an increasing velocity downwards (positive acceleration) therefore there exists a net force somewhere in the system. In this case, negative acceleration would indicate that . In another example, suppose that two bodies A and B having masses and , respectively, are connected with each other by an inextensible string over a frictionless pulley. There are two forces acting on the body A: its weight () pulling down, and the tension in the string pulling up. Therefore, the net force on body A is , so . In an extensible string, Hooke's law applies. Strings in modern physics String-like objects in relativistic theories, such as the strings used in some models of interactions between quarks, or those used in the modern string theory, also possess tension. These strings are analyzed in terms of their world sheet, and the energy is then typically proportional to the length of the string. As a result, the tension in such strings is independent of the amount of stretching. See also Continuum mechanics Fall factor Surface tension Tensile strength Hydrostatic pressure References Solid mechanics