[I think the teacher is being silly about the digital, but it never helps to argue with the folks with the grade book.] All the suggestions above about borrowing and working together are good. Perhaps the teacher is being vague about the assignment purposely to see what creative things happen? It sounds as if you already have some good ideas about how to show motion. If you already have a good digital camera, where you can adjust the focus, shutter speed, and aperature, I would not go out and buy anything expensive just for a class. Is this just the first assignment, and the rest of the time you'll be using the digital? If so, I'd just borrow or get a dispossable for this first assignment. Maybe also try shooting the same things with the digital and compare results? People find it difficult to believe sometime, but the creative idea is usually more important than the cost of the equipment.