Section outline

  • Developing Understanding

    In Unit 6, students will apply their understanding of energy and momentum to the context of rotating systems. Much like the approach taken with translational energy and momentum in Units 3 and 4, this unit emphasizes a conceptual grasp of how angular momentum and rotational energy respond to external torques acting on a system. It is equally important for students to articulate the specific conditions under which a system’s rotational energy or angular momentum will remain unchanged, as this forms the basis for tackling more complex scenarios.

    Students will draw upon the content and skills from both Units 5 and 6 to deepen their exploration into phenomena such as the motion of orbiting satellites and rolling without slipping.

    Building the Science Practices

    Unit 6 offers students opportunities to:
    •    Compare physical quantities either between different scenarios or at various times within a single scenario (Practice 2.C).
    •    Determine new values for quantities by analyzing how variables functionally depend on one another (Practice 2.D).

    Students are encouraged to make and justify claims rooted in these physical principles and functional relationships (Practices 3.B, 3.C). For instance, they might be tasked with conceptually explaining the effect of moving the pivot point on a system’s rotational inertia, and then justifying the resultant changes in angular acceleration.

    By the conclusion of the unit, students should feel comfortable assessing the reasonableness of their claims and justifications, while grounding their arguments in the fundamental principles of physics (Practices 2.D, 3.C).

    Preparing for the AP Exam

    Both the multiple-choice and free-response sections of the AP Physics C: Mechanics Exam require students to describe relationships between physical quantities and to analyze the effects of changing one quantity in a given scenario. Practice in investigating system changes and applying fundamental physics principles will help students decide whether a particular quantity increases, decreases, or remains constant as other variables shift.

    Additionally, when constructing justifications for claims, students should avoid referencing equations, laws, or principles in a superficial way. For example, merely stating that one disk rolls faster than another due to “conservation of energy” does not provide a complete answer on the exam’s free-response section. Students must clearly and concisely lay out the logical steps that connect the physical principle to the justification of their claim.

    • 6.1 Rotational Kinetic Energy Assignment

      6.1.A Describe the rotational kinetic energy of a rigid system in terms of the rotational inertia and angular velocity of that rigid system.

    • 6.2 Torque and Work Assignment

      6.2.A Describe the work done on a rigid system by a given torque or collection of torques. 

    • 6.3 Angular Momentum and Angular Impulse Assignment

      6.3.A Describe the angular momentum of an object or rigid system.

      6.3.B Describe the angular impulse delivered to an object or rigid system by a torque.

      6.3.C Relate the change in angular momentum of an object or rigid system to the angular impulse given to that object or rigid system.

    • 6.4 Conservation of Angular Momentum Assignment

      6.4.A Describe the behavior of a system using conservation of angular momentum.

      6.4.B Describe how the selection of a system determines whether the angular momentum of that system changes.

    • 6.5 Rolling Assignment

      6.5.A Describe the kinetic energy of a system that has translational and rotational motion.

      6.5.B Describe the motion of a system that is rolling without slipping.

      6.5.C Describe the motion of a system that is rolling while slipping.

    • 6.6 Motion of Orbiting Satellites Assignment

      6.6.A Describe the motions of a system consisting of two objects or systems interacting only via gravitational forces.