Conservation of angular momentum | Torque and angular momentum | AP Physics 1 | Khan Academy

TL;DR

Angular momentum is conserved when there is no external torque applied to a system, leading to phenomena such as an ice skater's increased angular speed when tucking their arms or a closer orbiting object rotating faster.

Transcript

• [Instructor] Let's talk a little bit about the conservation of angular momentum. And this is going to be really useful, because it explains diverse phenomena in the universe. From why an ice skater's angular speed goes up when they tuck their arms or their legs in, all the way to when you have something orbiting around a star, and the closer and ... Read More

Key Insights

• 📐 Conservation of angular momentum applies when no external torque is applied to a system.
• 📐 When two objects collide and combine, their combined angular momentum remains constant.
• 📐 Decreasing the moment of inertia leads to an increase in angular speed to maintain constant angular momentum.
• 🐎 Angular speed increases as the radius decreases in an orbiting system.

Questions & Answers

Q: How does angular momentum change when two objects collide and combine?

When two objects collide and combine, if no external torque is applied, the angular momentum of the system remains constant. The combined system rotates with the same angular momentum as the individual objects before the collision.

Q: Why does an ice skater's angular speed increase when they tuck in their arms?

When an ice skater tucks in their arms, the moment of inertia decreases. Since angular momentum is conserved with no external torque, the angular speed must increase to compensate for the decreased moment of inertia.

Q: Can angular momentum change if only one object in a system experiences a torque?

Yes, if only one object in a system experiences a torque, the angular momentum of that specific object can change. However, if the entire system (including all objects and their torques) is considered, the total angular momentum will remain constant.

Q: How does the moment of inertia affect angular momentum?

The moment of inertia, which depends on the mass and distribution of mass in an object, determines how much torque is needed to produce angular acceleration. A higher moment of inertia requires more torque to change the angular velocity, affecting angular momentum.

Summary & Key Takeaways

• Angular momentum remains constant in a system as long as no external torque is applied.

• When two objects collide and combine, their angular momentum will remain the same.

• Angular speed increases when the moment of inertia decreases, such as when an ice skater tucks in their arms.