# 32.4 Angular Momentum of Circular Motion | Summary and Q&A

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June 2, 2017
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MIT OpenCourseWare
32.4 Angular Momentum of Circular Motion

## TL;DR

Angular momentum in circular motion is calculated using the radius, mass, and angular velocity of the particle.

## Key Insights

• 😵 Angular momentum in circular motion is calculated using the cross product of the radius and momentum vectors.
• 📐 Cylindrical coordinates simplify the calculation of angular momentum in circular motion.
• 💆 The magnitude of momentum in circular motion is equal to the product of mass, radius, and angular velocity.
• ✈️ Angular momentum is always perpendicular to the plane of motion in circular motion.
• 📐 The angular momentum is directly proportional to the angular velocity in circular motion.
• 🫱 The direction of angular momentum in circular motion is determined by the right-hand rule.
• 😒 Circular motion with a central point allows for the use of cylindrical coordinates in the calculation of angular momentum.

## Transcript

Well, we've been calculating angular momentum about a point. Recall our definition of angular momentum. We've been looking at cases where the particle is moving linearly. Now let's look at a case where the particles are undergoing circular motion. So suppose we have a particle that's undergoing circular motion m. And let's choose some axes. And our... Read More

### Q: How is angular momentum calculated in circular motion?

Angular momentum in circular motion is calculated using the cross product of the radius vector and the momentum vector.

The cross product is simplified by using cylindrical coordinates and the unit vectors r hat and theta hat.

### Q: Why do we use cylindrical coordinates in circular motion calculations?

Cylindrical coordinates are used in circular motion calculations because they simplify the cross product calculation. By defining an angle theta and unit vectors r hat and theta hat, we can easily express the vectors involved in circular motion.

### Q: How does the magnitude of momentum in circular motion relate to the angular velocity?

The magnitude of momentum in circular motion is equal to the product of mass, radius, and angular velocity.

Mathematically, the magnitude of momentum (|p|) is equal to m * r * ω, where m is the mass of the particle, r is the radius of the circle, and ω is the angular velocity.

### Q: Why is angular momentum perpendicular to the plane of motion in circular motion?

Angular momentum is perpendicular to the plane of motion in circular motion because it is defined as the cross product of the radius vector and the momentum vector.

Since both the radius vector and the momentum vector are in the plane of motion, their cross product (angular momentum) has to be perpendicular to that plane.

## Summary & Key Takeaways

• Angular momentum is defined as the cross product of the vector radius and the vector momentum in circular motion.

• Cylindrical coordinates, including an angle theta and unit vectors r hat and theta hat, are used to simplify the calculation.

• The angular momentum is directly proportional to the angular velocity in circular motion.