There are
predefined variables in Mathcad, which keep built-in units of physical values
in different systems (see the sketch «Mathcad and physical values» (http://twt.mpei.ac.ru/ochkov/Units/unit.htm),
as well as one of the books of the given series – «Physical values in
Mathcad»). But user has the right to extend this list and to introduce into
calculation *user's* units (see fig. 16), which are connected with built-in ones via
corresponding coefficients[1].

At
the beginning of a calculation user is not able to know what new units will be
necessary. They can be introduced into the calculation as required: for
example, it is necessary to output time in weeks – it is enough to come back to
beginning of the calculation and to write week:=7 day and
etc.

(exactly like this: 7 day, but not 7day – see tip 9).

The
operator ì := m is handily in one more case. It is possible to fall into a reverie and to assign the
variable m (but it is very popular) some new value (see tip 1) in
a calculation. This operation is able to break all harmonious system of
physical values laid in Mathcad. But the operator ì := m releases the variable m for keeping
something else.

We could find the continuation of the theme in
the sketch «Mathcad and physical values» (http://twt.mpei.ac.ru/ochkov/Units/unit.htm).