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Distance and Displacement
By Ron Kurtus (updated 9 January 2023)
The separation of two objects or points in space is called the distance between those objects. The separation of the end points of an object is called the length of that object.
In order to give a numeric value to distance or length, you must define a unit length. Then the measurement of length or distance is a multiple (or fraction) of that unit length. In other words, distance is relative to the unit length.
If an object travels in a straight line from point A to point B, the distance traveled is called the displacement of the object. Displacement is a vector quantity, since the motion is in a defined direction. The distance traveled between two points is not necessarily the distance between the points, since the path may not be on a straight line.
Questions you may have include:
- What is the difference between distance and length?
- How do you measure distance?
- When is distance traveled a displacement?
This lesson will answer those questions. Useful tool: Units Conversion
Distance and length
The separation of two objects or points in space is called the distance between those objects. The separation of the end points of an object is called the length of that object.
Measuring distance
In order to measure distance or length, you need to define a unit length that does not vary.
The meter (m) is the international unit of length and is defined as the length of the path traveled by the light in vacuum during a time interval of 1/299,792,458 of a second. In the English system, used in the United States, the unit of length is the foot, which is 0.39 meters in length.
Distance or length is then indicated in multiples or fractions of the unit length.
Distance traveled
When an object changes positions over a period of time, it is considered in motion.
When an object moves or travels in a straight line from point A to point B, the distance traveled is called the displacement of the object. Since the motion is in a specific direction, displacement can be represented as a vector.
It is possible that the distance traveled from point A to point B may be on a path that is not a straight line. For example, the odometer on a car will show the distance traveled from one place to another, no matter what the path is. In this case, the distance traveled is not a displacement or a vector.
Summary
The separation of two objects or points in space is called the distance between those objects. The separation of the end points of an object is called the length of that object.
A unit length is defined in order to give a numeric value to distance or length. Measurement of length or distance is a multiple (or fraction) of that unit length.
If an object travels in a straight line from point A to point B, the distance traveled is called the displacement of the object. Displacement is a vector quantity, since the motion is in a defined direction. The distance traveled between two points is not necessarily the distance between the points, since the path may not be on a straight line.
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Resources and references
Websites
Distance - Wikipedia
Length - Wikipedia
SI Units: Length - National Institute of Standards & Technology (NIST)
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Distance and Displacement