Newton's first law of motion states that if the sum of all the forces acting on an object is zero, it remains unaccelerated. As a result, the object attains equilibrium. A car moving at a constant velocity is in equilibrium as the net external force acting on it is zero. To identify forces on an object, construct a free-body diagram, considering the object as a particle. If the force is applied only in one direction, the object is said to be in one-dimensional equilibrium. Consider a mass hanging through a massless string from the ceiling. The forces acting on the mass are in the vertical direction. Tension in the string acts in the positive y-direction, while its weight acts in the negative y-direction. Since the mass is at rest, the net magnitude of force acting on it should be zero.