Equilibrium and Net Force:
In mechanics, equilibrium refers to a state in which a body is at rest or moving with a constant velocity. For a body to be in equilibrium, the net force acting on it must be zero. This means that the forces acting in different directions on the body must cancel each other out, resulting in a balanced force system.

Explanation:
When a body is in equilibrium, it implies that it is either at rest or moving at a constant velocity. In both cases, the net force acting on the body is zero. This can be understood by considering Newton's second law of motion, which states that the net force acting on a body is equal to the product of its mass and acceleration (F = ma).

If a body is at rest, its acceleration is zero. Therefore, according to Newton's second law, the net force acting on the body must also be zero. In this case, the forces acting on the body in different directions are balanced and cancel each other out.

Similarly, if a body is moving with a constant velocity, its acceleration is also zero. Again, according to Newton's second law, the net force acting on the body must be zero. This means that the forces acting on the body are balanced and do not cause any change in its velocity.

Example:
Consider a box placed on a horizontal surface. If the box is at rest, the gravitational force acting downwards is balanced by the normal force exerted by the surface in the upward direction. These two forces cancel each other out, resulting in a net force of zero.

Similarly, if the box is sliding with a constant velocity, the frictional force acting in the opposite direction to its motion balances the applied force in the forward direction. Again, the net force is zero, and the box remains in equilibrium.

Conclusion:
In conclusion, for a body to be in equilibrium, the net force acting on it must be zero. This implies that the forces acting in different directions on the body are balanced and cancel each other out. Whether the body is at rest or moving with a constant velocity, the net force must be zero for equilibrium to be maintained.