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

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: human reaction time (if timing by hand) can be large; reduce this with video analysis.
• Systematic: include buoyancy by using $\rho_\text{sphere}-\rho_\text{oil}$ rather than $\rho_\text{sphere}$ alone.
• Wall effect: use a sufficiently wide cylinder and release the sphere centrally to reduce wall interactions.
• Temperature drift: viscosity changes with temperature; record temperature and keep it stable during runs.

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. $\eta = \frac{2(2.5 \times 10^{-3})^2 (1380-920) \space 9.81}{9 \times 0.0694}=6.9 \times 10^{-2} \space \pu{Pa.s}$

Compare your result against a reference value for the specific oil and temperature you used (manufacturer data can differ from generic “literature values”).

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?
If you’re doing density/viscosity work, re-weigh the sphere after the run (or keep runs short) and note any mass change in your evaluation.

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. Measuring Rolling Friction with Plasticine | The Physics Teacher (AIP)
2. H2 Physics Syllabus 9478 | SEAB
3. O-Level Physics Syllabus 6091 | SEAB
4. Air-Track Inelastic Collisions Demo 24.12 | UCSB Physics

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

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