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- Units
### Units

## Writing units

In the

**Formulas**question, the writing of the unit symbols differs slightly from that in the**SI**, in order to simplify the entry of units on the computer. The rules for entering units in the**Formulas**question are described below.- The multiplication of unit symbols is indicated by a space, as prescribed in Section 5.1 of
**SI**. The half-high, i.e. centered dot, (⋅) is. For example, input 50 kN m (≡ 50 kN⋅m), as follows:*not supported*

50 kN m -
Division of unit symbols is indicated by a solidus (oblique stroke, /) or by negative exponents.For example, input 10 m/s, or 10 m s
^{-1}, as follows:

10 m/s or 10 m s^(-1) -
Exponentiation is indicated by the circumflex accent (^, UTF-8 ﹠#94; or ASCII code 94 (alt+94)). For example, input 4.7 m
^{2}, as follows:

4.7 m^2 -
Parentheses are required when there is a risk of misinterpretation. For example,
8 kN m^-2 but rather:*do not input*

8 kN m^(-2) -
Permutations of the unit symbols are allowed. For example, the following two entries are considered identical:

3 m kg s^(-2) or 3 kg m/s^2

- The multiplication of unit symbols is indicated by a space, as prescribed in Section 5.1 of
**Note:**Unit symbols are case sensitive. For example, kilonewton is written kN, not KN, kn or Kn, megapascal is written MPa, not Mpa, MPA or mPa, etc.## Defining units

Units are defined on the Formulas question page, in the following three places:

- For each part, in the
**Unit**field. - For each part, in the
**Other rules**field. - Under
**Extra options**, in the**[Global] - Basic conversion rule**field.

- For each part, in the
### Unit

In the

**Unit**field, you can specify a single or a combined unit. For example:- kg N m/s^2
You can also specify single and combined units separated by =.

#### Example

In order to accept N and kg m/s

^{2}as valid units, define**Unit**as follows:N = kg m/s^2

Note that /s^2 must be placed at the end of the unit. Hence, N = /s^2 kg m and N = kg /s^2 m will lead to an error.

With the above setting, the following units will be considered correct:

N kg m/s^2 kg m /s^2 m kg /s^2 m kg s^(-2) s^(-2) kg m s^(-2) m kg#### Example

In order to accept W, J/s and m

^{2}kg s^{-3}as valid units, specify:W = J/s = m^2 kg s^(-3)### Other rules

Add prefixes that are not defined in the

**[Global] - Basic conversion rule**by writing units followed by a list of prefixes.#### Example

Specify that meganewton (MN) is an acceptable unit by writing the following:

N: M;Add conversions of single units by equating their values.

#### Example

1 m = 39.3700787 in;

With the above conversion rule, the following answers are equivalent:

10 in 0.254 m 25.4 cm 254 mm### Extra options

Select a basic conversion rule. Prefixes and conversion factors for different basic rules are defined in the

**conversion_rules.php**file. In the file downloaded from the**Moodle plugins directory**, two rules are defined:**None**and**Common SI units**.#### None

Select

**None**if other rules contradicts the rule defined in**[Global] - Basic conversion rule**if any.#### Common SI units

The

**Common SI units**rule defines prefixes and conversion factors commonly used by physicists. The rule is as follows:- //Prefixes m: k d c m u n p f; s: m u n p f; g: k m u n p f; mol: m u n p; N: k m u n p f; A: m u n p f; J: k M G T P m u n p f; eV: k M G T P m u; W: k M G T P m u n p f; Pa: k M G T P; Hz: k M G T P E; C: k m u n p f; V: k M G m u n p f; ohm: m k M G T P; F: m u n p f; T: k m u n p; H: k m u n p; //Conversion factors J = 6.24150947e+18 eV;
Note that, for example, m: k c d m u n p f; is equivalent to:

- 1 m = 1e-3 km = 1e1 dm = 1e2 cm = 1e3 mm = 1e6 um = 1e9 nm = 1e12 pm = 1e15 fm;
Because the

**Common SI units**prefixes and conversion factors concern mainly physicists and other scientists, the rule could be renamed**Scientific Units**.#### Example

With the

**Common SI units**rule, the following answers are equivalent:5 s, 5000 ms, 5e9 ns 0.2 m/s, 200 mm/s 1 m^2, 10000 cm^2, 1e-6 km^2## User defined rules

Other rules can be defined in the

**conversion_rules.php**file.#### Example

Define a rule for structural engineering in the

**SI**and**US**system of units:$basic_unit_conversion_rules[101] = array('Structural Engineering',' // Prefixes m: k c m ; N: k M; Hz: k; s: m; // Conversion factors 1 m = 3.28084 ft; 1 ft = 12 in; 3 ft = 1 yd; 1 kg = 0.001 t; 1 N = 0.22481 lb; 1 lb = 0.001 kip; etc. ');