physics

Kilogram-force

Kilogram-force is a physics 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 Kilogram-force rather than just read about it. In short: The kilogram-force (kgf or kgF), or kilopond (kp, from Latin: pondus, lit. 'weight'), is a non-standard gravitational metric unit of force. It is not accepted for use with the International System of Units (SI) and is deprecated for most uses, superseded by the newton.

Key takeaways

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

Reference excerpt

The kilogram-force (kgf or kgF), or kilopond (kp, from Latin: pondus, lit. 'weight'), is a non-standard gravitational metric unit of force. It is not accepted for use with the International System of Units (SI) and is deprecated for most uses, superseded by the newton. The kilogram-force is equal to the magnitude of the force exerted on one kilogram of mass in a 9.80665 m/s2 gravitational field (standard gravity, a conventional value approximating the average magnitude of gravity on Earth). That is, it is the weight of a kilogram under standard gravity. One kilogram-force is defined as 9.80665 N. Similarly, a gram-force is 9.80665 mN, and a milligram-force is 9.80665 μN.

History The gram-force and kilogram-force were never well-defined units until the CGPM adopted a standard acceleration of gravity of 9.80665 m/s2 for this purpose in 1901, though they had been used in low-precision measurements of force before that time. Even then, the proposal to define kilogram-force as a standard unit of force was explicitly rejected. Instead, the newton was proposed in 1913 and accepted in 1948. The kilogram-force has never been a part of the International System of Units (SI), which was introduced in 1960. The SI unit of force is the newton. Prior to this, the units were widely used in much of the world. They are still in use for some purposes; for example, they are used to specify tension of bicycle spokes, draw weight of bows in archery, and tensile strength of electronics bond wire, for informal references to pressure (as the technically incorrect kilogram per square centimetre, omitting -force, the kilogram-force per square centimetre being the technical atmosphere, the value of which is very near those of both the bar and the standard atmosphere), and to define the "metric horsepower" (PS) as 75 metre-kiloponds per second. In addition, the kilogram force was the standard unit used for Vickers hardness testing.

In 1940s, Germany, the thrust of a rocket engine was measured in kilograms-force, in the Soviet Union it remained the primary unit for thrust in the Russian space program until at least the late 1980s. Dividing the thrust in kilograms-force on the mass of an engine or a rocket in kilograms conveniently gives the thrust to weight ratio, dividing the thrust on propellant consumption rate (mass flow rate) in kilograms per second gives the specific impulse in seconds. The term "kilopond" has been declared obsolete.

Related units The tonne-force, metric ton-force, megagram-force, and megapond (Mp) are each 1000 kilograms-force. The decanewton or dekanewton (daN), exactly 10 N, is used in some fields as an approximation to the kilogram-force, because it is close to the 9.80665 N of 1 kgf. The gram-force is 1⁄1000 of a kilogram-force.

Worked examples

Example 1 — a first encounter with Kilogram-force

Start with the simplest possible case. Write down what Kilogram-force claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In physics, 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 Kilogram-force 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 Kilogram-force 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 Kilogram-force

In research: Kilogram-force appears in physics 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 Kilogram-force 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: Kilogram-force is common in secondary-school and first-year university syllabi. It links to neighbouring topics Non-SI metric units, Units of force, so understanding it makes those chapters shorter.
In everyday life: Look for Kilogram-force 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 Kilogram-force in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Kilogram-force 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 Kilogram-force out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Kilogram-force in simple terms?

Why does Kilogram-force matter?

Because it connects several physics 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 Kilogram-force?

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 Kilogram-force.