Conservation of momentum and energy question

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Discussion Overview

The discussion revolves around the conservation of momentum and energy in the context of a gun and bullet scenario. Participants explore the physical implications of kinetic energy and momentum during the firing process, particularly focusing on inelastic collisions and the role of gunpowder in propelling the bullet.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions why the kinetic energy equation does not hold when objects are at rest, seeking a physical explanation for the loss of energy in this scenario.
  • Another participant asks where the energy to propel the bullet originates, suggesting it comes from gunpowder.
  • A follow-up post reiterates the question about the source of energy and discusses the recoil of the gun, questioning why it recoils if it hasn't lost or gained energy.
  • Some participants note that mechanical energy is not conserved in this process, indicating that the conservation of momentum can still be applied.
  • One participant reflects on the relationship between force, momentum, and the interaction between the gun and bullet, suggesting that the gun's recoil is a consequence of the momentum change imparted by the bullet.

Areas of Agreement / Disagreement

Participants express differing views on the conservation of mechanical energy versus momentum, with some agreeing that mechanical energy is not conserved while others explore the implications of this in the context of the firing mechanism. The discussion remains unresolved regarding the detailed mechanics of energy transfer and the nature of forces involved.

Contextual Notes

Participants highlight the dependence on definitions of energy conservation and the complexities of momentum transfer, indicating that assumptions about energy loss and force interactions may not be fully articulated.

sameeralord
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Hello everyone,

Let's say there was gun and a bullet with a mass of 1kg at rest. The bullet and guns speeds were 2 m/s in opposite directions after firing if I right the equations.

Kinetic energy

0.5m1v12 + 0.5m2v12 = 0.5m1v22 + 0.5m2v22 Why doesn't this equation work when objects are at rest. Mathematically I can understand but why physcially. Can anyone explain what is happening to kinetic energy in this scenario. Why does conservation of momentum equation hold in in inelastic collisions but not this one. I understand some energy is lost but why is it not affecting momentum.
 
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Where does the energy to propel the bullet come from?
 
Vanadium 50 said:
Where does the energy to propel the bullet come from?

Ok let's say it came from gun powder. Then energy from gun powder would be given to the bullet, making it fly. Now the gun hasn't lost any energy or gained any but still it recoils. Why does it recoil?
 
sameeralord said:
Ok let's say it came from gun powder. Then energy from gun powder would be given to the bullet, making it fly. Now the gun hasn't lost any energy or gained any but still it recoils. Why does it recoil?
The point is that mechanical energy is not conserved, so your equation doesn't apply. (But you can write an equation for conservation of momentum.) The explosion of the gun powder provides energy to both the bullet and the gun.
 
Doc Al said:
The point is that mechanical energy is not conserved, so your equation doesn't apply. (But you can write an equation for conservation of momentum.) The explosion of the gun powder provides energy to both the bullet and the gun.

Thanks for the reply. Why does gun powder give a forward force to the bullet and backward force to the gun. Also why is this question not about gun powder and bullet. If gun powder provides the force, the reaction force should be towards gun powder, not the gun.

EDIT: Let me think this again. If force is rate of change of momentum. Since gun powder is part of the gun, the gun changed the momentum of the bullet. Oh so you can't change the momentum of something else without that momentum coming from you, meaning losing it. Oh so the gun retaliates. Oh so basically if you want to displace something, the person who is trying to displace the object is going to get displaced as well atleast a tiny bit. May be that is what momentum is saying. Is this r
 
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