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Geology 2112 – Principles and Applications of Geophysical Methods WEEK 5
4
Newton’s argument for the reduction in gravity near the equator, which he had proven was further from the centre of the earth, relied on his Law of Universal Gravitation:
Where F is the force of gravity, m1 and m2 are the masses of two objects that are attracting each other, r is the distance between them, and G is the universal gravitational constant:
G = 6.67 x 10‐11 m3/kg∙s2
• G is determined empirically
• With a fairly straight forward approach, we can get a formula for the acceleration due to gravity at the earth’s surface:
• Note that the negative sign for the acceleration term just means that it is directed toward the centre of the earth, whereas the radius is measured outward (positive)
• g is approximately equal to 9.81 m/s2 at the earth’s surface
The basis for all (most) gravity studies, both in determining the exact shape of the earth and in finding things underground, is the change in the value of g from place to place on the earth’s surface
• We often use the c.g.s. unit for g (cm/s2)because it is a bit more convenient to handle the small variations in its value
• The unit called a ‘Gal’ is = 1 cm/s2, named after Galileo
• Often milligal (mgal) is the most convenient unit for gravity studies
We can now estimate a few more quantities of the earth:
• Substitute our known values into that and we get 5.97 x 1024 kg for the mass of the earth
• We also know the average radius, so we can get the volume, and with the mass and volume we can get the density
• The average density of the earth – determined using no info that Newton didn’t have – is approximately 5515 kg/m3
• Newton and others might have noticed that the average density of bedrock near the surface is more like 2500 kg/m3, water is 1000 kg/m3, etc.
• They would have then realized that the density (and possibly the composition) of the earth must be much greater the deeper one goes
• Figuring out how the earth changes with depth has kept geophysicists busy for a few hundred years!
Geology 2112 – Principles and Applications of Geophysical Methods WEEK 5
5