Chapter 11 Work

The sum of kinetic and potential energy is the system’s mechanical energy. The term mechanical designates this form of energy as being due to motion and mechanical effects, such as stretching springs, rather than chemical or heat.

A systems energy (Esys) includes: Thermal energy (Etherm) is the total energy of the moving atoms and stretched bonds inside of an object. A systems energy (Esys) includes: E mech + E therm = K + U + Etherm

Energy exchanges within a system is called energy transformation and do not change the value of the system energy. Energy that exchanges between the system and the environment occur in two forms, mechanical and non-mechanical. Work, mechanical transfer, includes pushes and pulls. Non-mechanical transfer is heat.

W>0 when the environment does work on the system and energy increases. W<0 when the system does work on the environment and energy decreases. ΔEsys= ΔK + ΔU + ΔEtherm=W

ΔEsys= ΔK + ΔU + ΔEtherm=W NRG transferred to the system by doing work means Δesys = W NRG transferred within the system as K↔U↔Etherm=ΔEsys=0

ΔK = W This equation shows that force (W=Fd) transfers kinetic energy to a particle and thus changes the particles kinetic energy by pushing or pulling. This energy transfer, by mechanical means, is known as work. If there is no force perpendicular to the displacement, no work is done.

Work-Kinetic Energy Theorem 1 or more forces act on a particle as it displaces from position initial to position final, the net work done on the particle causes the kinetic energy to change. A system gains or loses KE when work transfers energy between the environment and the system.

If a force acts over time, we call it an impulse that changes a particles momentum. If a force acts over a distance, we say work has been done and a particles kinetic energy changes. KE= ½ mv2 = (mv)2/(2m) = p2/(2m)

Homework: Stop and Think 11.2 and #43 page 332 Do both on a piece of paper to turn in.