Clearly define your system of objects. The system should include all objects tha

Question

Clearly define your system of objects. The system should include all objects  that collide.

It should not include objects that do not collide.Clearly state the initial and final condition of the system.

The initial  condition should be just before the collision and the final condition should be  just after the collision.

State whether or not the system has a significant net external force on it  during the collision. There are three key

words here, "significant", "net", and  "external". Significant means that the force is not a small fraction of other  forces

involved in the collision. Net means that the force is not cancelled out  by any other external force (sometimes weight

and normal cancel, for example).  External means that the force is exerted by an agent that is not in your list of  objects

involved in the collision. Internal forces (exerted by and upon a member  of the system) should not be listed here.

State whether or not conservation of momentum applies based on your answer  to #3. If there is a significant net external

force on the system, then  conservation of momentum does not apply. If there is not a significant net  external force, then

conservation of momentum does apply.

I used someone running into a fence cemented into the ground and said that conservation of momentum should apply.

However, I was wrong. I now understand that the momentum has changed, but what I don't get is what the significant net external

force is. Is this a force on the wall? Any help would be appreciated.

Explanation / Answer

The system of objects includes all the objects which are/will collide i.e. it is the collection of all colliding objects.

Initial conditions that must be specified are: mass and velocitiy of each object prior to collision

Coefficient of restitution must also be given

Final conditions (i.e. velocities) can be calculated based on the principle of conservation of momentum and energy.

No, there is no net signigicant external force acting on the system of objects as a whole. As a result momentum remains conserved for the entire system. However, if you fix your attention to one of the objects, and forget about others, there is an impulse acting on it due to collision which causes the momentum to change.

Conservation of momentum applies to the entire system of object only and since there is a net external force on objects individually, conservation of momentum cannot be applied to individual objects

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