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Hence, with respect to an inertial frame, an object or body accelerates only when a physical force is applied, and (following Newton's first law of motion), in the absence of a net force, a body at rest will remain at rest and a body in motion will continue to move uniformly—that is, in a straight line and at constant speed.
The center of momentum frame is defined as the inertial frame in which the sum of the linear momenta of all particles is equal to 0. Let S denote the laboratory reference system and S′ denote the center-of-momentum reference frame. Using a Galilean transformation, the particle velocity in S′ is
In case of land vehicles like cars, tanks etc., which use the ENU-system (East-North-Up) as external reference (World frame), the vehicle's (body's) positive y- or pitch axis always points to its left, and the positive z- or yaw axis always points up. World frame's origin is fixed at the center of gravity of the vehicle. [3]
A rotating frame of reference is a special case of a non-inertial reference frame that is rotating relative to an inertial reference frame. An everyday example of a rotating reference frame is the surface of the Earth. (This article considers only frames rotating about a fixed axis. For more general rotations, see Euler angles.)
This special frame where this occurs is also called the center of momentum frame, and is defined as the inertial frame in which the center of mass of the object is at rest (another way of stating this is that it is the frame in which the momenta of the system's parts add to zero). For compound objects (made of many smaller objects, some of ...
An observational frame (such as an inertial frame or non-inertial frame of reference) is a physical concept related to state of motion. A coordinate system is a mathematical concept, amounting to a choice of language used to describe observations. [3]
The Earth frame is also useful in that, under certain assumptions, it can be approximated as inertial. Additionally, one force acting on the aircraft, weight, is fixed in the +z E direction. The body frame is often of interest because the origin and the axes remain fixed relative to the aircraft.
The perifocal coordinate system may also be used as an inertial frame of reference because the axes do not rotate relative to the fixed stars. This allows the inertia of any orbital bodies within this frame of reference to be calculated. This is useful when attempting to solve problems like the two-body problem. [6]