Impulse and Momentum of a bullet

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Homework Help Overview

The problem involves a bullet striking a wooden block in a ballistic pendulum setup, focusing on the concepts of impulse and momentum. Participants are tasked with determining the speed of the bullet/block combination after the collision and the height it rises to afterward.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss the definitions of initial and final velocities in the context of the collision, questioning whether the given speed of the bullet is before or after the impact. There is an exploration of the distinction between the bullet's velocity before the collision and the combined velocity after the collision.

Discussion Status

The discussion is actively clarifying the terminology related to initial and final velocities, with some participants suggesting that the terms may be misleading in this context. There is a focus on understanding the implications of these definitions for applying conservation of momentum.

Contextual Notes

Participants note the potential confusion arising from the use of "initial" and "final" in relation to a sudden event like a collision, as opposed to a gradual process. This highlights the need for careful consideration of terminology in physics problems.

Cheddar
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Homework Statement


A 2.5 g bullet, traveling at a speed of 425 m/s, strikes the wooden block (mass = 215 g) of a ballistic pendulum. (A) Find the speed of the bullet/block combination immediately after the collision. (B) How high does the combination rise above its initial position?


Homework Equations


initial velocity = (mass1 + mass2 / mass1) * final velocity
final velocity = square root of (2 * gravity * final height)

The Attempt at a Solution


is the 425 m/s the initial or final velocity?
 
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Cheddar said:
is the 425 m/s the initial or final velocity?


Well, is it the velocity before or after the collision?
 
It is the velocity before the collision. If it is the velocity when it strikes the block then it is the final velocity in its own equation. Final velocity before impact and final velocity of the bullet/block combination are two differenct variables in two different equations.
 
Initial and final are relative to the collision. The 425 m/s is the initial velocity of the bullet (before the collision). That's it.

The final velocity of the bullet would be the velocity of the bullet after the collision (which would be the same as the velocity of the block after the collision, since they are now combined.

EDIT: "before and after" may have been better words to use than initial and final (which suggests "at the start", and "at the end"), the reason being that we are not talking about a gradual process that starts and ends (like acceleration), but rather a sudden event (like a collision). But now I'm talking about semantics, not physics. I am taking initial = before and final = after in this context.

Also, saying that the 425 m/s is the final velocity of the bullet "in its own equation" makes no sense, because the only relevant equation here is the one that arises from conservation of momentum.
 
Last edited:

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