These definitions revise the classical force definition. In classical mechanics, the definition is 𝐹 = ( ) 𝑣 ⃗. It states that once the momentum of an object changes, there must be a force exerting on the object. This definition describes the result but does not tell the source, purpose, and attribute of force, which has confused physicists for more than three centuries. In contrast, Eq. 8 and Eq. 9 clearly describe all force characteristics. In summary, force is the strength of heat transfer. There must be a donner and an acceptor. Moreover, force is unidirectional. Therefore, the system work is indispensable in any equation describing the energy state of a system. Perhaps the work and potential of the cosmos are the socalled "dark energy" or "dark substance."