5.2.1 Gravity

Gravity—the only long-range force we will encounter in the next few chapters—keeps you in your chair and the planets in their orbits around the sun. We’ll have a thorough look at gravity in Chapter 13. For now we’ll concentrate on objects on or near the surface of the earth (or other planet).

The pull of a planet on an object on or near the surface is called the gravitational force. The agent for the gravitational force is the entire planet. Gravity acts on all objects, whether moving or at rest. The symbol for gravitational force is F⃗G. The gravitational force vector always points vertically downward, as shown in FIGURE 5.3.

FIGURE 5.3 Gravity.

The figure illustrates the action of gravitational force on a block.
FIGURE 5.3 Full Alternative Text

NOTE   We often refer to “the weight” of an object. For an object at rest on the surface of a planet, its weight is simply the magnitude FG  of the gravitational force. However, weight and gravitational force are not the same thing, nor is weight the same as mass. We will briefly examine mass later in the chapter, and we’ll explore the rather subtle connections among gravity, weight, and mass in Chapter 6.