If the string has curvature, then the two pulls on a segment by its two neighbors will not add to zero, and there will be a net force on that segment of the string, causing an acceleration. This net force is a restoring force, and the motion of the string can include transverse waves that solve the equation central to Sturm–Liouville theory:  where formula_2 is the force constant per unit length [units force per area] and formula_3 are the eigenvalues for resonances of transverse displacement formula_4 on the string, with solutions that include the various harmonics on a stringed instrument. Tension is also used to describe the force exerted by the ends of a three-dimensional, continuous material such as a rod or truss member. Such a rod elongates under tension. The amount of elongation and the load that will cause failure both depend on the force per cross-sectional area rather than the force alone, so stress = axial force / cross sectional area is more useful for engineering purposes than tension. Stress is a 3x3 matrix called a tensor, and the formula_5 element of the stress tensor is tensile force per area, or compression force per area, denoted as a negative number for this element, if the rod is being compressed rather than elongated. Thus, one can obtain a scalar analogous to tension by taking the trace of the stress tensor. A system is in equilibrium when the sum of all forces is zero. For example, consider a system consisting of an object that is being lowered vertically by a string with tension, "T", at a constant velocity. The system has a constant velocity and is therefore in equilibrium because the tension in the string, which is pulling up on the object, is equal to the weight force, mg ("m" is mass, "g" is the acceleration caused by the gravity of Earth), which is pulling down on the object. A system has a net force when an unbalanced force is exerted on it, in other words the sum of all forces is not zero. Acceleration and net force always exist together. For example, consider the same system as above but suppose the object is now being lowered with an increasing velocity downwards (positive acceleration) therefore there exists a net force somewhere in the system. In this case, negative acceleration would indicate that formula_9. In another example, suppose that two bodies A and B having masses formula_11 and formula_12, respectively, are connected with each other by an inextensible string over a frictionless pulley.