System of units
Unit system
It allows you to test the knowledge of students about unit. Also, it allows the student to use alternative units for the same answer, provided that they are convertible to each other.
You can specify mark fraction deduction for a wrong unit. The wrong unit here means that the unit that is not convertible to the correct unit, under conversion rules. Suppose a student get 2 marks for this subquestion answer. If the unit penalty = 0.2 and she give wrong unit, then the student can get 2(1-0.2) = 1.6 mark for this submission. (See Grading scheme).
If a teacher does not specify any unit for the subquestion, then the unit box will not be displayed and there is no need for students to enter it. However, if the student enters anything after their answer, it is usually considered to be incorrect so the mark of unit will be deduced. Hence, in the above case, it is better to set 1 for mark deduction.
Nevertheless, if you do not want to penalize the student entering arbitrary string at the end of answer, set 0 for mark deduction. i.e. their unit does not count toward their grade, but you still allow student use any convertible unit. Note that you have assumed a default answer that does not require unit.
Unit and format
A unit is either a base unit or a composite unit:
Base unit: a simple alphabetic string without exponent (e.g. ^2) Composite unit: compose of base unit separated by space, possibly together with exponents. Note that the ordering of base unit is not important For a given physical quantity, the unit name can be entered using the following rules:
Base unit: e.g. length (m), it should be simply entered as m Positive exponent: e.g. area (m2), it should be entered as m^2. Negative exponent: e.g. wavenumber (m-1), it should be entered as m^(-1). Compose unit: e.g. velocity (m s-2), it should be entered as m s^(-2) with the base unit separated by the space (representing multiplication). Alternative for negative exponent: One preceding division operator can also be used, say m/s^2 which is equivalent to m s^(-2). Students need to enter the same format for unit (i.e. they need to know the above rule). Examples:
Unit Description m a base unit, meter, for the dimension length kg a base unit, kilogram, for the dimension mass N a base unit (it is called derived unit in SI), newton, for the physical quantity force m/s a composite unit, for the physical quantity velocity kg m/s^2 a composite unit, with the same dimension as newton Examples of answer with unit:
1 m 0.1 m^2 20 m s^(-1) 400 kg m/s 100 kW
Conversion rules
Conversion rules allow students to use alternative unit. For the details of the rule, See UnitConversion.
Other rules: You can specify the conversion of the base unit by equating their values under different units. Examples:
1 m = 100 cm = 1000 mm; 1 cm = 0.3937 inch; With the above conversion rule, the following responses are completely equivalent:
10 inch 25.4 cm 254 mm 0.254 m For the SI prefix, an alternative syntax can be used: base unit followed by a list of SI prefix name.
W: k M G T; which is equivalent to 1 W = 1e-3 kW = 1e-6 MW = 1e-9 GW = 1e-12 TW .
Basic conversion rule: For the conversion between common SI unit with different prefix, there are a set of predefined conversion rules for them. The only thing you need to do is to choose the 'Common SI unit' in the 'Basic conversion rule'.
The following answers in the each group
5 s 5000 ms 5e9 ns 0.2 m/s 200 mm/s 1 m^2 10000 cm^2 1e-6 km^2 are all equivalent to each other. So if one of the answer is correct, all other are also correct answers.
Note that you may also use 'None' if you find it contradicting with your own rules. If you want to define you own, see the file conversion_rules.php.
Identifying composite unit: Note that, the base units and composite unit cannot be identified in the conversion rules above. To specify a list of allowable composite unit, you can use separator '=' in the 'Unit'. For example
N = kg m s^(-2) It identity the unit Newton to its SI counterpart. Hence, both composite unit are correct.
