Conversion from Decijoules to Decajoules

Information about the Decijoule (dJ)

The decijoule is a unit of energy equal to one tenth of a joule, or 0.1 joules. Its symbol is dJ. It is formed exactly as the decimetre and the decilitre are, and it is a legitimate part of the metric system, but it is written far less often than either of them because the quantity it names is usually reported as a decimal fraction of a joule instead.

The scale is easy to feel. An apple of a hundred grams falling ten centimetres arrives with about a decijoule. A spring-loaded mousetrap stores a few, a strong finger flick delivers roughly one, and a small pebble tossed underarm carries several. This is the first region on the way up from the millijoule where a person can sense the energy directly as a push rather than as a click.

Product safety uses the range even when it does not use the name. Toy projectile rules are written in joules and tenths of a joule, air-powered replicas are limited to about one joule in several countries, and impact tests on small plastic specimens report results in tenths. The figures are given as 0.3 J or 0.5 J rather than as 3 dJ or 5 dJ, but the quantity being controlled is precisely what the decijoule names.

Deci has survived in other units for reasons of convenience rather than logic. The decilitre is a standard kitchen measure across Scandinavia, Switzerland and much of central Europe, and the cubic decimetre is exactly the litre, which is why the prefix is quietly built into a unit almost everyone uses. The decigram appears in pharmacy and in precious-metal trading, and the decibel exists because the bel itself proved too coarse a step for describing sound.

With the joule, none of those pressures applied. Engineering had already fixed on prefixes stepping by a thousand, and the decimal point does the rest of the work: 0.4 joules is no harder to write than 4 decijoules and fits the same table as 40 joules or 4 kilojoules without a change of unit. A prefix earns its place by shortening what people write, and here it does not.

Encountering the unit is therefore mostly a matter of older literature, of translated technical documents and of teaching material that walks through the whole prefix ladder. The conversion is trivial in either direction: multiply by a hundred for millijoules, divide by ten for joules, and the value drops straight into whatever notation surrounds it.

One decijoule equals 0.1 joules, about 0.0239 calories, or about 0.0000278 watt-hours.

Information about the Decajoule (daJ)

The decajoule is a unit of energy equal to ten joules. Its symbol is daJ, and the spelling dekajoule with a k is also accepted. It is the least written of all the joule multiples, not because the quantity is obscure but because deca is the least used prefix in the entire metric system, and understanding that is more useful than memorising the unit itself.

The quantity is one of the easiest in physics to picture. Lifting a one-kilogram bag of sugar by one metre takes 9.81 joules, so almost exactly one decajoule. A heart beats about a joule per stroke, so ten beats. A rifle pellet from an unlicensed air rifle in Britain carries up to 16 joules, or a little over 1.6 decajoules. These are quantities the body understands directly as effort.

Deca survives in ordinary life in only a handful of places, and almost all of them involve mass. In Austria, Poland, Hungary and the Czech lands, delicatessen counters still sell by the dekagram, so a customer asks for twenty deka of ham rather than two hundred grams. The decanewton is used for rope and cable ratings and, in France, for towing capacities, because it happens to be close to the weight of a kilogram.

Elsewhere the prefix simply lost. Engineering standardised on steps of a thousand, which places kilo directly above the base unit and leaves nothing for deca to do that a two-digit number does not already do better. Writing 40 joules is shorter than writing 4 decajoules, and it sorts correctly in a table alongside 400 joules and 4 kilojoules without any mental arithmetic.

The scale itself is worth knowing even under another name, because it is the scale of single human actions. Throwing a ball, closing a heavy door, lifting a full kettle onto a hob, drawing a bow: each of these costs a few tens of joules. Impact tests on packaging and protective equipment are specified here too, and so are the energy limits written into rules for airguns and industrial fastening tools.

When a decajoule appears in a document, converting it takes one step in either direction. Multiply by ten for joules, divide by a hundred for kilojoules. Both forms are exact, and the joule form is the one that will match every other figure in a modern specification.

One decajoule equals 10 joules, about 2.39 calories, or about 0.00278 watt-hours.