suppose an insect dropped off a leaf and falls to the ground. consider the forces acting on the insect when it starts to fall, when it falls at a constant speed, and when it lands. when is there a net force acting and in what direction? What force pair is there when the incest lands?

A) When the insect starts to fall.

Assuming that the little insect can not do anything (he/she does not have wings or anything like that) the only force acting on the insect will be:

Gravitational force: Is the force that pulls down the insect.

Air resistance: As the insect starts to move, the air will try to oppose to that motion.

B) When the insect falls at a constant speed.

This will happen when the air resistance equals in magnitude (but in opposite direction) the gravitational force.

Now the net force is zero, so the insect is not accelerated anymore, which means that the insect moves at a constant speed.

And the fact that the net force is zero does not mean that the gravitational force and the air resistance are "not acting" on the insect, this means that those forces are not having any effect in the dynamics of the insect's motion.

C) When it lands.

When the insect lands, there is a new force that appears, the normal force of the ground that opposes to the movement of the insect through it.

As the insect comes to stop (because he hits the ground) the air resistance does not longer act on it, so now the normal force of the ground is equal in magnitude (but in opposite direction) to the gravitational force, then the net force is zero.

D) When is there a net force acting and in what direction?

The only time that is a net force acting, is when the insect starts to fall, where the gravitational force is accelerating the insect downwards.

E) What force pair is there when the insect lands?

The force pair is the gravitational force, and the reaction, the normal force of the ground.