# DD.2.1 Position in the CM Frame | Summary and Q&A

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June 2, 2017
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MIT OpenCourseWare
DD.2.1 Position in the CM Frame

## TL;DR

The video explains how to describe the motion of two particles relative to their center of mass using position vectors and introduces the concept of reduced mass.

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### Q: What is the significance of finding the relative position vector r1,2?

The relative position vector r1,2 allows us to determine the relative velocity of the particles. Knowing the relative position vector, we can separately calculate the locations of the two objects in the center of mass frame.

### Q: How do the expressions for r1 prime and r2 prime change if the indices 1 and 2 are interchanged?

If the indices 1 and 2 are interchanged, the expressions for r1 prime and r2 prime remain the same, except that r2 prime has a negative sign. This reflects the change in position between the two particles when viewed from the perspective of the other particle.

### Q: What is the reduced mass, and how is it related to the masses of the particles?

The reduced mass, denoted as mu, is the product of the masses of the particles divided by the sum of their masses. It is given by mu = (m1 * m2) / (m1 + m2). The inverse of the reduced mass is equal to the sum of the inverses of the individual masses.

### Q: How are the position vectors r1 prime and r2 prime related to the reduced mass?

The position vector r1 prime is equal to the reduced mass multiplied by the relative position vector r1,2, divided by the mass of particle 1. Similarly, the position vector r2 prime is equal to the negative of the reduced mass multiplied by the relative position vector r1,2, divided by the mass of particle 2.

## Summary & Key Takeaways

• The video introduces two particles and their center of mass, explaining that the goal is to describe the motion of each particle relative to the center of mass.

• The position vectors r1 prime and r2 prime are introduced, representing the position of each particle in the reference frame of the center of mass.

• The video derives expressions for r1 prime and r2 prime in terms of the positions of r1 and r2, using the center of mass position formula and the relative position vector r1,2.