Summary of 2018 AP Physics 1 Free Response #3 (2024)

Takeaways

Q & A

In this educational video, Alan from Bothell STEMcoach discusses AP Physics 1, specifically focusing on the 2018 free response question number three. The scenario involves a spinning disk with frictional torque exerted by an axle, leading to a constant angular deceleration until the disk comes to rest. Alan explains the relationship between torque, moment of inertia, and angular acceleration, emphasizing that with a constant torque, the angular acceleration must also be constant. He guides viewers through the process of sketching graphs for angular velocity and acceleration, illustrating how the disk's motion can be represented over time. The video also covers deriving an equation for the rotational inertia of the disk in terms of given parameters, such as the initial angular velocity, torque, and time to rest.

The second paragraph of the video script delves into a follow-up experiment where oil is applied to reduce friction between the axle and the spinning disk. As the friction decreases, the torque exerted by the axle also diminishes, leading to a reduction in the disk's angular acceleration. Alan sketches a graph to represent the change in angular velocity over time, suggesting that the acceleration curve will decrease as friction is reduced. He discusses the student's attempt to model the torque over time with two equations, one suggesting an increase and the other a decrease in torque. Alan concludes that the equation where torque decreases over time is the correct model for this experiment, aligning with the observed reduction in friction. The video ends with a quick review of the scoring for the question and an invitation for viewers to check the solution keys on the website, encouraging comments, likes, or subscriptions for further educational content.

💡AP Physics 1

AP Physics 1 is a college-level course and examination offered by the College Board as part of the Advanced Placement Program. It is designed to provide students with a foundational understanding of physics concepts. In the video, the theme revolves around solving a free response question from the 2018 AP Physics 1 exam, which is a significant part of the educational content.

💡Free Response Question

In the context of AP exams, a free response question is an open-ended problem that requires students to provide a detailed answer, often involving calculations or explanations. The video focuses on question number three of the 2018 AP Physics 1 exam, which is a free response type, requiring the application of physics principles to solve.

💡Torque

Torque is a measure of the force that can cause an object to rotate about an axis. It is a vector quantity and is calculated as the cross product of the radius vector and the force vector. In the video, torque is discussed in relation to the friction between the axle and the disc, which exerts a constant torque on the spinning disk.

💡Angular Velocity

Angular velocity is the rate of change of an angle with respect to time and is a vector quantity that represents how fast an object is rotating. In the video, the initial counter-clockwise angular velocity of the disk is denoted as Omega naught, and the problem involves sketching a graph of angular velocity as a function of time until the disk comes to rest.

💡Rotational Inertia

Rotational inertia, also known as moment of inertia, is a measure of an object's resistance to changes in its rotation. It depends on the mass distribution of the object and is used in the formula relating torque and angular acceleration. The video explains that if the torque is constant, the angular acceleration must also be constant, indicating the disk's rotational inertia.

💡Friction

Friction is the force that resists the relative motion of two surfaces in contact. In the context of the video, friction between the axle and the disc is responsible for the constant torque acting on the spinning disk, which ultimately brings the disk to rest.

💡Graph

A graph is a visual representation of data, typically involving a set of axes representing variables. In the video, the task is to sketch graphs representing the disk's angular velocity and angular acceleration as functions of time, which helps visualize the physical behavior of the system.

💡Kinematic Equation

Kinematic equations relate the motion of an object to its initial conditions and the forces acting upon it. In the video, the angular velocity equation, which is the rotational equivalent of the linear kinematic equation, is used to describe the change in angular velocity over time due to constant angular acceleration.

💡Oil

In the second part of the video, oil is introduced to the contact surface between the axle and the disc to reduce friction. This action changes the torque exerted on the disc over time, which is then reflected in the graph of angular velocity as a function of time.

💡Constant

In the context of the video, a constant refers to a value that does not change over time. The video mentions a constant torque exerted by the frictional force at the beginning and later discusses how the introduction of oil changes this scenario, leading to a non-constant torque.

💡Acceleration

Acceleration is the rate of change of velocity over time. In the video, angular acceleration is discussed as the constant rate of change of angular velocity, which is represented by the slope of the graph for the disk's motion. The reduction of friction due to oil application affects this acceleration, causing it to decrease over time.

Alan introduces the AP Physics 1 2018 free response question number three.

The disk spins around an axle with friction between them, exerting a constant torque.

At time T equals zero, the disk has an initial counter clockwise angular velocity.

Students are asked to sketch a graph of the disk's angular velocity over time until it comes to rest.

The relationship between torque and angular acceleration is discussed, with torque being constant.

The angular acceleration is negative and constant, indicating the disk is slowing down.

The magnitude of the frictional torque is derived using the equation tau equals negative tau over I.

An equation for the rotational inertia I of the disk is derived in terms of given variables.

The acceleration is the slope of the angular velocity graph, which is negative due to the disk slowing down.

The torque exerted on the disk decreases as oil is dripped on the axle-disc contact surface.

A new graph is sketched to represent the disk's angular velocity as friction is reduced by oil.

The torque decreases over time as more oil reaches the contact surface.

Two equations are presented to model the torque over time, with one being correct.

Equation two is identified as the correct model, showing torque decreasing over time.

The scoring criteria for the question are briefly reviewed, ensuring all points are covered.

A summary of the solution process is provided for review and understanding.

Alan encourages viewers to check the solution key on the website for further review.

The video concludes with a prompt for comments, likes, or subscriptions for the next video.

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AP PhysicsFriction StudyTorque AnalysisAngular VelocityRotational InertiaKinematic EquationsEducational VideoPhysics ExperimentBothell StemAlan Coach