For mathematics and physics, the 'unit' of spaced repetition should be skills, not questions. You would need a program which can generate questions that test a

For mathematics and physics, the 'unit' of spaced repetition should be skills, not questions. You would need a program which can generate questions that test a particular skill, such as partial differentiation or factorizing quadratics. If you get a question wrong, that indicates that you need to spend more time practicing that type of question.