Gravity

About the gravity

According to Newton's law of gravity, all masses in the universe attract each other with the so-called gravitational force. In this context, gravity means the (vector) sum of the gravitational force and the centrifugal force caused by the Earth's rotation.

However, when we study the Earth's gravitational field in geodesy, the unit of force (Newton) is not used, but the unit of acceleration. This is motivated by the fact that we are interested in the force that affects a unit mass, which according to Newton's second law (force = mass * acceleration) is equal to the acceleration. According to this convention, we thus treat gravity g as an acceleration with the unit m / s². However, i gravity measurement, the older unit Gal, named after Galileo Galilei (1564–1642), is still commonly used. (1 Gal = 1 cm/s².)

Gravity variation

Gravity (g) is not constant but varies on the Earth's surface; partly with the situation, partly with time. A g-value gets higher the closer the Earth's middle one gets. Then the g-value also depends on what is below the Earth's surface at the place you are measuring, i.e. bedrock, gas and oil deposits and more, as well as on some other less significant factors such as terrain around. The distance from the center of the Earth is the most decisive for the force of gravity, which means that the lowest g-values on the Earth's surface occur at the equator, on the highest mountain there (the Andes or Kilimanjaro). This is because the Earth is flattened at the poles, which means that you get further from the center of the Earth the further from the pole you are.

For example, gravity at a point in Smygehuk is measured at 9,815 220 m / s² and on Treriksröset to 9,823,944 m / s². This difference in gravity is composed of the following effects:

• An increase of g northwards due to reduced centrifugal force.
• An increase of g northwards due to a shorter distance to the center of the Earth (due to the flattening of the Earth at the poles).
• A decrease of g as the height above sea level increases (= the sum of the decrease due to increased distance to the Earth center and the increase due to more soil mass below the surface).
• An increase or decrease of g due to different densities of the bedrock under the points.

The change over time is partly due to the gravity from the moon and the sun, partly due to the movements of the Earth's crust (for example the land uplift).

A calculation example that describes gravity

As the acceleration of gravity decreases to the south, your weight is less in Smygehuk than at Treriksröset ("unfortunately" it is only your weight and not your weight which decreases as you go south). Let's formulate the question like this:

How much can you, weighing 70 kg, gain mass on a trip from Treriksröset to Smygehuk without your weight increasing (without the bathroom scale you usually carry with you showing an increased weight)?
Weight (Treriksröset) = 70 kg * 9,824 m / s² = 687.68 N
gives weight (Smygehuk) = 687.68 N / 9,815 m / s² = 70,064 kg
Result: You can revel so you increase 64 grams on the trip.
From the North Pole to the Equator, the corresponding effect is 423 grams.