CAIE A-Level Physics Choosing Formulae with Physical Reasoning

Practise deciding whether a supplied equation fits the physical situation, checking variables and units, rearranging accurately, and interpreting the calculated value in context.

Exam points

select an appropriate supplied formula for the physical situation and identify each variable
rearrange a supplied formula and convert units before substitution
interpret the physical meaning and units of the calculated result rather than substituting mechanically

[Maximum number: 2]

The drag force $F_{\mathrm{D}}$ acting on a sphere falling through a liquid is given by

F_{\mathrm{D}}=6 \pi \eta r v

where r is the radius of the sphere,
v is the speed of the sphere in the liquid and
$\eta$ is a property of the liquid called the viscosity.

The sphere has a radius of 3.0 cm and is falling vertically downwards at a terminal velocity of $2.0 \mathrm{~ms}^{-1}$ through the liquid. The drag force acting on the sphere is 0.096 N .

Calculate the viscosity of the liquid.
viscosity = $\mathrm{kg} \mathrm{m}^{-1} \mathrm{~s}^{-1}$

viscosity $=0.096 /(6 \times \pi \times 0.03 \times 2.0)$

C1

$=0.085 \mathrm{~kg} \mathrm{~m}^{-1} \mathrm{~s}^{-1}$

A1

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D.4-Choosing formulae without losing physical reasoning question 1