Problem 3. (a) Show (as I will have done in class) that for two particles with only a force between them (and the force of 2 on 1 we call f) then the motion is described by an "internal" Newton's 2nd law, Vem = const, f= µÝrel where vrel = V1 – v2 = r1 - r2, and mįvi + m2V2 m1 + m2 mim2 cm mi + m2 (Since Taylor calls ri - r2 simply r in Chapter 8, he would probably also call Vrel simply v.) (b) Write an internal Lagrangian that yields this equation of motion.

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**Problem 3.**

(a) Show (as I will have done in class) that for two particles with only a force between them (and the force of 2 on 1 we call \( \mathbf{f} \)) then the motion is described by an "internal" Newton’s 2nd law,

\[
\mathbf{v}_{cm} = \text{const.}, \quad \mathbf{f} = \mu \dot{\mathbf{v}}_{rel}
\]

where \( \mathbf{v}_{rel} = \mathbf{v}_1 - \mathbf{v}_2 = \dot{\mathbf{r}}_1 - \dot{\mathbf{r}}_2 \), and

\[
\mathbf{v}_{cm} = \frac{m_1 \mathbf{v}_1 + m_2 \mathbf{v}_2}{m_1 + m_2}, \quad \mu = \frac{m_1 m_2}{m_1 + m_2}
\]

(Since Taylor calls \( \mathbf{r}_1 - \mathbf{r}_2 \) simply \( \mathbf{r} \) in Chapter 8, he would probably also call \( \mathbf{v}_{rel} \) simply \( \mathbf{v} \).)

(b) Write an internal Lagrangian that yields this equation of motion.
Transcribed Image Text:**Problem 3.** (a) Show (as I will have done in class) that for two particles with only a force between them (and the force of 2 on 1 we call \( \mathbf{f} \)) then the motion is described by an "internal" Newton’s 2nd law, \[ \mathbf{v}_{cm} = \text{const.}, \quad \mathbf{f} = \mu \dot{\mathbf{v}}_{rel} \] where \( \mathbf{v}_{rel} = \mathbf{v}_1 - \mathbf{v}_2 = \dot{\mathbf{r}}_1 - \dot{\mathbf{r}}_2 \), and \[ \mathbf{v}_{cm} = \frac{m_1 \mathbf{v}_1 + m_2 \mathbf{v}_2}{m_1 + m_2}, \quad \mu = \frac{m_1 m_2}{m_1 + m_2} \] (Since Taylor calls \( \mathbf{r}_1 - \mathbf{r}_2 \) simply \( \mathbf{r} \) in Chapter 8, he would probably also call \( \mathbf{v}_{rel} \) simply \( \mathbf{v} \).) (b) Write an internal Lagrangian that yields this equation of motion.
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