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I know that momentum conserves when no external forces interfere with a system. But I don’t completely understand it, here’s two examples that I don’t fully get:

  1. Two objects collide on a plane that has friction- therefore momentum doesn’t conserve because there’s an external force applying of the system> the friction force.

  2. Two objects collide on a frictionless plane as described: first object has a spring attached to its side and the second object collide into the spring that is attached to the first one changing the spring’s length (causing to elastic force) >the momentum still remains the same, the elastic force of the spring doesn’t count as an external force.

How can I know unambiguously what an external force is, what counts as one so the conservation is canceled?

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  • $\begingroup$ Related? It is all to do with the relative sizes of the impulses and the duration of the collision physics.stackexchange.com/q/310540 $\endgroup$
    – Farcher
    Commented Apr 28, 2018 at 20:42

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There is a simple solution to your question. You must analyze what belongs to the system and what doesn't. In both cases, the ground isn't a part of the system, hence if the ground has any interactions with the system, then you may not conserve momentum. However, in the second case, the spring is a part of the system, i.e. the forces on the blocks due to springs are internal forces, whose effects don't affect the force in Newton's second law, or just that there is no net unbalanced force. Thus you may conserve momentum in the second case but not in the first case.

So here are the conclusions

  1. Conservation of Momentum may be applied only if the net external force is equal to zero.
  2. If any source of force is in the system, it is considered to be an internal force, and hence the net effect of these forces cancel out.

Hope this helps

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The link provided by @Farcher answers (1).

As for (2), the first step in resolving a momentum related problem is to determine the elements that are included in your system. If your selected system comprises of the two objects plus spring, than there are no external forces and the momentum of this system is conserved.

You can select your system as the second object alone. In that case, the first object and spring indeed establish an external force and momentum of the second object is not conserved.

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  • $\begingroup$ Can I include as many objects as I like in my system? And if I’ll include in my first example the plane along with the two objects as one system, according to this logic, the momentum will conserve? $\endgroup$
    – Ozk
    Commented Apr 29, 2018 at 5:24
  • $\begingroup$ Yes, you can include the plane. There are many typical problems where the plane is allowed to move and friction between plane and object becomes an internal force. You ca even include the whole planet, however, this huge mass would barely move and you will not gain much insight into the kinetics of the problem. $\endgroup$
    – npojo
    Commented Apr 29, 2018 at 6:47

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