Assume that the Earth is a uniform conducting sphere of mass $5.98 \times 10^{24} \mathrm{~kg}$. The surface of the Earth carries a charge of $-4.80 \times 10^{5} \mathrm{C}$ that is evenly distributed.

Define gravitational field.

A spherical planet can be considered as a point mass at its centre.

The Earth has a mass of $5.98 \times 10^{24} \mathrm{~kg}$ and a radius of $6.37 \times 10^{6} \mathrm{~m}$.

The Moon has a mass of $7.35 \times 10^{22} \mathrm{~kg}$ and a radius of $1.74 \times 10^{6} \mathrm{~m}$.
The Earth and the Moon can both be considered as point masses at their centres. Their centres are a distance of $3.84 \times 10^{8} \mathrm{~m}$ apart.

The Earth has a mass of $5.98 \times 10^{24} \mathrm{~kg}$ and a radius of $6.37 \times 10^{6} \mathrm{~m}$.

The Moon has a mass of $7.35 \times 10^{22} \mathrm{~kg}$ and a radius of $1.74 \times 10^{6} \mathrm{~m}$.
The Earth and the Moon can both be considered as point masses at their centres. Their centres are a distance of $3.84 \times 10^{8} \mathrm{~m}$ apart.

The point P in Fig. 1.1 represents a point mass.
On Fig. 1.1, draw lines to represent the gravitational field around P.
- P

Fig. 1.1

State what is meant by a field of force.

Gravitational fields and electric fields are two examples of fields of force. State one similarity and one difference between these two fields of force.
similarity:
difference:

The Earth E and the Moon M can both be considered as isolated point masses at their centres. The mass of the Earth is $5.98 \times 10^{24} \mathrm{~kg}$ and the mass of the Moon is $7.35 \times 10^{22} \mathrm{~kg}$. The Earth and the Moon are separated by a distance of $3.84 \times 10^{8} \mathrm{~m}$, as shown in Fig. 2.1.

P is a point, on the line joining the centres of E and M, where the resultant gravitational field strength is zero. Point P is at a distance x from the centre of the Earth.