What has to be true to use conservation of momentum vs. energy

Click For Summary

Discussion Overview

The discussion revolves around the conditions under which conservation of momentum and conservation of energy can be applied in physics, particularly in the context of collisions and external forces. Participants seek clarification on the principles governing these conservation laws and examples illustrating their application.

Discussion Character

  • Conceptual clarification
  • Technical explanation
  • Homework-related

Main Points Raised

  • One participant states that conservation of momentum applies when there are no external forces acting on the system, emphasizing the need to check the net force in each direction.
  • Examples of momentum conservation include collisions and scenarios involving frictionless surfaces, such as ice skaters or boats pushing away from each other.
  • Another participant notes that while momentum is conserved in all collisions, kinetic energy is only conserved in elastic collisions, with inelastic collisions resulting in a loss of kinetic energy due to deformation.
  • A participant expresses appreciation for the explanations provided, indicating that the information is helpful.

Areas of Agreement / Disagreement

Participants generally agree on the conditions for applying conservation of momentum and energy, but there is a distinction made regarding the conservation of kinetic energy in different types of collisions, which remains a point of clarification rather than disagreement.

Contextual Notes

The discussion does not address specific mathematical formulations or detailed examples of inelastic versus elastic collisions, leaving some assumptions and definitions implicit.

Who May Find This Useful

Students learning about conservation laws in physics, particularly those in introductory courses, may find this discussion beneficial for understanding the conditions for applying these principles.

hydralisks
Messages
19
Reaction score
0
We are currently learning about conservation of momentum and conservation of energy in my intro physics class @ berkeley.

What must be true for me to use one of the other or both (I'd appreciate some common examples of when to use/when not to use them too, if possible)
 
Physics news on Phys.org
Conservation of momentum holds for a system when there are no external forces on the system. Since momentum is a vector, we need to check the net force in each direction, and if the net force is zero then the momentum for that direction is conserved.

The primary examples in intro physics are collisions (between blocks, disks, sports equipment, bullets and targets, etc) and also frictionless surface problems like ice skaters who push away from each other or people in boats that push away from each other, etc.

Conservation of energy in a system occurs whenever there is no external work done on the system. Typical causes of external work are friction, air resistance, or normal pushing / pulling.
 
Momentum is conserved in all collisions. Kinetic energy is only conserved in elastic collisions. In the case of inelastic collisions, kinetic energy is lost due to deformation of the objects involved in the collision.
 
Thanks so much for the fast responses! Makes sense!
 

Similar threads

  • · Replies 21 ·
Replies
21
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 30 ·
2
Replies
30
Views
3K
  • · Replies 53 ·
2
Replies
53
Views
5K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 9 ·
Replies
9
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 2 ·
Replies
2
Views
1K
  • · Replies 7 ·
Replies
7
Views
2K