Weight is a special application of the concept of force. It is defined as the force exerted on an object by the gravitational field of the earth, or more specifically the pull of the earth on the body.

where:

Note that g,, has the same numerical value as the acceleration of gravity at sea level.

The mass of a body is the same wherever the body is, whether on the moon or on the earth. The weight of a body, however, depends upon the local acceleration of gravity. Thus, the weight of an object is less on the moon than on the earth because the local acceleration of gravity is less on the moon than on the earth.

Example:

Calculate the weight of a person with a mass of 185 lbm. W = mg

Example:

Calculate the weight of a person with a mass of 185 lbm on the moon. Gravity on the moon is 5.36 ft/sec².

With the idea of mass and weight understood, especially their differences, the concept of gravitational force is more easily explained. Any object that is dropped will accelerate as it falls, even though it is not in physical contact with any other body. To explain this, the idea of gravitational force was developed, resulting in the concept that one body, such as the earth, exerts a force on another body, even though they are far apart. The gravitational attraction of two objects depends upon the mass of each and the distance between them. This concept is known as Newton's law of gravitation, which was introduced in an earlier chapter.