How Does Bullet Speed Change After Hitting a Pendulum?

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Homework Help Overview

The discussion revolves around a physics problem involving a bullet colliding with a pendulum. The original poster presents a scenario where a bullet passes through a pendulum, and participants explore the implications of energy and momentum conservation in this inelastic collision context.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants discuss using conservation of energy and momentum to analyze the collision, questioning the validity of energy conservation due to the inelastic nature of the collision.
  • Some participants attempt to calculate the height the pendulum reaches after the collision to find its velocity, while others express confusion about the relationship between the bullet's and pendulum's kinetic energies.
  • There are inquiries about the correct application of momentum equations and the implications of the bullet not remaining in the pendulum.

Discussion Status

The discussion is ongoing, with various interpretations being explored. Some participants suggest using both energy and momentum principles, while others emphasize the need to treat the bullet and pendulum as separate entities post-collision. There is a recognition that energy is not conserved during the collision itself, but may be conserved in the system afterward.

Contextual Notes

Participants are navigating the complexities of inelastic collisions, questioning assumptions about energy conservation, and discussing the specific conditions of the problem, such as the bullet passing through the pendulum rather than sticking to it.

  • #31
zaddyzad said:
because with that I am not mixing bullet with the pendulum, and there is a form of energy that did give the pendulum height, and that's its initial velocity.

Yes.
 
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  • #32
gneill said:
Because energy is not conserved over the collision. Before and after the collision it is, for both objects separately.?

Can you explain this please ?
 
  • #33
I got the answer though :D
 
  • #34
And what are some other possibilities of finding the initial velocity of the pendulum. why didnt 300(0.01) = (2.21)V' work? Because for a brief instant that the bullet does hit the block or the slight second its leaving the bullet is in the block, and momentum should be transferred no ?
 
  • #35
zaddyzad said:
Because energy is not conserved over the collision. Before and after the collision it is, for both objects separately.

Can you explain this please ?

Before the collision the bullet has a constant KE assuming that there's no air friction.
After the collision the bullet has a constant KE assuming that there is no air friction.

Before collision the pendulum bob has zero velocity and a constant height, so its KE and PE are constant.
After collision the pendulum is moving in a conservative field (gravitation), so total energy is conserved.
 
  • #36
zaddyzad said:
And what are some other possibilities of finding the initial velocity of the pendulum. why didnt 300(0.01) = (2.21)V' work? Because for a brief instant that the bullet does hit the block or the slight second its leaving the bullet is in the block, and momentum should be transferred no ?

The bullet and block are never one object moving with the same velocity. Simply occupying the same space does not count :wink:
 
  • #37
I don't fully understand why EK(bullet) = EK(bullet) + EP(bullet) doesn't work. The kinetic energy before and after the collision is constant, and so it the EP that the pendulum has. Why isn't the energy of the system being conserved.
 
  • #38
whats the answer you got finally ?
 
  • #39
zaddyzad said:
I don't fully understand why EK(bullet) = EK(bullet) + EP(bullet) doesn't work. The kinetic energy before and after the collision is constant, and so it the EP that the pendulum has. Why isn't the energy of the system being conserved.

Energy is only conserved for perfectly elastic collisions. Otherwise, energy is always lost in the collision to various "loss" pathways such as frictional heating, sound, plastic deformation of materials, breaking of atomic bonds(tearing, breaking), and so on.
 
Last edited:
  • #40
gneill said:
Energy is only conserved for perfectly inelastic collisions. .

Though kinetic energy is only conserved in perfect elastic collision?
 
  • #41
zaddyzad said:
Though kinetic energy is only conserved in perfect elastic collision?

D'oh! My bad. Of course it's ELASTIC collisions only. I fixed the text in the original.
 

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