What are the condition((s) necessary for two aluminum balls to be in equilibrium?

Question

Use the momentum principle to balance forces. Account for three forces acting on the first Al ball.

Answer 1

The first requirement is that the vector sum of the forces must be zero. You can have a lot of forces acting on the object but they have to point in opposite directions so there is no movement, like a draw in arm wrestling or tug of war. But that is not enough, since the forces could be opposite and cause an object to spin. The second condition therefore is to have all the stories cancel, like two kids balancing a seesaw.