What is the net force acting on a yo-yo when it doesn't move?
The net force acting on the yo-yo when it doesn't move is 0 N.
We can know this because Newton's first law of motion states that a body at rest remains at rest and a body in motion remains in motion at constant velocity, unless acted upon by an unbalanced force. When a body is at rest or is in motion at a constant velocity, there is no acceleration. The acceleration is 0.
We get the formula Force = mass x acceleration