Torque and Rotational Dynamics · ⁨Mô-men lực và động học quay⁩

Learning Objective 5.1.A: Describe the rotation of a system with respect to time using angular displacement, angular velocity, and angular acceleration.

• 5.1.A.1 Angular displacement is the measurement of the angle, in radians, through which a point on a rigid system rotates about a specified axis.
  • Equation: $\Delta\theta = \theta - \theta_0$
  • 5.1.A.1.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.
  • 5.1.A.1.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.
  • 5.1.A.1.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.
• 5.1.A.2 Average angular velocity is the average rate at which angular position changes with respect to time.
  • Equation: $\omega_{\text{avg}} = \dfrac{\Delta\theta}{\Delta t}$
• 5.1.A.3 Average angular acceleration is the average rate at which the angular velocity changes with respect to time.
  • Equation: $\alpha_{\text{avg}} = \dfrac{\Delta\omega}{\Delta t}$
• 5.1.A.4 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.
  • 5.1.A.4.i For constant angular acceleration, the mathematical relationships between angular displacement, angular velocity, and angular acceleration can be described with the following equations:
    • Equation: $\omega = \omega_0 + \alpha t$
    • Equation: $\theta = \theta_0 + \omega_0 t + \dfrac{1}{2}\alpha t^2$
    • Equation: $\omega^2 = \omega_0^2 + 2\alpha(\theta - \theta_0)$
  • 5.1.A.4.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: Descriptions of the directions of rotation for a point or object are limited to clockwise and counterclockwise with respect to a given axis of rotation.

Learning Objective 5.2.A: Describe the linear motion of a point on a rotating rigid system that corresponds to the rotational motion of that point, and vice versa.

• 5.2.A.1 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 $\Delta\theta$ is given by the equation $\Delta s = r\Delta\theta$.
• 5.2.A.2 Derived relationships of linear velocity and of the tangential component of acceleration to their respective angular quantities are given by the following equations:
  • Equation: $s = r\theta$
  • Equation: $v = r\omega$
  • Equation: $a_T = r\alpha$
• 5.2.A.3 For a rigid system, all points within that system have the same angular velocity and angular acceleration.

Boundary statement: Descriptions of the directions of rotation for a point or object are limited to clockwise and counterclockwise with respect to a given axis of rotation.

Learning Objective 5.3.A: Identify the torques exerted on a rigid system.

• 5.3.A.1 Torque results only from the force component perpendicular to the position vector from the axis of rotation to the point of application of the force.
• 5.3.A.2 The lever arm is the perpendicular distance from the axis of rotation to the line of action of the exerted force.

Learning Objective 5.3.B: Describe the torques exerted on a rigid system.

• 5.3.B.1 Torques can be described using force diagrams.
  • 5.3.B.1.i Force diagrams are similar to free-body diagrams and are used to analyze the torques exerted on a rigid system.
  • 5.3.B.1.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.
• 5.3.B.2 The magnitude of the torque exerted on a rigid system by a force is described by the following equation, where $\theta$ is the angle between the force vector and the position vector from the axis of rotation to the point of application of the force.
  • Equation: $\tau = rF_{\perp} = rF\sin\theta$

Boundary statement: While AP Physics 1 expects students to mathematically manipulate the magnitude of torque using vector conventions, the direction of torque is beyond the scope of the course.

Learning Objective 5.4.A: Describe the rotational inertia of a rigid system relative to a given axis of rotation.

• 5.4.A.1 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.
• 5.4.A.2 The rotational inertia of an object rotating a perpendicular distance $r$ from an axis is described by the equation
  • Equation: $I = mr^2$
• 5.4.A.3 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:
  • Equation: $I_{\text{tot}} = \sum I_i = \sum m_i r_i^2$

Learning Objective 5.4.B: Describe the rotational inertia of a rigid system rotating about an axis that does not pass through the system's center of mass.

• 5.4.B.1 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.
• 5.4.B.2 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:
  • Equation: $I' = I_{\text{cm}} + Md^2$

Boundary statement: AP Physics 1 only expects students to calculate the rotational inertia for systems of five or fewer objects arranged in a two-dimensional configuration.

Boundary statement: Students do not need to know the rotational inertia of extended rigid systems, as these will be provided within the exam. 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 disk of the same mass and radius.

Learning Objective 5.5.A: Describe the conditions under which a system's angular velocity remains constant.

• 5.5.A.1 A system may exhibit rotational equilibrium (constant angular velocity) without being in translational equilibrium, and vice versa.
  • 5.5.A.1.i Free-body and force diagrams describe the nature of the forces and torques exerted on an object or rigid system.
  • 5.5.A.1.ii Rotational equilibrium is a configuration of torques such that the net torque exerted on the system is zero.
    • Equation: $\sum \tau_i = 0$
  • 5.5.A.1.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.
• 5.5.A.2 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.

Boundary statement: AP Physics 1 does not expect students to simultaneously analyze rotation in multiple planes.

Learning Objective 5.6.A: Describe the conditions under which a system's angular velocity changes.

• 5.6.A.1 Angular velocity changes when the net torque exerted on the object or system is not equal to zero.
• 5.6.A.2 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.
  • Equation: $\alpha_{\text{sys}} = \dfrac{\sum \tau}{I_{\text{sys}}} = \dfrac{\tau_{\text{net}}}{I_{\text{sys}}}$
• 5.6.A.3 To fully describe a rotating rigid system, linear and rotational analyses may need to be performed independently.