A uniform spherical planet has radius R. The gravitational potential at its surface is −Φ.
P is a point on a line that passes through the centre of the planet, at variable displacement x from the centre, as shown in Figure 1.1.
Figure 1.2 shows the variation with x of the gravitational potential ϕ due to the planet in the region outside the planet close to its surface.
Explain why ϕ is negative for all values of x.