Gravitational metric system facts for kids
The gravitational metric system is an older way of measuring things. It's not the main system used around the world today, which is called the International System of Units (SI). This older system was built using three main measurements: length (like how long something is), time (like seconds), and force (like a push or pull).
Its main units were the metre for length, the second for time, and the kilopond for force. You might see it called MKpS, MKfS, or MKS (which stands for mètre–kilogramme-poids–seconde in French). Be careful, though, because "MKS" can also mean a different system that uses mass (how much "stuff" is in something) instead of force.
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Why It's Not Used Much Anymore
Today, scientists are very careful to tell the difference between an object's mass and its weight. Mass is how much material an object has, and it stays the same no matter where you are. Weight, however, is the force of gravity pulling on that mass. So, your weight would be different on the Moon than on Earth, but your mass would be the same.
Historically, the unit "kilopond" was sometimes just called "kilogram." Later, people realized they needed to separate "kilogram-mass" (what we call a kilogram today) from "kilogram-force" (what we call a kilopond today). A kilopond was originally defined as the weight of a one-kilogram mass.
The problem is that gravity isn't exactly the same everywhere on Earth. This means a kilopond would have slightly different values depending on where you measured it. To fix this, the kilopond was later defined using a specific "standard gravity" value of 9.80665 m/s2.
Another issue with this system was that some units, like horsepower, didn't fit together perfectly. Also, it didn't connect well with units for electricity, magnetism, or heat.
In Germany, the kilopond stopped being an official unit of force on January 1, 1978. That's when they officially switched to the SI system. You can change a kilopond into the modern SI unit of force, the newton, by multiplying it by the standard gravity value:
- 1 kp = 9.80665 N
Units in the System
This old system had its own names for different measurements.
Force
In English, people often added "force" to a mass unit, like "gram-force" (gf) or "kilogram-force" (kgf). This was similar to how "pound-force" (lbf) was used. In other countries, special names like pond (p) or kilopond (kp) were more common.
- 1 p = 1 gf = 980.665 dyn
- 1 kp = 1 kgf = 9.80665 kg⋅m/s2
Mass
The unit of mass in this system was called the hyl, metric slug, or TME (which means "technical mass unit"). This unit was defined as the amount of mass that would speed up by 1 m/s2 when a force of 1 kgf was applied to it. This unit is very old and not used anymore.
- 1 TME = 9.80665 kg
- 1 hyl = 9.80665 kg
Pressure
The unit of pressure in this system was the technical atmosphere (at). It was defined as the force of one kilogram-force (1 kgf) pushing down on an area of one square centimetre.
- 1 at = 98.0665 kPa
Energy
There wasn't a special name for the unit of energy. It was simply called the "kilopond-metre" (kpm).
- 1 kpm = 9.80665 J
Power
In the 1800s in France, there was a unit of power called the poncelet. It was the power needed to lift a mass of 1 quintal (which is 100 kg) at a speed of 1 m/s. The German or metric horsepower (PS, from Pferdestärke) was set to be three-quarters of a poncelet.
- 1 poncelet = 0.980665 kW
- 1 PS = 0.73549875 kW
See also
- List of metric units
