in water and glycerine to highlight viscosity contrast and reinforce concept transfer.

4 Common mistakes & lightning fixes

5 5-Day micro-practice sprint

Tick each box, snap a photo, post to class chat—peer accountability matters.

6 Quick FAQ

Q Why does IP love “plasticine questions”?
Because the same blob lets exam writers weave density, kinematics and mechanics into one neat package, mirroring the cross-topic flavour of A-Level practicals.

Q Won't the clay absorb oil and change mass?
Mass change over a \(\pu{30 s}\) run is \(< 0.1 \%\) — well inside typical measurement uncertainty.

Q Is Blu-Tack a valid substitute?
Blu-Tack is visco-elastic; rebound spoils perfectly inelastic assumptions. Stick to plasticine.

7 Further reading & ready-to-borrow ideas

1. Stokes' law | Wikipedia
2. Measuring Rolling Friction with Plasticine | The Physics Teacher (AIP)
3. H2 Physics Syllabus 9478 | SEAB
4. H2 Physics Practical Discussion | Reddit r/SGExams
5. Density of Plasticine Lab Report | Scribd
6. Hooke's Law and Plastic Materials | Physics Forums
7. Air-Track Inelastic Collisions Demo 24.12 | UCSB Physics
8. Momentum Conservation: Shooting Clay Balls at a Cart | YouTube - Zak's Lab
9. Centre of Gravity Balance Trick | YouTube - Science Experiments

Next step: grab \(\pu{50 g}\) of plasticine, run one experiment tonight, and log your biggest surprise.