When it comes to the phone when I'm writing, I'm all about hitting the mute button. But I usually check the number, just to be sure Crash hasn't had another gaga injury, or The Boy hasn't thrown up. It was my sister's cell number. Worried that something was wrong (it was the middle of the school day for her), I picked up. Quickly reassured that everything was OK, she wondered if she could ask me a question. "Sure." "It's about physics." Physics would not be my sister's forte, and I couldn't imagine what she wanted to know. "Go ahead," my curiousity was piqued. "Ok, you have two cars driving toward each other head on, they collide and stick together, how fast is the wreckage moving?" Huh??? "I'm trying to help a student."
I explain that she needs to use conservation of momentum, grateful that the vectors in this case are anti-parallel and not at some funky angle, and I walk her through setting up the momentum of the cars before the crash. Yes, she knows the masses of both cars -- and their speeds. Great, we're on a roll here. Then we start with the other side. I note we multiply the mass of the wreck by its velocity. "But I don't know the mass of the wreck," she moaned. "Sure you do, sweetie," I said encouragingly, "what happens when they collide? and stick?" She's my sister after all, and even if she's not a math maven, she's smart -- and she got it. It was then that she told me I was on the speaker phone, and her whole class could hear me. Eeek!
If you want to do the problem: Car A has a mass of 1500 kg and is moving at 50 mph head on toward Car B, which has a mass of 1400 kg and is moving at 10 mph (which is not a Honda Fit and does not contain any stained glass). How fast is the wreckage moving? In a frictionless universe, of course!