Newton’s Law of Gravitation

hurricanes in the northern hemisphere to rotate in the counterclockwise direction, while in the southern hemisphere, they rotate in a clockwise direction.

NEWTON’S LAW OF GRAVITATION

Gravitational force affects all things on or near the earth, including the Moon, which is a distance away from the earth. The force of gravity supplies the necessary centripetal force to allow for the rotation of the moon. These are the identical forces that cause the earth to rotate around the sun and for galaxies to rotate around a specific point. You should know that it is the weakest of the four basic forces found in nature (such as electromagnetic forces, weak nuclear forces, and strong nuclear forces). Unlike other forces, it is always attractive. It is along a line that joins the centers of mass of two objects. Figure 27 shows gravitational forces:

Figure 27.

The magnitude of force is equal in each direction, regardless of the mass of the object because of Newton’s third law. If the forces were different, the moon would leave the earth’s orbit or would crash into the earth. The center of mass is used in equations regarding this because these bodies tend to be quite large.

This brings us to Newton’s Universal Laws of Gravitation, which states that force equals the gravitational constant multiplied by the value of both masses, divided by the radius (the distance between the two masses) squared. Figure 28 shows this equation:

Figure 28.

The gravitational constant is believed to be the same throughout the universe and has been measured to be .7 x 10-11. This is a really small force that means that gravitational forces only really apply and are only really important in larger objects like planetary objects. A person’s weight on earth is dependent upon the mass of the earth, which is 6 x 1024 kilogram. The moon’s mass is just about 1 percent of the earth’s mass.

You should remember that people do not weigh the same at different points in the earth. The presence of mineral deposits or ice sheets beneath the earth and the presence of mountains will affect a person’s weight. The earth is also not completely round so it is thicker at the equator than it is at the poles.

Ocean tides are one observable effect of the moon’s gravity acting on the earth. Because water easily flows on the earth’s surface, a high tide is created on the side of the earth nearest to the moon, where the moon’s gravitational pull is the strongest. There are tides on the earth’s surface opposite to the moon because the earth is pulled toward the moon more than the water on its far side because earth’s center is closer to the moon than the water on the far side of the moon.

As the earth rotates, the tidal bulge keeps its orientation with the moon, leading to two tides per day (about 12 hours and 25.2 minutes every day). So, water is attracted to the moon more than earth on the near side of the moon, while the earth is attracted to the moon more than water on the far side of the moon. There is a tidal factor based on the sun but it is half the affect of the moon, depending on the angles of the earth, moon, and sun together. This leads to spring tides and neap tides with spring tides being highest and neap tides being lowest.