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In the gravitational metric system(s) the base unit of force is not normalised to one mass unit (gram or kilogram) times one length unit (metre or centimetre) per time unit squared (second) as in the SI, but it depends on a selected or locally measured gravitational constant gn. This constant is usually set to an acceleration of 9.80665 m/s² on Earth. Other derived units inherit this factor. Sometimes alternately the base unit of mass is the one carrying the constant.
Where the difference between weight and mass is not important or not understood, gravitational units are still common despite adoption of the SI, an absolute metric system.
Units
[] Force
In English contexts the unit of force is usually formed by simply appending “force” to the unit of mass, thus gram-force (gf) or kilogram-force (kgf), which follows the tradition of pound-force (lbf). In other, international contexts the special name pond (p) or kilopond (kp) respectively is more frequent.
- 1 p = 1 gf
- = 1 g · gn = 9.80665 g·m/s² = 980.665 g·cm/s² = 980.665 dyn
- 1 kp = 1 kgf
- = 1 kg · gn = 9.80665 kg·m/s² = 980 665 g·cm/s²
Three approaches to mass and force units
| System |
Gravitational |
Engineering |
Absolute |
| Force (F) |
F = m·a |
F = m·a/gc = w·a/g |
F = m·a |
| Weight (w) |
w = m·g |
w = m·g/gc ˜ m |
w = m·g |
|
| Units |
English |
Metric |
English |
Metric |
English |
Metric |
| Acceleration (a) |
ft/s2 |
m/s2 |
ft/s2 |
m/s2 |
ft/s2 |
m/s2 |
| Mass (m) |
slug |
hyl |
pound-mass |
kilogram |
pound |
kilogram |
| Force (F) |
pound |
kilopond |
pound-force |
kilopond |
poundal |
newton |
Mass
The hyl, metric slug (mug), or TME (German: technische Masseneinheit, technical mass unit), is the mass that accelerates at 1 m/s² under a force of 1 kgf.[1] The hyl is has also been used as the unit of mass in a meter-gram force-second(mgfs) system.[2]
- 1 TME
- = 1 kp / 1 m/s² = 1 kp·s²/m = 9.806 65 kg
- 1 hyl
- = 1 kp·s²/m = 9.806 65 kg or
- 1 hyl (alternate definition - mgfs)
- = 1 p·s²/m = 9.806 65 g
Pressure
The (only) gravitational unit of pressure is the technical atmosphere (at). It is the gravitational force of one kilogram, i.e. 1 kgf, exerted on an area of one square centimetre.
- 1 at
- = 1 kp/cm² = 10 000 × gn kg/m² = 98 066.5 kg/(m·s²) = 98.066 5 kPa
Energy
There is no dedicated name for the unit of energy, “metre” is simply appended to “kilopond”, but usually the symbol of the kilopond-metre is written without the middle dot.
- 1 kpm
- = 1 kp·m = gn kg·m = 9.806 65 kg·m²/s² = 9.806 65 J
Power
In 19th-century France there was as a unit of power, the poncelet, which was defined as the power required to raise a mass of 1 quintal (1 q = 100 kg) at a velocity of 1 m/s. The German or metric horsepower (PS, Pferdestärke) is arbitrarily selected to be three quarters thereof.
- 1 pq
- = 1 qf·m/s = 100 kp·m/s = 100 × gn kg·m/s = 980.665 kg·m²/s³ = 0.980 665 kW
- 1 PS
- = 3/4 pq = 75 kp·m/s = 75 × gn kg·m/s = 735.498 75 kg·m²/s³ = 0.735 498 75 kW
References
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