M= \sum F_x=0

or

M= \sum F_y=0

The answer to this question depends on the system of forces that you are considering. If all of the forces are acting in the same plane, then you can use either equation and obtain the correct answer. However, if the forces are acting in different planes, then you must use the moment equilibrium equation that corresponds to the plane in which you are interested. For example, if you are considering a beam that is supported at two points and you want to determine the forces in the beam, you would use the equation

M= \sum F_y=0

since the forces in the beam are all acting in the y-direction.

## Other related questions:

### Q: What is the moment equilibrium equation?

A: The moment equilibrium equation states that the sum of the moments (or torque) around any point must be zero.

### Q: What are equilibrium equations in engineering mechanics?

A: The equilibrium equations in engineering mechanics are a set of six equations that describe the equilibrium of a body. They are:

1. The sum of the forces on the body must be zero.

2. The sum of the moments of the forces on the body must be zero.

3. The sum of the products of the forces and their respective distances from a given point must be zero.

4. The sum of the products of the moments of the forces and their respective distances from a given point must be zero.

5. The sum of the products of the forces and their respective distances from a given line must be zero.

6. The sum of the products of the moments of the forces and their respective distances from a given line must be zero.

### Q: What is moment in equilibrium?

A: In mechanics, equilibrium is a state of rest or uniform motion in which the sum of all forces acting on a body is zero.