Earlier, we discussed the fact that field forces are forces that act at a distance. It can easily be demonstrated that the gravitational force is a field force. For example, when any object is held above the Earth's surface, the Earth's gravitational field will exert an attractive force on the object regardless of the medium that separates the Earth and the object. According to the field concept, any object of mass m creates a gravitational field around itself. As the mass of the object increases, the gravitational field around the object will also increase. Any other object placed in that field will then feel a force of gravitational attraction. The gravitational field is a vector quantity that is always directed towards the center of the mass of field's origin. For example, let us look at the Earth. Imagine the Earth as an isolated mass. If an object is brought near the Earth, a force of attraction is exerted on it. The direction of the force points radially in towards the center of the Earth, and the magnitude of the force is mg, where g is called the acceleration due to gravity. So, every point in the space around the Earth, has a corresponding vector g, which is the acceleration that a body would experience if it were located at that point. This vector g is called the gravitational field at the particular point. The gravitational field is defined as force per unit mass at any point outside a massive object, or g is equal to Force divided by mass. For example, a 1 kg object near the surface of the Earth will experience a gravitational force of 9.8 N while a 2 kg object will experience a gravitational force of 19.6 N. In other words, the gravitational field at a given point in space tells us the force an object will experience per kilogram of its mass. You should note that the units force per mass simplify to give the unit of acceleration. Newton's Universal Law of Gravitation provides us with an equation to calculate the force of attraction between two objects. Thus, we can apply this equation to a planet, let us call the planet m1, and an object near the planet, let us call this object m2. It is very important to note that this gravitational force of attraction between the planet and the object is simply the object's weight. Recall that an object's weight is the gravitational force that acts on it and equals mg. Setting these two equations equal to one another allows us to derive an equation for the acceleration of gravity at a given point away from a planet. Note that r must be measured from the center of the planet and that g is a vector quantity that is always directed toward the center of the Earth. Also note that the strength of the gravitational field is independent of the mass that is placed at that location. Let us see an example. Find the value of g that the Moon experiences due to the Earth. ...

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