Never round float results, use test.approx_equals. https://docs.codewars.com/authoring/recipes/floating-point

-1 is a valid result and should not be used as an error indicator. In case of error the function should raise an exception (better), or return a standard "no result" value such as None.

calling acceleration_calc(None) doesn't make sense; if there aren't args the correct call is acceleration_calc().