IB Physics HL A 2 10 Viscous Drag Questions

Practise IB Physics HL A.2.10 by analysing viscous drag, force balance and quantitative terminal-speed models.

Exam points

Apply Stokes’ law for viscous drag on a small sphere, Fd = 6πηrv, with drag opposite the motion.
Relate viscosity, sphere radius and speed to drag or terminal motion using complete mark-scheme evidence and correct units.

[Maximum number: 4]

A solid sphere is released from rest below the surface of a fluid and begins to fall.

(a)

Explain why the sphere will reach a terminal speed.

[ 2 ]

(b)

The weight of the sphere is 6.16 mN and the radius is $5.00 \times 10^{-3} \mathrm{~m}$ . For a fluid of density $8.50 \times 10^{2} \mathrm{~kg} \mathrm{~m}^{-3}$ , the terminal speed is found to be $0.280 \mathrm{~ms}^{-1}$ . Calculate the viscosity of the fluid.

[ 2 ]

any evidence (numerical or algebraic) of a realisation that
$\begin{aligned}6\pi\eta rv+\rho gV&=W \\ \eta&=\frac{6.16\times10^{-3}-4.366\times10^{-3}}{6\pi\times0.005\times0.280} \\ &=0.0680\ \mathrm{Pa\,s}\end{aligned}$

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IB Physics HL A 2 10 Viscous Drag Questions question 1

Question (a)

Question (b)