If the sum of all the forces on a ball is zero, what is the ball doing

The ball is moving in a straight line, at a constant speed (which could be zero but doesn't have to be).

If there are no forces acting on an object, or there ARE some but they add up to zero, then the object is not accelerating.

The net force on the ball will be zero and the ball will remain in its initial state either moving or at rest.

Explanation :

We all familiar with Newton first law of motion which states, an object remains either at rest or moving with a uniform speed; until some external force hinders its motion or state. This is called law of inertia.

Now, if there are multiple force acting on a body and all of them are equal and opposite to one another then, the resultant force will be zero. This can be formulated as,

Where,

- Total amount of force acting on a body

- Number of forces acting on a body

Considering a ball with multiple forces acting on it and all of them being equal and opposite, there will be no change in its state either it's in the rest position or moving in same velocity.