Explain what is meant by a field in physics, and describe how field lines are used to represent both the direction and strength of a gravitational field around a planet.

A field is a region of space in which a force acts on an object that has the relevant property (e.g. mass for gravity). Field lines are drawn so that a test mass placed on a line would feel a force in the direction of the line. For a gravitational field around a planet the lines point radially inward towards the planet’s centre. The density of the lines represents the field strength – lines are close together where the field is strong (near the surface) and become more widely spaced as the distance increases, showing the field weakens with distance.