Is there a difference in meaning between super and supra (both with accusative)? Would one indicate motion and the other one position?
When both prepositions are used with accusative, the difference is not large. Compare these two dictionary entries:
- [of place] over, above, on the top of, upon, on
- [of place] above, beyond
- [of time] during, at
- [of measure] over, above, beyond, in addition to
- [figuratively] of official position, over, in charge of
- [in the phrase, super omnia] above all, before all
- [of place] above, over
- [in the phrase supra caput] close, clinging, burdening, oppressing
- [of geographical position] above, beyond
- [figuratively] of time, before
- [of number] over, above, beyond, more than
- [of quality or degree] above, beyond, superior to
There are some differences, but the overall spirit is the same. In particular, I see no hint of one meaning position and the other meaning movement.
There is one difference in general tone (thanks TKR!): supra is only "over, above (and not touching)" while super can also be "on top of, resting on". That is, super has a broader meaning, allowing direct contact.
This was for accusative use only, and I will not try to discuss the difference between accusative and ablative uses of super. That accusative describes motion and ablative describes position is a useful rule of thumb, but it is better to treat is as a tendency than a rule. This rule works well for in, but it does not really describe the meanings of e(x) and prope.
The short answers are "no" and "yes", respectively.
Supra appears to be a contraction of supera, the fem. sing. abl. of superus, used adverbially (the a is long). There are plenty of analogous adverbial uses, e.g. qua, una. Unlike super, it almost never appears in compounds. As a preposition, it is found only with the accusative.
There are two useful bits of doggerel about common prepositions. First, with accusative:
Ante, apud, ad, adversus,/ Clam, circum, circa, citra, cis,/ Contra, inter, erga, extra,/ Infra, intra, iuxta, ob,/ Penes, pone, post and praeter,/ Prope propter, per, secundum,/ Supra, versus, ultra, trans :/ Add super, subter, sub and in,/ When motion 'tis, not state they mean.
A, ab, absque, coram, de,/ Palam, clam, cum, ex and e,/ Sine, tenus, pro and prae : Add super, subter, sub and in,/ When state, not motion, 'tis they mean.
These were long ago hammered into pupils in England, presumably to induce a more-or-less automatic, correct usage