In classical physics, kinetic energy is a concept in which the amount of energy due to motion is calculated based upon the mass and velocity of an object. For example, the mass of water is known and its velocity can be measured with precision relative to the ground it is flowing over. Similarly its velocity can also be measured as it flows inside a pipe or conduit. This allows engineers to estimate the amount of energy produced by a hydro-electric project. However classical models only work when an observer can choose an arbitrary point of reference such as a point inside a pipe that the water is flowing past. When no such frame of reference exists, it is impossible to measure velocity, and therefore impossible to determine the object's kinetic energy. For example, consider two identical objects which appear to be approaching one another in open space. Although it is apparent that both objects are on a collision course, it is impossible to know which one is moving. Depending upon the point of view of the observer, either one could be at rest. Therefore it is possible that one of them may have no kinetic energy at all.
Adding the Earth into the above example would help define the movement of the objects relative to Earth, but we are still no closer to determining the absolute speed of either object. That is because the Earth itself is moving through space relative to the Sun and the Sun is moving around the center of our Galaxy and so on into infinity. Problems like this gave rise to Relativity Theory. Using a relativistic approach, velocity is seen as the rate of change in the distance between an object and a given coordinate, which often corresponds to another object. The Earth need not be any part of the calculation.