Marking system for ballistics pendulum? Does anyone have any ideas for a low resistance marking system for a ballistics pendulum? The professor I am working for wants me to build a ballistic pendulum (as a demonstration apparatus) for a very weak airsoft gun. He said there must be a marking system (can't just eyeball it). Even marking with charcoal or felt tip pens drastically skews the results (pendulum indicates lower velocity than it should). I had some crazy idea that there might be some type of exposure paper that is sensitive to red laser (laser pointer) light, which I could use for this application (resistance-free marking). But then I realized that if this were feasible, people would already be walking around with this paper taking notes with their laser pens, no? Plus I googled it (in many formats) and didn't return any such products (doesn't mean they don't exist, but...). I suppose the actual problem is not so much classical physics specific, but since the ballistics pendulum is a classical physics device... (let me know if I posted this in the wrong place!) Thanks!