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5.2: Gravitational Field

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  • The region around a gravitating body (by which I merely mean a mass, which will attract other masses in its vicinity) is a gravitational field. Although I have used the words “around” and “in its vicinity”, the field in fact extents to infinity. All massive bodies (and by “massive” I mean any body having the property of mass, however little) are surrounded by a gravitational field, and all of us are immersed in a gravitational field.

    If a test particle of mass \(m\) is placed in a gravitational field, it will experience a force (and, if released and subjected to no additional forces, it will accelerate). This enables us to define quantitatively what we mean by the strength of a gravitational field, which is merely the force experienced by unit mass placed in the field. I shall use the symbol \(\textbf{g}\) for the gravitational field, so that the force \(\textbf{F}\) on a mass \(m\) situated in a gravitational field \(\textbf{g}\) is

    \[\textbf{F} = m \textbf{g}. \label{5.2.1} \tag{5.2.1}\]

    It can be expressed in newtons per kilogram, \(\text{N kg}^{-1}\). If you work out the dimensions of \(g\), you will see that it has dimensions \(\text{LT}^{−2}\), so that it can be expressed equivalently in \(\text{m s}^{−2}\). Indeed, as pointed out in section 5.1, the mass \(m\) (or indeed any other mass) will accelerate at a rate \(g\) in the field, and the strength of a gravitational field is simply equal to the rate at which bodies placed in it will accelerate.

    Very often, instead of using the expression “strength of the gravitational field” I shall use just “the gravitational field” or perhaps the “field strength” or even just the “field”. Strictly speaking, the “gravitational field” means the region of space surrounding a gravitating mass rather than the field strength, but I hope that, when I am not speaking strictly, the context will make it clear.