Question

When is the net force on an object equal to zero?

Answer 1

There are all kinds of conditions that could bring that about.

1. The most common one is friction. An object is moving that it is not accelerating. That means the frictional force is equal to the force causing motion.

2. The downward gravitational force is being negated by a rope suspending an object. The upward pull by the rope cancels the downward pull of gravity.

3. The rules of the game prohibit motion. A ballplayer would be stupid to leave second base while the pitcher has the ball in his hand (a physics joke. Please ignore).

4. A normal force upwards by the earth's surface is balanced by the weight of an object laying on the ground.

5. Speaking of baseball, there is no acceleration in the horizontal direction, so the net force on the ball horizontally is 0. 

6. An elastic band held and aimed horizontally. The force of the elastic to launch a spitball vs the hand holding the elastic back waiting for the best time to let the spitball go.

That should get you going.

Answer 2

When ALL the forces are equal, or in general when forces in the opposite direction are equal

so we can have newton example:

10 newtons X -10 newtons

netforce = 0 newtons