This ^^^ is why I hated physics class. Im going to go pedal my mt bike, with disc brakes. And practice my nose wheelies on some downhill switchbacks. There is a thing of beauty in a properly executed nose wheelie within a tight downhill turn. Get the rear wheel up about 1-2', then perform a hip flick to what ever direction you wish to turn. Release the front brake in the new line your bike is now pointed. Way cool when performed correctly. Serious results may occur with poor execution. And gravity is a harsh mistress. Balance , concentration and focus needed, physics, not so much.