Angular Velocity of Disk After Bullet Impact: Calculation Explanation

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Homework Help Overview

The problem involves a disk with a bullet impacting it tangentially, raising questions about the resulting angular velocity of the disk after the collision. The subject area pertains to angular momentum and rotational dynamics.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the conservation of angular momentum as a method to analyze the problem. There are attempts to calculate the angular momentum of the bullet before impact and the combined system after the collision. Questions arise regarding the expression for angular momentum of the rotating disk and how the bullet affects the mass distribution.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of angular momentum and its implications for the system. Some guidance has been offered regarding the need to clarify the angular momentum of the disk alone and how the bullet's impact alters the system's dynamics.

Contextual Notes

Participants are navigating through assumptions about the system's mass distribution and the definitions of angular momentum in the context of a rotating disk with an additional mass. There is an emphasis on understanding the implications of the bullet's impact on the overall dynamics.

khurram usman
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A disk of radius R and mass M is mounted with its axis vertical.It is inially at rest.A bullet of mass m and velocity v is fired horzentally and tengential to disk .It enters the disc at its perimeter.What angular velocity will be acquired by disk?
1=mv/(m+M)R
2=mv/(2M+m)R
3=Mv/mR
4=2mv/(MR+2mR)
5=(m+2M)v/MR

please explain how u compute the answer
 
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Have you considered applying the conservation of angular momentum?

Hint: what is the angular momentum of the bullet around the disc's centre just prior to impact? What is the angular momentum due to the combined rotation of the disc+bullet around the disc's centre after the collision?
 
cepheid said:
Have you considered applying the conservation of angular momentum?

Hint: what is the angular momentum of the bullet around the disc's centre just prior to impact? What is the angular momentum due to the combined rotation of the disc+bullet around the disc's centre after the collision?

ok...i see the approach
the angular momentum of bullet at tangent to disk will be mvR
now how to calculate velocity?
 
Last edited by a moderator:
khurram usman said:
the angular momentum of bullet at tangent to disk will be mvR
Good.
now how to calculate velocity?
First answer cepheid's second question.
 
Doc Al said:
Good.

First answer cepheid's second question.

well as i see it the combined angular momentum after collision will be (m+M)Rv'
correct??
 
khurram usman said:
well as i see it the combined angular momentum after collision will be (m+M)Rv'
correct??
No, not quite. How would you express the angular momentum of a rotating disk? How does the presence of the bullet stuck onto the disk change that?
 
Doc Al said:
No, not quite. How would you express the angular momentum of a rotating disk? How does the presence of the bullet stuck onto the disk change that?

maybe the bullet changes mass distibution
i am not quite getting it
 
khurram usman said:
maybe the bullet changes mass distibution
Sure it does.

But before worrying about disk + bullet, how would you express the angular momentum of a rotating disk alone?
 

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