science

MKS units

MKS units is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand MKS units rather than just read about it. In short: The metre, kilogram, second system of units, also known more briefly as MKS units or the MKS system, is a physical system of measurement based on the metre, kilogram, and second (MKS) as base units. Distances are described in terms of metres, mass in terms of kilograms and time in seconds.

Key takeaways

MKS units belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect MKS units to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of MKS units from memory before moving on to harder problems.

Reference excerpt

The metre, kilogram, second system of units, also known more briefly as MKS units or the MKS system, is a physical system of measurement based on the metre, kilogram, and second (MKS) as base units. Distances are described in terms of metres, mass in terms of kilograms and time in seconds. Derived units are defined using the appropriate combinations, such as velocity in metres per second. Some units have their own names, such as the newton unit of force which is defined as kilogram times metres per second squared. The modern International System of Units (SI, from the French name Système international d'unités) was originally created as a formalization of the MKS system. The SI has been redefined several times since then and is now based entirely on fundamental physical constants, but still closely approximates the original MKS units for most practical purposes.

History By the mid-19th century, there was a demand by scientists to define a coherent system of units. A coherent system of units is one where all units are directly derived from a set of base units, without the need of any conversion factors. The United States customary units are an example of a non-coherent set of units. In 1874, the British Association for the Advancement of Science (BAAS) introduced the CGS system, a coherent system based on the centimetre, gram and second. These units were inconvenient for electromagnetic applications, since electromagnetic units derived from these did not correspond to the commonly used practical units, such as the volt, ampere and ohm. After the Metre Convention of 1875, work started on international prototypes for the kilogram and the metre, which were formally sanctioned by the General Conference on Weights and Measures (CGPM) in 1889, thus formalizing the MKS system by using the kilogram and metre as base units. In 1901, Giovanni Giorgi proposed to the Associazione elettrotecnica italiana (AEI) that the MKS system, extended with a fourth unit to be taken from the practical units of electromagnetism, such as the volt, ohm or ampere, be used to create a coherent system using practical units. This system was strongly promoted by electrical engineer George A. Campbell. The CGS and MKS systems were both widely used in the 20th century, with the MKS system being primarily used in practical areas, such as commerce and engineering. The International Electrotechnical Commission (IEC) adopted Giorgi's proposal as the M.K.S. System of Giorgi in 1935 without specifying which electromagnetic unit would be the fourth base unit. In 1939, the Consultative Committee for Electricity (CCE) recommended the adoption of Giorgi's proposal, using the ampere as the fourth base unit. This was subsequently approved by the CGPM in 1954. The rmks system (rationalized metre–kilogram–second) combines MKS with rationalization of electromagnetic equations. The MKS units with the ampere as a fourth base unit is sometimes referred to as the MKSA system. This system was extended by adding the kelvin and candela as base units in 1960, thus forming the International System of Units. The mole was added as a seventh base unit in 1971.

Derived units

Mechanical units

Electromagnetic units

Worked examples

Example 1 — a first encounter with MKS units

Start with the simplest possible case. Write down what MKS units claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to MKS units before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about MKS units ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of MKS units

In research: MKS units appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses MKS units in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: MKS units is common in secondary-school and first-year university syllabi. It links to neighbouring topics Metric system, Systems of units, so understanding it makes those chapters shorter.
In everyday life: Look for MKS units outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study MKS units in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what MKS units means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain MKS units out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is MKS units in simple terms?

Why does MKS units matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study MKS units?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on MKS units.