# How are rest and movement relative terms?

## Movement is relative

RELATIVITY PRINCIPLE.INFO

The principle of relativity not only says that there is no center of the universe, it also says that movements are relative. In this context, relative means that the movement of an object is always relative to another object or a System of objects can be measured. Within a system, all known physical processes depend only on the movements of the objects in relation to one another, but not on the movement of the system in relation to external objects. If you want to estimate the effects of a car accident, the relative speed of the vehicles is decisive for the severity of the damage. On the other hand, the speed relative to an outside observer or the roadway is not so important.

The principle of relativity can easily be checked in everyday life. It is just as easy to throw a ball forward as it is backward in one evenly fast move. Although in the first case it becomes faster relative to the earth, in the second case it becomes slower. The principle of relativity goes beyond this purely mechanical example. All physical laws should only depend on the relative movements of the objects involved. It is therefore not possible to install a device in a train that can measure the speed of the train without reference to the outside world. All physical laws are identical in a moving train to those in a stationary one.

While speeds are relative, changes in speed are not. Acceleration or braking can easily be measured on the basis of the inertial forces that occur. To do this, it is not necessary to use information from the outside world. If you are in an accelerating vehicle, inertial forces inevitably arise from which the change in speed can be calculated. Rotations also generate inertial forces that are responsible, for example, for the directions of rotation of cyclones on the rotating earth. So speed is relative, while acceleration and rotation are absolute.

On the next page I will briefly outline the history of the principle of relativity.

Last change: 06/22/2011