phys16 – lecture 14
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PHYS16 – Lecture 14. Ch. 7 Work and Energy. Announcements. Interested in Biophysics Research? Come hear me give a talk on Thursday 4:45 pm in Merrill 2 “Using Lasers to Observe the Mechanics of Life”. Ch. 7 Work and Energy. Mechanical Energy Potential Energy Kinetic Energy Work - PowerPoint PPT PresentationTRANSCRIPT
PHYS16 – Lecture 14
Ch. 7 Work and Energy
Announcements
• Interested in Biophysics Research?– Come hear me give a talk on Thursday 4:45 pm in
Merrill 2– “Using Lasers to Observe the Mechanics of Life”
• Mechanical Energy– Potential Energy– Kinetic Energy
• Work– Constant Force → Dot Product– Variable Force → Integral
• Work-Mechanical Energy theorem
Ch. 7 Work and Energy
Energy pre-question
• A projectile is shot at an angle of θ with an initial speed of v0. What is the kinetic energy of the projectile at the top of the arc?
A)
B)
C)
D)E) None of the above
0
)(cos21
)(sin2121
220
220
20
mv
mv
mv
Energy pre-question
• Some forces in nature depend on the inverse-square of the distance between objects (1/r2). If you were to calculate the potential for this force it would depend on:
A) 1/rB) 1/r3
C) rD) r2
E) None of the above
Cheat Sheet
• Mechanical Energy– Potential Energy, Gravitational– Potential Energy, Spring– Kinetic Energy
• Work– Constant Force– Variable Force
cosxFW
...dyFdxFW yx
2
21 mvK
mghU 2
21 kxU
Review of Work
• Back to Jorge and his crate… Jorge pushes on a 10-kg crate 5 m with a 50 N force at 21 degrees. The coefficient of kinetic friction is 0.2. What is the total work?
http://www.wsu.edu/~jtd/Physics205/Chap4/Image189.gif
Discussion: Does the normal force do work?
• How much work is done by friction in the Jorge/crate example? How much work is done by the normal force?
• I push on a wall. Does the normal force do work? Why am I tired?
• I push on a wall while on a skateboard, does the normal force do work?
• Does the normal force ever do work?
Discussion: Mechanical Advantage and Work
• If I increase my mechanical advantage do I decrease the amount of work I do?
• What is the mechanical advantage for the incline? What is the work?
Discussion: Mechanical Advantage and Work
• What about for a pulley?
F
θ = 60 degrees
mass
Work-Mechanical Energy Theorem
Energy can be transformed
• Wyle E. Coyote• http://www.youtube.com/watch?v=Jnj8mc04r9E&feature=related
Work – Mechanical Energy Theorem
• Work = the transfer of Energy• Energy = the ability to do work
UWKW
Work done by system is negative
Work and grav. potential energy• If I lift an object, how much work did I do on the object?• Use work-energy theorem to derive gravitational potential
energy
Uyymg
Udymg
UxdF
UW
if
y
y
f
i
)(
)(
Notice Sign
Discussion: Denver vs. Boston
• So if I drop a 1-kg ball from 2 m then the change in the potential energy is 20 J. What if this classroom where in Denver instead of in Boston, a mile higher, would the potential energy change?
http://upload.wikimedia.org/wikipedia/commons/c/c8/ReliefUSA_map.gif
Zero of Potential Energy
• Wherever you set your zero point for potential energy will determine your coordinate system– U=0 J at y=0 point for gravitational pot. energy– U=0 J at equilibrium point for spring pot. Energy
• All calculations of ΔU will be relative so it won’t matter where you put 0 J just make sure not to switch back and forth!!!
Conclusion
• Mechanical Energy– Potential Energy, Gravitational– Potential Energy, Spring– Kinetic Energy
• Work– Constant Force– Variable Force
• Work-Mechanical Energy Thereom
cosxFW
...dyFdxFW yx
2
21 mvK
mghU 2
21 kxU
UWKW