Work, Energy, and Machines

14 terms

work

W the transfer of energy by mechanical means
unit: N m or J (joule)
W = FΔd (F is the component of the force which acts in the same direction as the displacement) Could be FcosΘ or FsinΘ
the work done on a system is the change in KE of the system
W = Δ KE

kinetic energy

KE the energy of an object resulting from its motion

energy

E the ability of an object to change itself or its environment
units: J (joule), c (calorie) C (1000 c)

power

P the rate at which work is done, the rate at which energy is transfered unit: W (watt) or (joules/sec)
P = W/Δt ( = FΔd/Δt = FΔv)

kilowatt-hour

kWhr an energy unit used by electric companies to measure energy sales; 1 kWhr = 1000 W delivered continuously over 1 hr

machine

a tool that makes work easier (but does not change the amount of work) by changing the magnitude or direction of the force used to do work
examples: lever, wheel/axle, inclne plane, pulley, wedge, screw

MA = Fr/Fe the ratio of resistance force to effort force

resistance force

Fr the force exerted by a machine

effort force

Fe the force a person exerts on a machine

IMA = de/dr for an ideal machine, the ratio of the displacement of the effort force to the displacement of the load

resistance displacement

dr the displacement of the load of a machine

effort displacement

de the displacement a person exert on a machine

efficiency

e the ratio of the output work of a machine to the input work exerted on the machine
e = Wo/Wi • 100 (expressed as a percentage) or
e = MA / IMA • 100

compound machine

a machine consisting of two (or more) simple machines linked together such that the resistance force of one machine becomes the effort force of the second machine