How Does a Bullet Impact the Kinetic Energy of a Rotating Disk?

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Homework Help Overview

The problem involves a bullet impacting a rotating disk, examining the effects on the disk's kinetic energy. The disk is described as solid, with specific dimensions and mass, and the bullet's mass and velocities are provided. The scenario raises questions about the interaction between the bullet and the disk, particularly regarding angular momentum and rotational motion.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • Participants discuss the conservation of angular momentum, questioning how it applies to the bullet and disk system. Some suggest considering the torque generated by the bullet's impact and its effect on the disk's rotation. Others question the conditions under which the disk should rotate, particularly the forces acting on it.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the problem. Some have offered insights into the relationship between angular momentum and kinetic energy, while others are questioning the fundamental assumptions about the forces involved in the scenario.

Contextual Notes

Participants are navigating the implications of the bullet's impact on the disk's rotation, considering the initial conditions and the nature of the forces at play. There is an emphasis on understanding the underlying physics rather than reaching a definitive solution.

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Homework Statement



A bullet with 5 g of mass and a velocity of 100m/s goes through a disk that was rested initially the disk is solid and as a radius r=20cm. It's 2cm thick and as a 2kg mass that can spin with no friction in an axis that goes through the center. The bullet goes trough the disk perpendiculary to the axis and in between its thickness at a distance d=10cm of its center with a final speed of 80m/s.

Calculate the kinetic energy of rotation of the disk
 
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EBBAzores said:

Homework Statement



A bullet with 5 g of mass and a velocity of 100m/s goes through a disk that was rested initially the disk is solid and as a radius r=20cm. It's 2cm thick and as a 2kg mass that can spin with no friction in an axis that goes through the center. The bullet goes trough the disk perpendiculary to the axis and in between its thickness at a distance d=10cm of its center with a final speed of 80m/s.

Calculate the kinetic energy of rotation of the disk
What have you done so far? Hint: what can you say about the angular momentum (ie of the bullet before the collision and the bullet+disk after) relative to the disk axis?

AM
 
The angular momentum will variate it self so by saying that, there will be a torque due to the variation and with the torque we can calculate de work of the bullet and by that the kinetic energy of rotation
 
Why should the disk rotate at all? It appears that the only force on the is parallel to the axis.
 

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