Because in the universe, there is no absolute reference frame, there is also no absolute motion. it means that, motion is relative, so the term relative motion is usually used. A body which is motionless relative to a given reference frame, may be moving relatively to infinitely many other frames. As a result, we can conclude that “everything in this universe is moving”.
Basically, motions are distinguished into translation, rotation, and vibration. In this topic, we will only discuss translation, namely the movement of a body in a straight line so that every point on the body follows a parallel path and no rotation takes place. In kinematics, an object or body is simplified by assuming it as a particle, so the aspects of rotation and vibration of the object are negligible size. In other words, the use of the term particles enables us to use the concepts of kinematics to solve many physics problems dealing with the state of the motion of an object, especially translation. Meanwhile, the problems dealing with the rotation and vibration of an object can be solved using another consept.
If an object is in a motion, then it will undergo the change in position. The cange in position of an object at a certain time is called displacement. In other words, displacement is also defined as the amount of movement of an object measured in a particular direction. Meanwhile, the length of the actual path (trajectory) travelled by an object during its motion is called distance.
To be able to distinguish distance and displacement, consider the following figure showing someone who walks starting at one side of a table to the other side. How far has the person walked?.
Basically,
there are two ways to answer the question above. Firstly, the person made a
walk of six meters (two meters east, two meters south, and two meters west). Secondly,
the person made a walk of two meters to the south of his starting point. So which
answer is correct?. In this case, the two answers are correct depending on
wether the questioner meant to ask for the distance or displacement. If the
questioner asks the distance, the first answer is correct, while if the
questioner asks the displacement, the second answer is correct.
Based
on the explanation above, distance is scalar measure of the interval between
two locations measured along the actual path connecting them, while
displacement is a vector measure of the interval between two locations measured
along the shortest path connecting them. Generally, distance is represented by
S, while displacement is represented by ⧍x, where :
𝜟x = xfinal - xinitial
Where
:
x
= the position of the object
if
a motion consist of many parts,then the change is displacement is the sum of
the displacements in each part of the motion.