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Q5.8(b):
Answer carefully, with reasons : Is the total linear momentum conserved during the short time of an elastic collision of two balls ?

Solution :

Theoretical Foundation: The Principle of Conservation of Linear Momentum

According to Newton's Second Law of Motion in its generalized form, the force acting on a system is equal to the time rate of change of linear momentum ($F = \frac{dp}{dt}$). For a system to conserve its linear momentum, the net external force acting on the system must be zero ($\sum F_{ext} = 0$).

Step 1: Analyzing the Forces during Elastic Collision

When two balls (let them be sphere $A$ and sphere $B$) undergo an elastic collision, they exert equal and opposite impulsive forces on each other over a very short time interval $\Delta t$.

  • Let $\vec{F}_{BA}$ be the force exerted by ball $B$ on ball $A$.
  • Let $\vec{F}_{AB}$ be the force exerted by ball $A$ on ball $B$.
  • By [Newton’s Third Law of Motion], $\vec{F}_{BA} = -\vec{F}_{AB}$.

Step 2: Evaluating External Influences

The collision process involves internal forces (the contact forces between the two balls). While gravity and normal forces from the ground may act on the balls, their magnitudes are typically negligible compared to the impulsive forces generated during the instantaneous collision, or they effectively cancel out if the collision is treated as a horizontal interaction [Impulse-Momentum Theorem].

A B $\vec{p}_A$ $\vec{p}_B$

Step 3: Logical Synthesis

Since the internal forces ($F_{AB}$ and $F_{BA}$) exist only between the bodies of the system and sum to zero, the change in total momentum ($\Delta \vec{P}_{total}$) is:

$\Delta \vec{P}_{total} = \int_{t_i}^{t_f} (\vec{F}_{internal} + \vec{F}_{external}) \, dt$

Given that the interaction is isolated and internal to the system of the two balls, and considering the short time duration $\Delta t$, the effect of any external force is effectively zero. Thus, the total linear momentum is conserved.

Analytical Conclusion

In any collision, whether elastic or inelastic, the total linear momentum of a system is conserved if no net external force acts on the system. The "elastic" nature of the collision specifically dictates the conservation of total kinetic energy, but the conservation of linear momentum holds independently of the energy transformation type [Law of Conservation of Momentum].

Final Solution: Yes, the total linear momentum is conserved during the short time of an elastic collision of two balls because the impulsive forces acting between the balls are internal to the system, and the net external force on the system is zero.


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