5 Torque and Rotational Dynamics

Describe the rotation of a system with respect to time using angular displacement, angular velocity, and angular acceleration.

• Angular displacement is the measurement of the angle, in radians, through which a point on a rigid system rotates about a specified axis. Relevant equation:
  • i. A rigid system is one that holds its shape but in which different points on the system move in different directions during rotation. A rigid system cannot be modeled as an object.
  • ii. One direction of angular displacement about an axis of rotation—clockwise or counterclockwise—is typically indicated as mathematically positive, with the other direction becoming mathematically negative.
  • iii. If the rotation of a system about an axis may be well described using the motion of the system’s center of mass, the system may be treated as a single object. For example, the rotation of Earth about its axis may be considered negligible when considering the revolution of Earth about the center of mass of the Earth–Sun system. TOPIC 5.1 Rotational Kinematics
• Angular velocity is the rate at which angular position changes with respect to time. Relevant equation:
• Angular acceleration is the rate at which angular velocity changes with respect to time. Relevant equation:
• Angular displacement, angular velocity, and angular acceleration around one axis are analogous to linear displacement, velocity, and acceleration in one dimension and demonstrate the same mathematical relationships.
  • i. For constant angular acceleration, the mathematical relationships between angular displacement, angular velocity, and angular acceleration can be described with the following equations:
  • ii. Graphs of angular displacement, angular velocity, and angular acceleration as functions of time can be used to find the relationships between those quantities. BOUNDARY STATEMENT AP Physics C: Mechanics expects students to be able to mathematically manipulate the magnitudes of angular displacement, angular velocity, and angular acceleration using vector conventions. However, the directions of said vectors will not be assessed on the exam. Descriptions of the directions of rotational kinematics quantities for a point or rigid body are limited to clockwise and counterclockwise with respect to a given axis of rotation.

Describe the linear motion of a point on a rotating rigid system that corresponds to the rotational motion of that point, and vice versa.

• For a point at a distance r from a fixed axis of rotation, the linear distance s traveled by the point as the system rotates through an angle is given by the equation
• Derived relationships of linear velocity and of the tangential component of acceleration to their respective angular quantities are given by the following equations:
• For a rigid system, all points within that system have the same angular velocity and angular acceleration. TOPIC 5.2 Connecting Linear and Rotational Motion TOPIC 5.3 Torque

Identify the torques exerted on a rigid system.

• T orque results only from the force component perpendicular to the position vector from the axis of rotation to the point of application of the force.
• The lever arm is the perpendicular distance from the axis of rotation to the line of action of the exerted force.

Describe the torques exerted on a rigid system.

• T orques can be described using force diagrams.
  • i. Force diagrams are similar to free-body diagrams and are used to analyze the torques exerted on a rigid system.
  • ii. Similar to free-body diagrams, force diagrams represent the relative magnitude and direction of the forces exerted on a rigid system. Force diagrams also depict the location at which those forces are exerted relative to the axis of rotation.
• The torque exerted on a rigid system about a chosen pivot point by a given force is described by
  • i. The cross -product between two vectors, A and B, results in a vector quantity of magnitude
  • ii. The direction of the vector resulting from the cross-product of vectors A  and B  is perpendicular to both vectors A  and B  and therefore is normal to the plane defined by vectors A  and B  .
  • iii. The direction of the vector resulting from the cross-product of vectors A  and B  can be qualitatively determined by applying the appropriate right-hand rule.

Describe the rotational inertia of a rigid system rotating about an axis that does not pass through the system’s center of mass. TOPIC 5.4 Rotational Inertia

• A rigid system’s rotational inertia in a given plane is at a minimum when the rotational axis passes through the system’s center of mass.
• The parallel axis theorem uses the following equation to relate the rotational inertia of a rigid system about any axis that is parallel to an axis through its center of mass: BOUNDARY STATEMENT AP Physics C: Mechanics only expects students to use calculus in the derivations of the rotational inertia of thin rods of uniform or nonuniform density about an arbitrary axis perpendicular to the rod, as well as derivations of the rotational inertia of a thin cylindrical shell, disk, or rigid bodies that can be considered to be made up of coaxial rings or shells about an axis that passes through their centers (e.g., annular rings). Students should have a qualitative understanding of the factors that affect rotational inertia; for example, how rotational inertia is greater when mass is farther from the axis of rotation, which is why a hoop has more rotational inertia than a solid puck of the same mass and radius.

Describe the rotational inertia of a rigid system relative to a given axis of rotation.

• Rotational inertia measures a rigid system’s resistance to changes in rotation and is related to the mass of the system and the distribution of that mass relative to the axis of rotation.
• The rotational inertia of an object rotating a perpendicular distance r from an axis is described by the equation Im r2= .
• The total rotational inertia of a collection of objects about an axis is the sum of the rotational inertias of each object about that axis.
• For a solid that can be considered as a collection of differential masses, dm, the solid’s rotational inertia can be calculated using the equation where r is the perpendicular distance from dm to the axis of rotation.

Describe the conditions under which a system’s angular velocity remains constant.

• A system may exhibit rotational equilibrium (constant angular velocity) without being in translational equilibrium, and vice versa.
  • i. Free-body and force diagrams describe the nature of the forces and torques exerted on an object or rigid system.
  • ii. Rotational equilibrium is a configuration of torques such that the net torque exerted on the system is zero. Relevant equation:
  • iii. The rotational analog of Newton’s first law is that a system will have a constant angular velocity only if the net torque exerted on the system is zero.
• A rotational corollary to Newton’s second law states that if the torques exerted on a rigid system are not balanced, the system’s angular velocity must be changing.

Describe the conditions under which a system’s angular velocity changes.

• Angular velocity changes when the net torque exerted on the object or system is not equal to zero.
• The rate at which the angular velocity of a rigid system changes is directly proportional to the net torque exerted on the rigid system and is in the same direction. The angular acceleration of the rigid system is inversely proportional to the rotational inertia of the rigid system. Relevant equation:
• T o fully describe a rotating rigid system, linear and rotational analyses may need to be performed independently. TOPIC 5.6 Newton’s Second Law in Rotational Form