IB Physics SL A.2.10 Viscous Drag

Practise applying Stokes' law to spheres in fluids and explaining changing acceleration or terminal speed through the balance of weight, buoyancy and speed-dependent drag.

Exam points

calculate Stokes drag with 6πηrv using SI radius, viscosity and speed, directed opposite motion
set weight against buoyancy and drag at terminal speed, where resultant force and acceleration are zero
predict how radius, fluid viscosity or density changes the terminal speed of a sphere or bubble

[Maximum number: 4]

A spherical oil droplet is released from rest at the bottom of a column of water.

The graph shows the variation with time t of the vertical velocity of the oil droplet.

Figure for Question A.2.10—Viscous drag question 1 — IB Physics SL

(a)

Describe why the acceleration of the oil droplet changes.

[ 2 ]

(b)

Explain why the velocity of the oil droplet is constant for $t>3 \mathrm{~s}$ .

[ 2 ]

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A.2.10—Viscous drag question 1

Question (a)

Question (b)