From introductory physics: some familiarity with angular momentum, torque, Newton's 2nd law for rotations, vector cross product, dot product, gradient.
Equipment: Bicycle wheel with rope
A particle moves under the influence of a central force directed toward a fixed origin $O$.
Give a Conceptual Explanation: Why is the angular momentum about $O$ constant for the particle? (Remember, conceptual explanations can and should include equations and diagrams.)
Give a Conceptual Explanation: Why must the particle’s orbit make a plane containing $O$?