Antes del metro...
¿Cómo medían los romanos?

Levantar campamentos, almacenar suministros o construir calzadas no se hacía "a ojo": necesitaban medir.
Llamas magnitudes a todas aquellas propiedades o características que puedes medir (como la longitud, la masa o el tiempo).
Medir es comparar una de estas magnitudes con una unidad de referencia para conocer su valor exacto.
Mucho antes del Sistema Métrico Decimal (SMD), el Imperio romano ya trazaba obras perfectas.
¿Su secreto? Usar su propio cuerpo como cinta métrica.
Por ejemplo, dedo, palmo, pie y paso eran unidades de referencia para las longitudes.
El contador de las calzadas
¿Te has preguntado de dónde viene la palabra "milla"?
Viene de Mille Passus (mil pasos).
Cada vez que una legión caminaba mil pasos dobles, los ingenieros colocaban una piedra gigante llamada miliario.
Los miliarios son como los "carteles de autovía" de la antigüedad.
En ellos podías leer la distancia hasta la ciudad más próxima y quién había pagado la carretera.
¿Por qué cambiar?
El problema de medir con "pies" es que no todos los pies son iguales.
Por eso, hoy usas el Sistema Métrico Decimal (SMD), donde un metro es igual en todo el mundo.
Esto te permite hacer cálculos de longitud, masa, volumen y capacidad con total precisión.
Lectura facilitada
¿Cómo medían los romanos?
Los romanos construían campamentos, almacenes y carreteras perfectas.
Para lograrlo, necesitaban medir.
- Magnitud: Es todo lo que se puede medir. Por ejemplo: la longitud, la masa o el tiempo.
- Medir: Es comparar algo con una medida de referencia para saber su valor exacto.
Los romanos no tenían reglas de medir.
Su secreto era usar su propio cuerpo.
Usaban cuatro medidas principales de longitud: el dedo, el palmo, el pie y el paso.
Los miliarios
La palabra milla viene del latín Mille Passus y significa mil pasos.
Cada mil pasos dobles que caminaban soldados, ingenieros colocaban una piedra gigante llamada miliario.
Los miliarios eran los carteles de las carreteras antiguas.
Tenían dos datos importantes:
- La distancia hasta la ciudad más cercana.
- El nombre de la persona que pagó la carretera.
El cambio al sistema actual
Medir con el cuerpo tenía un problema: los cuerpos de las personas no son iguales.
El pie de cada persona mide algo diferente.
Por eso hoy usas el Sistema Métrico Decimal (SMD).
En este sistema, un metro mide lo mismo en todo el mundo.
Esto permite medir la longitud, la masa y el volumen con total precisión.
¿Y ahora el SMD?
Prefijos
Estos son algunos de los prefijos más importantes para múltiplos y submúltiplos.
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Los prefijos se pueden indicar con potencias de base diez, con exponente positivo para los múltiplos y negativo para los submúltiplos.
A partir de \({\mathrm{10}}^\mathrm{3}\) los prefijos llevan un nombre específico si son múltiplos de tres:
- Mega (M) 106
- Giga (G) 109
- Tera (T) 1012
Lo mismo ocurre con los exponentes negativos, en ese caso son micro (\(\mu\)) para exponente -6, nano (n) para exponente -9 y pico (p) para exponente -12.
Longitud
La unidad de medida de longitud es el metro (m).
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Para pasar de unas unidades a otras se utiliza la multiplicación por 10 o la división entre 10.
También se pueden utilizar para los cambios de unidades los factores de conversión. En el apartado 3.1. se explica lo que son y cómo aplicarlos.
Masa
La unidad de medida de masa en el Sistema Internacional (SI) es el kilogramo (kg). En el siguiente esquema puedes ver el múltiplo y los submúltiplos más empleados.
Habitualmente utilizamos fracciones del kilogramo cuando vamos a comprar al supermercado; también empleamos los gramos (g) para compras más pequeñas (por ejemplo, loncheados) y, en el caso de medicamentos, se utilizan los miligramos (mg).
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Capacidad
La unidad de medida es el litro (l).
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Superficie
La unidad de medida es el metro cuadrado. En este mapa mental puedes ver la relación entre estas unidades y las unidades agrarias.
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Volumen
La unidad de medida es el metro cúbico. En este mapa mental puedes ver la relación entre estas unidades y las unidades de capacidad.
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