Somehow, even with the most difficult lit or history homework question, there are ways to sort through it, do a little more research, start writing and then find yourself with a plausible response.
That would not be the case with physics, or algebra II or calculus. So who ya gonna call? One site that is getting raves now is Khan Academy,
http://www.khanacademy.org/ It's a not-for-profit open source learning project with the mission of providing a world-class education to anyone, anywhere. It won't provide the exact answer to a specific problem, but it does offer a teaching video and then exercises centered on highly targeted areas, such as parametric equations or introduction to mechanical advantage.
Another well-thought of site is The Math Forum @ Drexel/Ask Dr. Math which also has an online tutoring component for help with a specific problem. There is a cost -- the fee starts at around $35 a month for unlimited, 24/7 assistance.
Don't do anything desperate when neither you nor your teen can figure out the velocity problem. Simply seek immediate professional help.