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Q6.6(c):
Underline the correct alternative : (c) The rate of change of total momentum of a many-particle system is proportional to the...

Solution :

Step 1: Identifying the System Dynamics

To determine the rate of change of the total momentum of a many-particle system, we define the total momentum $\mathbf{P}$ of a system consisting of $N$ particles as the vector sum of individual momenta: $$\mathbf{P} = \sum_{i=1}^{N} \mathbf{p}_i = \sum_{i=1}^{N} m_i \mathbf{v}_i$$ [By the definition of linear momentum in classical mechanics].

Step 2: Differentiating with respect to Time

The rate of change of the total momentum is given by the derivative of $\mathbf{P}$ with respect to time $t$: $$\frac{d\mathbf{P}}{dt} = \sum_{i=1}^{N} \frac{d\mathbf{p}_i}{dt}$$ [By the linearity property of the derivative operator].

Step 3: Applying Newton’s Second Law

According to Newton's Second Law for each particle $i$, the force acting on it is the sum of internal forces ($\mathbf{f}_{ij}$, force exerted by particle $j$ on $i$) and external forces ($\mathbf{F}_{i, ext}$): $$\frac{d\mathbf{p}_i}{dt} = \mathbf{F}_{i, ext} + \sum_{j \neq i} \mathbf{f}_{ij}$$ Substituting this into the summation: $$\frac{d\mathbf{P}}{dt} = \sum_{i=1}^{N} \mathbf{F}_{i, ext} + \sum_{i=1}^{N} \sum_{j \neq i} \mathbf{f}_{ij}$$

Step 4: Analyzing Internal Forces

Per Newton’s Third Law of Motion, for every internal force $\mathbf{f}_{ij}$, there exists an equal and opposite reaction force $\mathbf{f}_{ji}$ such that $\mathbf{f}_{ij} = -\mathbf{f}_{ji}$. When summing over all pairs $(i, j)$ in the system, these forces cancel out exactly: $$\sum_{i=1}^{N} \sum_{j \neq i} \mathbf{f}_{ij} = 0$$ Consequently, the internal forces contribute nothing to the change in total momentum of the system. We are left with: $$\frac{d\mathbf{P}}{dt} = \sum_{i=1}^{N} \mathbf{F}_{i, ext} = \mathbf{F}_{ext}$$ where $\mathbf{F}_{ext}$ is the vector sum of all external forces acting on the system.

Visual Representation of System Forces

External Force F1 External Force F2 Many-Particle System Boundary

Conclusion

The rate of change of total momentum is equal to (and thus proportional to) the sum of external forces acting on the system. Internal forces are irrelevant to the change in the total momentum of the system.

Final Solution: The rate of change of total momentum of a many-particle system is proportional to the external force acting on the system.


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