Download Energy and Work: ch 11

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