energy elastic potential energy systems power varying forces
TRANSCRIPT
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Energy
Elastic Potential EnergySystemsPower
Varying Forces
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Elastic Force, Fs, is Conservative• Force required to stretch/compress spring• Force is _____________ to the amount of
stretching or compression (____________)• Newton’s 3rd Law means the spring exerts an
______ ____ ___________ force• The force is described by Hooke’s Law• It is a ___________ force
15.5kxFs
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Potential Energy Stored in a Spring
• The average restoring force exerted by a spring when compressed (or stretched) is:
kxkx
F 21
2
0
xi = 0xf = −x
F1
F0 00 F kxF 1
221 kxxFWs
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Elastic Potential Energy• Include work done by spring in work-energy
theorem
gifnc PEKEkxkxW 2212
21
16.5221 kxPEs
17.5sgnc PEPEKEW
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Systems and Energy Conservation
• The total amount of energy in the universe remains _________.
• Energy can _______ _____, but it can never be created or destroyed.
• Apply to a ________ of objects that changes from initial state to final state.
• The system may, in general, have energy transferred into or out of it (___________).
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Systems
• Wc changes ____
• Rewrite in terms of mechanical energy
• If E changes, it goes somewhere
• For example, friction creates thermal energy
KEWW cnc
ififnc PEPEKEKEW
iiffnc PEKEPEKEW
EEEW ifnc
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Nonisolated Systems
Energy is transferred into or out of the system Ways to transfer energy
Work Heat Mechanical Waves Electrical Transmission Electromagnetic Radiation
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Power Power is the _____ _____ of energy transfer The energy transfer can change over time Δt so
we use the average power
23.5vFt
xF
t
W
P
Unit is Watt: 1 W = 1 J/s = 1 kgm2/s3
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Power
Energy (or work) is sometimes listed in terms of power · time
Electricity usually billed for kilowatt-hours (kWh)
J 1060.3s 3600 W10kWh 1 63
U.S. units are the horsepower (hp)
W746s
lbft550hp 1
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Varying Forces• For small displacements x,
Fx nearly __________
• Add up all small rectangles• Work is _____ under graph
xFW x1
321 WWWW
Fig. 5.28, p. 148
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For A Spring• Compress or stretch _________
(no heat)• Work is done by applied force• Spring does same work (but it’s
____________)• Change in potential energy is
negative of work done by spring
Fig. 5.29, p. 148
221
appkxWF
appFss WWPE