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Reference frames
Beginner
The coordinate system and clock from which motion is described.
A reference frame is a coordinate system with an origin, axes and a clock. Positions and velocities depend on the frame, but the physical laws (in inertial frames) do not. Choosing a convenient frame, such as one with the x-axis along the slope, can simplify a problem a lot.
For a ball thrown in a moving train, the train frame sees a simple vertical throw while the ground frame sees a parabola.
Mixing coordinates from two frames in one equation is a classic source of sign errors.
In more detail
A reference frame is a coordinate system with an origin and a clock used to describe positions and times. Choosing a convenient frame often makes a problem much simpler.
Real-world uses
- Air traffic control uses a frame fixed to the airport for aircraft positions.
- Astronomers use a frame centred on the Sun for planetary positions.
- Pilots use a runway-fixed frame for landing, while navigators on a ship use a frame fixed to the water when plotting a course.
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Frequently asked questions
Is the ground always a good frame?
For most everyday problems, yes. For a rotating planet or accelerating car, a different frame may be better.
What is an inertial frame?
A frame that is not accelerating, in which Newton's first law holds without extra fictitious forces.
How do I choose a good reference frame?
Pick a frame where the problem is simplest, usually one attached to a fixed object or to the moving body that you are tracking.
Related topics in Kinematics
Rest and motion · Distance vs displacement · Speed vs velocity · Average velocity