CAIE A-Level Physics 13.2.1 Uniform Sphere Point-Mass Model

Practise explaining why a uniform spherical mass can be treated as an equal point mass at its centre when considering exterior points.

Exam points

explain why the mass of a uniform sphere outside the sphere may be treated as a point mass at its centre

[Maximum number: 1]

The nearest star to the Sun is Proxima Centauri.

This star has a mass of $2.5 \times 10^{29} \mathrm{~kg}$ and is a distance of $4.0 \times 10^{13} \mathrm{~km}$ from the Sun. The Sun has a mass of $2.0 \times 10^{30} \mathrm{~kg}$ .

State why Proxima Centauri may be assumed to be a point mass when viewed from the Sun.

radius/diameter/size (of Proxima Centauri) << /is much less than $4.0 \times 10^{13} \mathrm{~km} /$ separation (of Sun and star)
or
(because) it is a uniform sphere

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13.2.1—That, for a point outside a uniform sphere, the mass of the sphere may be question 1