Energy and Work
Chapter 28: 1-3
Quiz
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1.
2.
3.
4.
M1 = 0.473 kg, M2=1.343 kg, R=6.56cm
Cylinder: I = ½MR2
What is the I for each disk?
What is the initial angular momentum of
each disk?
• 5. Is angular momentum conserved during this
“collision”?
• 6. What must be true if L is conserved?
Energy and Energy Transfers
Kinetic Energy
Thermal Energy
Chemical Energy
Potential Energy
Transfers:
Work—transfer of energy by forces
Heat—transfer of energy because of temperature
differences
How can we isolate a system?
Conservation of Energy
Isolated system: No energy exchanged between
system and the rest of the universe. Energy
exchanges occur between components of the
systems inside the box isolating system.
K  U g  U s  E th  E chem  ...  E  constant
Alternatively
 E  0   K   U g   U s   E th   E chem  ...  0
10.4 Work
Special Special Case: F constant and in direction
of the displacement.
W  Fd
Units: 1 N•m=1 J = 1 Joule
Special Case:
Constant force and a constant angle θ to the
displacement.
W  F/ / d  Fd cos 
What happens if θ = 00
Problem 10.1
During an etiquette class you walk slowly and
steadily at 0.20m/s for 2.5m with a 0.75 kg book
balanced on top of your head. How much work
does your head do on the book?
Problem 10.2
A 2.0 kg book is lying on a 0.75 m high table. You
pick up the book and place it on a bookshelf 2.3m
above the floor.
a) How much work does your hand do on the
book? (diagram)
b) How much work does gravity do on the book?