Any form of energy can be transformed into another form, but the total energy always remains the same. This principle, the conservation of energy, was first postulated in the early 19th century, and applies to any isolated system. Although the total energy of a system does not change with time, its value may depend on the frame of reference. For example, a seated passenger in a moving airplane has zero kinetic energy relative to the airplane, but non-zero kinetic energy relative to the Earth. The terms gravitation and gravity are mostly interchangeable in everyday use, however a distinction is made in scientific circles. Gravitation is a general term describing the phenomenon by which bodies with mass are attracted to one another, while gravity refers specifically to the force exerted by the Earth on objects in its vicinity, which depends on its mass as well as other factors, such as Earth's rotation. Modern physics describes gravitation using the general theory of relativity, in which gravitation is a consequence of the curvature of space-time which governs the motion of inertial objects.
Lecture Slides are screen-captured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.