⚙️AP Physics C: Mechanics (2025) Unit 6 – Rotating Systems: Energy & Momentum

Key Concepts

• Understand the relationship between linear and rotational motion involves translating linear quantities (displacement, velocity, acceleration) into their rotational counterparts (angular displacement, angular velocity, angular acceleration)
• Recognize that torque is the rotational equivalent of force and causes an object to rotate about an axis
  • Torque depends on the magnitude of the force and the perpendicular distance from the axis of rotation to the line of action of the force (moment arm)
• Identify the moment of inertia as a measure of an object's resistance to rotational acceleration
  • Moment of inertia depends on the mass distribution of the object and the axis of rotation
• Apply the rotational kinematic equations to solve problems involving angular displacement, angular velocity, and angular acceleration
• Understand that rotational kinetic energy is the energy associated with the rotational motion of an object
  • Rotational kinetic energy depends on the object's moment of inertia and angular velocity
• Recognize that angular momentum is a conserved quantity in the absence of external torques
  • Angular momentum depends on the object's moment of inertia and angular velocity

Rotational Kinematics

• Angular displacement $(\Delta \theta)$ measures the change in angular position of an object
  • Measured in radians (rad) or degrees (°)
• Angular velocity $(\omega)$ describes the rate of change of angular displacement
  • Measured in radians per second (rad/s) or degrees per second (°/s)
• Angular acceleration $(\alpha)$ describes the rate of change of angular velocity
  • Measured in radians per second squared (rad/s²) or degrees per second squared (°/s²)
• Rotational kinematic equations relate angular displacement, angular velocity, and angular acceleration
  • $\Delta \theta = \omega_0 t + \frac{1}{2} \alpha t^2$
  • $\omega_f = \omega_0 + \alpha t$
  • $\omega_f^2 = \omega_0^2 + 2\alpha \Delta \theta$
• Tangential velocity $(v_t)$ and tangential acceleration $(a_t)$ describe the linear motion of a point on a rotating object
  • $v_t = r\omega$
  • $a_t = r\alpha$

Torque and Rotational Dynamics

• Torque $(\tau)$ is the rotational equivalent of force and causes an object to rotate about an axis
  • Measured in newton-meters (N·m)
  • $\tau = rF\sin\theta$ where $r$ is the moment arm, $F$ is the force, and $\theta$ is the angle between the force and the moment arm
• Net torque $(\sum \tau)$ determines the angular acceleration of an object
  • $\sum \tau = I\alpha$ where $I$ is the moment of inertia and $\alpha$ is the angular acceleration
• Moment of inertia $(I)$ is a measure of an object's resistance to rotational acceleration
  • Depends on the mass distribution of the object and the axis of rotation
  • For a point mass: $I = mr^2$ where $m$ is the mass and $r$ is the distance from the axis of rotation
  • For extended objects, the moment of inertia is calculated by integrating over the mass distribution
• Parallel-axis theorem allows for calculating the moment of inertia about any parallel axis given the moment of inertia about the center of mass
  • $I = I_{CM} + Md^2$ where $I_{CM}$ is the moment of inertia about the center of mass, $M$ is the total mass, and $d$ is the distance between the parallel axes

Rotational Energy

• Rotational kinetic energy $(KE_{rot})$ is the energy associated with the rotational motion of an object
  • $KE_{rot} = \frac{1}{2}I\omega^2$ where $I$ is the moment of inertia and $\omega$ is the angular velocity
• Work-energy theorem for rotational motion relates the net work done by torques to the change in rotational kinetic energy
  • $W_{net} = \Delta KE_{rot}$
• Power in rotational motion is the rate at which work is done by a torque
  • $P = \tau\omega$ where $\tau$ is the torque and $\omega$ is the angular velocity
• Conservation of mechanical energy applies to rotational motion when no non-conservative forces are present
  • $KE_{rot,i} + PE_i = KE_{rot,f} + PE_f$

Angular Momentum

• Angular momentum $(L)$ is a vector quantity that describes the rotational motion of an object
  • For a point mass: $L = mvr$ where $m$ is the mass, $v$ is the velocity, and $r$ is the perpendicular distance from the axis of rotation
  • For an extended object: $L = I\omega$ where $I$ is the moment of inertia and $\omega$ is the angular velocity
• Angular impulse $(\Delta L)$ is the change in angular momentum caused by a torque acting over a time interval
  • $\Delta L = \int \tau dt$
• Conservation of angular momentum states that the total angular momentum of a system remains constant in the absence of external torques
  • $L_{i} = L_{f}$
• Rotational inertia $(I)$ determines an object's resistance to changes in angular velocity
  • Objects with larger rotational inertia require greater torques to achieve the same angular acceleration

Conservation Laws in Rotation

• Conservation of angular momentum is a fundamental principle in rotational motion
  • In the absence of external torques, the total angular momentum of a system remains constant
  • $L_{i} = L_{f}$
• Conservation of mechanical energy applies to rotational motion when no non-conservative forces are present
  • $KE_{rot,i} + PE_i = KE_{rot,f} + PE_f$
• When both angular momentum and mechanical energy are conserved, the initial and final states of a system can be related without considering the intermediate steps
• Conservation laws simplify problem-solving by reducing the number of variables and equations needed
• In real-world situations, non-conservative forces (friction, air resistance) may cause mechanical energy to dissipate, while external torques may change the angular momentum of a system

Applications and Problem-Solving

• Identify the relevant rotational quantities (angular displacement, angular velocity, angular acceleration, torque, moment of inertia, angular momentum) in a given problem
• Apply the appropriate equations and conservation laws to solve for unknown quantities
  • Rotational kinematic equations
  • Newton's second law for rotation $(\sum \tau = I\alpha)$
  • Work-energy theorem for rotational motion
  • Conservation of angular momentum and mechanical energy
• Consider the effects of non-conservative forces and external torques on the system
  • Friction and air resistance may cause mechanical energy to dissipate
  • External torques may change the angular momentum of the system
• Analyze the motion of connected objects (gears, pulleys, belts) by relating their angular velocities and torques
• Solve problems involving rolling motion by combining translational and rotational equations
  • For rolling without slipping: $v_{CM} = R\omega$ where $v_{CM}$ is the center of mass velocity, $R$ is the radius, and $\omega$ is the angular velocity

Common Misconceptions and Tips

• Remember that torque and angular acceleration are vector quantities with a specified direction (clockwise or counterclockwise)
• Pay attention to the axis of rotation when calculating moments of inertia and applying rotational equations
  • The moment of inertia depends on the axis of rotation and the mass distribution of the object
• Be careful when using the parallel-axis theorem to calculate moments of inertia
  • The theorem relates moments of inertia about parallel axes, not perpendicular axes
• Distinguish between angular velocity $(\omega)$ and angular speed $(|\omega|)$
  • Angular velocity is a vector quantity with a specified direction, while angular speed is a scalar quantity
• Remember that the moment of inertia is not always constant
  • If the mass distribution of an object changes during rotation (e.g., a figure skater extending their arms), the moment of inertia will change as well
• When applying conservation laws, clearly define your system and identify any external forces or torques acting on it
• Practice solving a variety of problems to develop a strong understanding of rotational motion concepts and problem-solving strategies