In Example 11.5 "A ballistic pendulum" , define the system to be the bullet + block. Of the following choices, when is the momentum conserved for this system? Choose all that apply.

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BlackTom AI · Accepted Answer

First, restating the prompt helps frame what we’re analyzing: In Example 11.5 "A ballistic pendulum," with the system defined as the bullet plus the block, determine when momentum is conserved for this system during the described process. The provided option is: 'At the moment when the bullet impacts the block.'

Consider the physics of the ballistic pendulum step by step. During the instantaneous collision between the bullet and the block, the external forces acting on the system (gravity and the pivot constraint) have negligible impulse over a very short time interval, so the horizontal momentum of the bullet+block system is conserved during that impact. This is the classic moment when the closed-system momentum conservation applies, because the collision time is short enough that external impulses can be ignored relative to the internal exchange of momentum between bullet and block.

After the collision, the bullet+block mass moves together as a single system and begins to swing as a pendulum. During this post-collision swing, the system is subjected to external forces: gravity acts downward and the pivot provides constraint forces. These external impulses, especially from the pivot, do not sum to zero in the horizontal direction over the swing, so horizontal momentum is no longer conserved. The motion is then governed by energy exchange (kinetic to potential) and rotational dynamics about the pivot, not by momentum conservation of the horizontal linear momentum of the entire system.

Because momentum conservation applies specifically where external impulses are negligible, and because the prompt defines the system as bullet+block, the key conservation occurs precisely during the short collision interval, not during the subsequent motion. Therefore, the momentum conservation condition is met at the instant of impact, while it is not maintained during the subsequent swing.

In summary, the reasoning supports the claim that momentum is conserved at the moment of impact due to negligible external impulse over that brief interval, while it is not conserved during the later swinging motion where external forces from gravity and the pivot play a role.

Note: If there were additional answer choices, we would assess each in a similar fashion, distinguishing the instantaneous collision (where momentum can be conserved) from the subsequent motion (where external impulses prevent momentum conservation).

PHYS2305-S2025-Nelson Quiz 08 - Chapter 11 (11.1-11.5)

In Example 11.5 "A ballistic pendulum" , define the system to be the bullet + block. Of the following choices, when is the momentum conserved for this system? Choose all that apply.

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