1. Work & Energy Ideas: ch 11 Work either adds or subtracts energy. We’ll only deal with mechanical work (not heat)

2. Work & Energy  Energy Transfer:  Mechanical means: pull, push, etc.  Heat: not covered  Work: positive or negative?  W>0 environment does work on the system and energy goes up.  W<0 system does work on the environment and energy goes down

3. Energy & Work  Work = F x Distance, it’s really the amount of force acting in the direction the object is moving, or W = FxcosΘ  When there is a variable force, then the Work done is the area under the Force vs. distance curve W = Fdx

4. Energy & Work  Special Case: what about a force acting perpendicular to the direction of motion.  This is “just like” circular motion where a net centripetal force is acting.  How much work does the centripetal force do?

5. Energy & Work  Potential Energy and conservative forces  We call gravity a conservative force. The work done by gravity is independent of the path taken…..it only depends on the starting and ending positions.  Spring force/energy is also conservative, does not depend on the path, only the starting and ending postions  Frictional force is not conservative: the work done by friction does depend on the path.

6. Energy & Work  Finding force from Potential Energy  W = F Δx = -U (work = -change in potential energy)  U = -FΔx  Or…F = -ΔU/Δx ….this is the slope of the U vs. x graph