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Physics

Unit of Distance in Physics: Explained with SI Units

Unit of Distance in Physics: Explained with SI Units

Q: 1. What is the fundamental SI unit of distance used in Physics?

The fundamental SI (International System of Units) unit of distance is the metre (m). It is one of the seven base units upon which all other SI units are built. As per the current definition, the metre is defined as the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second.

Q: 4. Since displacement is the shortest path, can the distance travelled ever be less than the magnitude of displacement?

No, the distance travelled can never be less than the magnitude of the displacement. The displacement is defined as the shortest straight-line path between two points. The distance is the actual path length taken. The only scenario where the distance is equal to the magnitude of displacement is when an object travels in a straight line without changing direction. In any other case involving a curved path or a change in direction, the distance will always be greater than the displacement's magnitude.

Q: 5. How do the SI and CGS units of distance relate to each other?

The SI unit of distance is the metre (m), while the CGS (Centimetre-Gram-Second) unit of distance is the centimetre (cm). The relationship between them is straightforward: 1 metre is equal to 100 centimetres. The CGS system was a precursor to the modern SI system and is still sometimes used in specific fields of physics, particularly for smaller-scale phenomena.

Reviewed by:

Gaurang Shah

Key SI Units of Distance and Real-Life Examples

What is Distance?

Distance is defined as the numerical measurement of how far apart the objects are from each other. In everyday life, when it comes to the mechanism of physics, distance refers to the physical length or estimation. In many of the cases, the distance between A and B is interchangeable (e.g., two countries over)with the distance between B and A.

In physics, distance acts as a function or metric in a simplification of the theory of physical distance. There is a generalized concept that describes what it means for elements of some space to be "close to" or "far away from" each other

In Psychology, distance is a non-numerical measurement. The different ways in which an object can move taking time as a reference quality in a social distance, space, hypothetically, and self, along a dimension, is known as psychological distance.

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The airline routes between Tokyo and Los Angeles usually follow a direct great circle route highlighted in the above diagram however use the jet stream route highlighted as a green route when heading towards the eastwards. Even though the shortest route appears to be the curve rather than the straight line as this map is Mercator projection, it does not measure all distances related to the real spherical surface of the Earth.

Unit of Distance

The SI unit of distance is meter according to the International System of Units. Using this system, many other base units and equations can be easily derived and are thus known as derived units.

The below shown figure is a grid Manhattan distance

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A physical distance can mean quite a few different things:

Distance Travelled: The length of the specific path traveled between two places. For example, the distance covered while navigating a maze.
Straight-Line Distance: This is the shortest possible path through space between two places that one can take, considering that there are no obstacles. This is otherwise usually known as Euclidean distance.
Geodesic Distance: The shortest path between two possible places while staying on the surface, such as the great circle distance along the curve of Earth.
The length of the specific path that returns to the starting point, for example, a ball thrown straight up in the air or the Earth when it completes one orbit.

Unit of Area

The phenomena that suggest the extent of a 2D shape, planar lamina, or shape in a plane is known as the area, and its analog on the 2D surface of a 3D object is known as the surface area. The area can be identified as the amount of material with a given thickness. It would be essential to fashion a model of the shape or the quantity of the paint required to cover the surface with a single coat. It is a two-dimensional analog of the length of a curve (a one-dimensional concept) or the volume of a solid (which is a three-dimensional concept).

Each and every unit of length has an equivalent unit of area, namely the area of a square is given with the help of a side length. The area can be measured in square meters (m2), square millimeters (mm2), square centimeters (cm2), square kilometers (km2), square yards (yd2), square miles (mi2), square feet (ft2), etc. Algebraically the mentioned units can be thought of as the square of the corresponding length units.

The SI unit of area is square meter, which is considered as an SI derived unit.

Parallactic second

Parallactic second is the unit of distance as the abbreviation of a parallactic second is of parsec.

Parsec = Parsec is a unit of larger distances. It is the distance at which stars would create parallax of one second of arc.

1 parsec = 3.26 light-years

The distance which is drawn as the line from the center of the Earth up to the Sun center, will cross subtending a 1-second angle with each other. It expresses the physical meaning of the parsec.

Parsec is the unit of the length used to explain the distance between stars to galaxies. Professional astronomers use it. It represents the distance from which the radius of the Earth's orbit decreases by one second of the arc. Therefore, a star at a parse distance has a parallax of one second, and the distance of an object in the parsec is the interval of its parallax in seconds of the arc.

For example, the nearest star that is part of the Alpha Centauri triple-star system is Proxima Centauri, which has an arc parallax of 0.769 seconds. Hence, its distance from the sun and Earth is 1.30 parsec. A parsec is equal to 3.26 light-years, equivalent to 3.09 × 1013 km (1.92 × 1013 mi).

FAQs on Unit of Distance in Physics: Explained with SI Units

1. What is the fundamental SI unit of distance used in Physics?

The fundamental SI (International System of Units) unit of distance is the metre (m). It is one of the seven base units upon which all other SI units are built. As per the current definition, the metre is defined as the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second.

2. What is the main difference between distance and displacement?

The main difference lies in their nature as physical quantities. Distance is a scalar quantity, meaning it only has magnitude and represents the total path length covered by an object. Displacement is a vector quantity, meaning it has both magnitude and direction, representing the shortest straight-line path between the initial and final points. For example, if you walk 5 metres east and then 5 metres west back to your starting point, the distance covered is 10 metres, but your displacement is zero.

3. What are some other common units of distance besides the metre?

Besides the metre, various other units are used depending on the scale of the measurement. These include:

Kilometre (km): Used for measuring geographical distances (1 km = 1000 m).
Centimetre (cm): A smaller CGS unit (1 m = 100 cm).
Millimetre (mm): Used for small-scale measurements (1 m = 1000 mm).
Inch, Foot, Yard, Mile: Imperial units of distance.
Astronomical Unit (AU): The average distance from the Earth to the Sun, used within our solar system.
Light-Year (ly): The distance light travels in one year, used for interstellar distances.
Parsec (pc): A larger astronomical unit, equivalent to about 3.26 light-years.

4. Since displacement is the shortest path, can the distance travelled ever be less than the magnitude of displacement?

No, the distance travelled can never be less than the magnitude of the displacement. The displacement is defined as the shortest straight-line path between two points. The distance is the actual path length taken. The only scenario where the distance is equal to the magnitude of displacement is when an object travels in a straight line without changing direction. In any other case involving a curved path or a change in direction, the distance will always be greater than the displacement's magnitude.

5. How do the SI and CGS units of distance relate to each other?

The SI unit of distance is the metre (m), while the CGS (Centimetre-Gram-Second) unit of distance is the centimetre (cm). The relationship between them is straightforward: 1 metre is equal to 100 centimetres. The CGS system was a precursor to the modern SI system and is still sometimes used in specific fields of physics, particularly for smaller-scale phenomena.

6. Why is the modern definition of a metre based on the speed of light instead of a physical object?

The modern definition is based on the speed of light because it provides a universal, unchanging, and precisely reproducible standard. The previous standard, a physical platinum-iridium bar kept in France, was susceptible to physical damage, degradation over time, and slight variations in length due to environmental factors. By defining the metre based on the speed of light, which is a fundamental constant of nature, scientists anywhere in the world can replicate the standard with extreme accuracy, ensuring global consistency in measurements.

7. How are units of distance applied when measuring microscopic scales like the size of an atom?

For measuring microscopic and atomic scales, standard units like metres or centimetres are too large to be practical. Instead, scientists use much smaller, specialised units. The most common units for these scales are:

Angstrom (Å): Equal to 10⁻¹⁰ metres. It is commonly used to express atomic radii and the wavelengths of X-rays.
Nanometre (nm): Equal to 10⁻⁹ metres. It is widely used in nanotechnology and to describe the dimensions of molecules and microorganisms.
Picometre (pm): Equal to 10⁻¹² metres. It is used for even smaller measurements, such as bond lengths between atoms in a molecule.