Law Of Conservation of Energy In a closed

Law Of Conservation of Energy In a closed system, the sum of the PE (gravitational and/or elastic), kinetic energy, and internal energy remains constant. l In a closed system, energy may be transformed from one type to another, but the total energy of the system remains the same. l These ideas are expressed as the law of conservation of energy, which states that energy can not be created or destroyed l

Nonideal Mechanical Systems l When a system is acted upon by a nonconservative force, such as friction, it is called a nonideal mechanical system. l Frictional force converts some or all of the kinetic energy of a moving object into internal energy (usually heat).

Nonideal Mechanical Systems l The total energy of a nonideal system is given by the equation ET = PE + KE + Q • • (Ref. Tables) where ET represents the total energy of the system PE is the potential energy of the system KE is the kinetic energy of the system Q is the internal energy of the system

Nonideal Mechanical Systems l In a nonideal system ΔME = Wnc • Where ΔME is the change in mechanical energy (ΔPE + ΔKE) • and Wnc is work done on or by a nonconservative force

Nonideal Mechanical Systems l Let’s re-arrange ΔME = Wnc ΔPE + ΔKE = Wnc (PEf – PEi) + (KEf – KEi) = Wnc PEf + KEf = PEi + KEi + Wnc • Wnc is positive for work done on the system.

Practice Problem l Page 91 Rev Book, #97

Gravitational PE – Another Look l The force due to gravity, then is considered a conservative force. When a conservative force acts on an object, the total mechanical energy of the object does not change. (Examples of conservative forces: gravity, spring forces) l By comparison, a non-conservative force changes the total mechanical energy of the object. (examples of non-conservative forces: friction, applied forces)