How do I find out if the normal reaction in a given situation is impulsive or not? For some cases, like two blocks colliding head-on, it is quite obvious that the normal reaction between the blocks would be impulsive in nature (and the normal force between the blocks and ground is non-impulsive).
However for some other cases, like an oblique collision with a particle stationary (or moving) on a plane incline, I am unable to figure out which normal forces are impulsive and which are not.
My reference book only states whether a given N is impulsive, and there's no explanation provided. Maybe this is intuitive, but if someone could provide a bit of logic to it or a standard method of solving this, it would be quite useful.

 A: Remember the rule: only impulsive forces can balance impulsive forces. You know for sure that the collision force between the bodies is impulsive. Draw the free body diagrams of the two bodies during collision, and see which other normal forces are balancing the collision force. Those normal forces will be impulsive.
In your example, the collision force on the bigger body has an upward component, so it will accelarate upwards without any resistance. Normal force does not oppose it, hence it is not impulsive. (In fact, it becomes 0 immediately after collision). On the smaller body however, the collision force has a downward component. But since the body cannot move downwards, the normal force must be balancing that component. Thus according to the rule, the normal force on the smaller body is impulsive.
This can be applied for tensile forces too.
A: As a general rule of thumb, if you can identify an impulsive force on a body, then the normal reaction will be impulsive if the dot product between the impulsive force and the normal force on that body is negative. So for the second example in your figure, you can see that according to that rule the N3 normal will be reactive and the N2 will not be.
In effect - you need a reactive force at N3 to counteract the downwards vertical component of the reactive force acting on the smaller ball. But the reactive force on the larger ball is upwards, so there is no reactive component at N2.

