There are two systems of measurement widely used in the world today:
- Metric system
- Imperial system
The metric system was developed in France in the 18th century and was adopted precisely in 1799. The system is currently used by most countries in the world. The imperial system was the original system used by the United Kingdom prior to 1965 when they adopted the metric system. Currently, three countries are widely known to use systems other than the metric: the USA, Liberia, and Myanmar. The metric system has seven (7) base units: metre, kilogram, second, Ampere, degree Kelvin, and mole. The imperial system has many units of measurement. The popular ones are pound, ounce, and ton for weight, gallon, pint, and fluid ounce for volume, acre, hectare for area and feet, and inches and yard for length.
Even though the metric system is the official system in Nigeria, imperial units are often used in construction works at sites, and practitioners are often faced with the need to convert from one unit to another, especially on the aspect of length. For conversions involving length, we should know that:
1 yard = 0.9144 metre
1 foot = 0.3048 metre
1 inch = 0.0254 metre
1 mile = 1609.344 metre
On the other hand, using only Imperial units
1 foot = 12 inches
1 yard = 3 feet
Such that 1 yard = (3 x 12) inches = 36 inches
We should know this and we can find various conversion rates on most websites. My major interest in this article is what students usually find challenging in their studies in school. This problem can be based on Imperial units, however, this is rare these days because we use more books written in the metric system than the Imperial system which was common some years ago.
Two types of quantities are basically used in the metric system: fundamental quantities and derived quantities. Fundamental quantities are measured by one of the base units identified above while derived quantities involve the combination of one or more of the fundamental quantities. One major advantage of the metric system is that it is easier to convert between scales because it uses the power of 10s. For instance
1 km = 106 mm
1 kg = 1000 g = 106 milligram
1 MN = 1000 kN = 106 N
This advantage should have made it possible for students to pass their exams very well, however, some students find it difficult to make the conversions properly and many fail their exams because of it. I would show a few examples of how this challenge can be overcome.
A. Given that a force of 30 kN was applied over an area of 5 m2, determine the pressure on this area and present your answer in Nmm-2
To make the solution easier, follow these two simple steps:
- Convert each quantity to an equivalent scale required in the unit of the answer
- Using the newly converted quantities, solve for the answer and present it in the required unit
For example, 1 above, the unit of the answer is Nmm-2
We know that
1 kN = 1000 N
It implies that 30 kN = (30 x 1000) N = 30, 000 N
1 m = 1000 mm
(12) m2 = (10002) mm2 = 106 mm2
It implies that 5 m2 = (5 x 106) mm2
Pressure = force/area
Using the original units, Pressures = 30 (kN) / 5(m2)= 6 kNm-2
Using the new units, pressure = 3000 (N) / (5 x 106) mm2 = 0.006 Nmm-2
From this, we can see that 1 Nmm-2 = 1000 kNm-2
In subsequent calculations, we can use these conversion rates to solve our problems but it is very important we know it from the first principle.
B. Given that a plane travels 700 km in 7 hours, determine the speed of the aeroplane in ms-1
Following the principle outlined above,
1 km = 1000 m
700 km = (700 x 1000) = 700, 000 m
1 hour = 60 minutes and 1 minute = 60 seconds
1 hour = (60 x 60) seconds = 3600 s
7 hours = (7 x 3600) = 25, 200 s
Speed = distance/time
Using the original unit, speed = 700 (km) / 7 (hours) = 100 kmhr-1
Using the new unit, speed = 700,000 (m) / 25,200 (s) = 27.778 ms-1
From this, we can see that 1 ms-1 = 3.6 kmhr-1
One may have more complex problems. In each case, apply the same principle outlined above and you would be sure to get the right answer.
Find attached an online converter for quick conversion from imperial units to metric or metric units to metric.