**17. Forces of gravity**

In addition to the attraction force of the earth mass, there is another force which effect the rotated earth called (Centrifugal Force). This force created due to the continuous rotation of the earth around its axis. The force related to the radius of rotation and the square of the angular velocity:-

Therefore

The total gravity force = Attraction Force + Centrifugal Force (FC)

$$F\mathbf{c}\_p = r\mathbf{w}^2 \text{COS}\boldsymbol{\theta} \tag{60}$$

P = point at the earth surface. ϴ = latitude Centrifugal force *Fc p* α r ω<sup>2</sup>

$$Fcp = mra^2\tag{61}$$

Where m = mass

r = radius of rotation *ω* ¼ angular velocity

$$\therefore \text{--} \mathbf{g} = -G \left[ \frac{1}{r^2} dm + m r o^2 \right. \tag{62}$$

The Centrifugal force contribute the total gravity effect on the earth surface by about 10�<sup>7</sup> –10�<sup>8</sup> gal. The force have its maximum value at the equator of the earth and reach zero value at the poles of the earth.

The absolute gravity value at the equator is about 978 gal, while its value at poles about 983 gal, The difference in gravity value between the equator and pole is range between (5.17 gal) or (5170 mgal).
