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Physics (Year 12) - Gravity and Motion

Dev Lohar

# Gravitational Fields

A gravitational field exists around each body that has mass. It is a region in which a gravitational force is exerted on all matter within that region.

(Image sourced from Sjlegg, Public domain, via Wikimedia Commons)

A gravitational field can be represented by drawing arrows towards the direction of the gravitational force, as shown in the diagram above. The space between the red lines indicate the magnitude of the gravitational force. Closely packed lines indicate a strong gravitational field whilst widely spaced lines indicate a weak gravitational field. Parallel lines indicate a uniform or constant gravitational field. Whilst drawing gravitational fields for a uniform mass, it is important to ensure that the arrows are approximately equally distanced apart.

# Gravitational Field Strength

When answering questions, it is necessary to keep in mind that r is the distance to the centre of mass, which is taken to be at the centre of the planet, and it’s not the distance from the point of measurement to the surface. In most questions, they only provide you with the distance to the surface; e.g. a satellite is orbiting the Earth at an altitude of 100 km, find the gravitational field strength at the altitude at which the satellite is orbiting. In this case, the value of r in the equation would be equal to the distance between the satellite and the surface (100 000 metres) plus the radius of the Earth (provided in the data sheet).

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