Frame of reference
In physics, a frame of reference (or reference frame) is the coordinate system from which the positions of objects are described.[1] All speeds and velocities are measured relative to some frame of reference.[2]
Everyday examples
A car's motion is measured relative to its starting point or the road, a thrown object's motion relative to the surface it was launched from, and a planet's orbit relative to the star it goes round.[3] In most everyday problems, the frame of reference is the Earth.[4]
The same object can have different velocities in different frames. Someone riding in a train moving at 10 metres per second east is moving at that speed relative to the ground. If another train passes at 15 metres per second east, the first rider's velocity relative to the second train is 5 metres per second west.[5]
Inertial frames
An inertial frame of reference is one in which an object at rest stays at rest, and a moving object keeps moving at a constant speed in a straight line, unless an outside force acts on it. The laws of physics seem simplest in inertial frames.[6] For example, in a plane flying at a steady height and speed, physics seems to work exactly as it does for someone standing on the ground.[7]
Frames of reference are central to Albert Einstein's theory of special relativity. Its first postulate states that the laws of physics are the same, and can be written in their simplest form, in all inertial frames of reference.[8][9]
See also
References
- ↑ "2.1 Relative Motion, Distance, and Displacement". Physics. OpenStax. Retrieved 20 October 2026.
The reference frame is the coordinate system from which the positions of objects are described.
- ↑ "28.1 Einstein's Postulates". College Physics 2e. OpenStax. Retrieved 20 October 2026.
All velocities are measured relative to some frame of reference.
- ↑ "28.1 Einstein's Postulates". College Physics 2e. OpenStax. Retrieved 20 October 2026.
a car's motion is measured relative to its starting point or the road it is moving over, a projectile's motion is measured relative to the surface it was launched from, and a planet's orbit is measured relative to the star it is orbiting around.
- ↑ "4.5 Relative Motion in One and Two Dimensions". University Physics Volume 1. OpenStax. Retrieved 20 October 2026.
When we say an object has a certain velocity, we must state it has a velocity with respect to a given reference frame. In most examples we have examined so far, this reference frame has been Earth.
- ↑ "4.5 Relative Motion in One and Two Dimensions". University Physics Volume 1. OpenStax. Retrieved 20 October 2026.
If you're riding in a train moving at 10 m/s east, this velocity is measured relative to the ground on which you're traveling. However, if another train passes you at 15 m/s east, your velocity relative to this other train is different from your velocity relative to the ground. Your velocity relative to the other train is 5 m/s west.
- ↑ "28.1 Einstein's Postulates". College Physics 2e. OpenStax. Retrieved 20 October 2026.
An inertial frame of reference is a reference frame in which a body at rest remains at rest and a body in motion moves at a constant speed in a straight line unless acted on by an outside force. The laws of physics seem to be simplest in inertial frames.
- ↑ "28.1 Einstein's Postulates". College Physics 2e. OpenStax. Retrieved 20 October 2026.
when you are in a plane flying at a constant altitude and speed, physics seems to work exactly the same as if you were standing on the surface of the Earth.
- ↑ "28.1 Einstein's Postulates". College Physics 2e. OpenStax. Retrieved 20 October 2026.
The first postulate upon which Einstein based the theory of special relativity relates to reference frames.
- ↑ "28.1 Einstein's Postulates". College Physics 2e. OpenStax. Retrieved 20 October 2026.
The laws of physics are the same and can be stated in their simplest form in all inertial frames of reference.