Momentum in Collisions Question

[justadream]

If I understand it correctly, in a collision of two masses (assuming the usual MCAT conditions), the impulse is conserved (sum up to 0).

But how about momentum after a collision? Are the momenta of the two objects after a collision necessarily equal (in magnitude)? If not, under what conditions might it be?

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[Cawolf]

The collision of two masses results in a conservation of momentum only for a short time "dt" before and after the collision. Outside of this short time other forces may act upon the objects and add other impulses to the model. Therefore when solving for conservation of momentum you can only describe what happens right before and right after the collision.

A general setup where momentum is conserved would be a "flat, frictionless surface without air resistance". Also problems set in space far away from any gravitational forces. In reality there will be forces that alter the direction and velocity of some object.