CHAPTER 6. MOTIONIN TWO DIMENSIONS 6.5
What is aframe of reference? ESCDH
DEFINITION: Frame of Reference
A frame of reference is the point of view from which a system is ob-
served.In practical terms, a frame of reference is a set of axes (specifying directions) with an
origin. An observer canthen measure the position and motion of all points in a system,
as well as the orientation of objects in the systemrelative to the frame of reference.
There are two types ofreference frames: inertial and non-inertial. Aninertial frame
of reference travels at aconstant velocity, whichmeans that Newton’s first law (iner-
tia) holds true. A non-inertial frame of reference, such as a moving car or a rotating
carousel, accelerates. Therefore, Newton’s first law does not hold true ina non-inertial
reference frame, as objects appear to acceleratewithout the appropriateforces.
Why are frames of reference impor-
tant?
ESCDI
Frames of reference areimportant because (as we have seen in the introductory exam-
ple) the velocity of a carcan differ depending onwhich frame of reference is used.
Extension: Frames of Reference andSpecial Relativity
Frames of reference areespecially important in special relativity, becausewhen
a frame of reference is moving at some significant fraction of the speed oflight,
then the flow of time inthat frame does not necessarily apply in another ref-
erence frame. The speed of light is considered to be the only true constant
between moving framesof reference.The next worked example will explain this.
Relative Velocity ESCDJ
The velocity of an object is frame dependent. More specifically, the perceived velocity
of an object depends onthe velocity of the observer. For example, a person standing