phys1441-spring08-031208-post

phys1441-spring08-031208-post - PHYS 1441 Section 002...

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Wednesday, Mar. 12, 2008 PHYS 1441-002, Spring 2008 Dr. Jaehoon Yu 1 PHYS 1441 – Section 002 Lecture #15 Wednesday, Mar. 12, 2008 Dr. Jae hoon Yu Work done by a constant force Work-Kinetic Energy Theorem Work with friction Potential Energy
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Wednesday, Mar. 12, 2008 PHYS 1441-002, Spring 2008 Dr. Jaehoon Yu 2 Announcements • Spring break next week – Mar. 17 – Mar. 21 – Have a safe break! • Term exam #2 – Wednesday, March 26, in class – Will cover CH4.1 – whatever we finish Monday, Mar. 24 • Colloquium today at 4pm SH101
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Wednesday, Mar. 12, 2008 PHYS 1441-002, Spring 2008 Dr. Jaehoon Yu 3 x y Work Done by a Constant Force A meaningful work in physics is done only when a sum of forces exerted on an object made a motion to the object. M F θ Free Body Diagram M d g F Mg = u ru r N F ur θ F Which force did the work? W = Force F JG How much work did it do? What does this mean? Physically meaningful work is done only by the component of the force along the movement of the object. Unit? Nm Work is an energy transfer !! ( ) F d = J GJ G cos Fd θ (for Joule) J = Why?
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Wednesday, Mar. 12, 2008 PHYS 1441-002, Spring 2008 Dr. Jaehoon Yu 4 WF s = 1 N m = Work Done by a Constant Force () 1 joule J
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Wednesday, Mar. 12, 2008 PHYS 1441-002, Spring 2008 Dr. Jaehoon Yu 5 W = cos0 = o Work done by a constant force ( ) cos F θ s cos90 = o cos180 = o 1 0 1
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Wednesday, Mar. 12, 2008 PHYS 1441-002, Spring 2008 Dr. Jaehoon Yu 6 Ex. 1 Pulling A Suitcase-on-Wheel Find the work done by a 45.0N force in pulling the suitcase in the figure at an angle 50.0 o for a distance s=75.0m. Does work depend on mass of the object being worked on? W = ( ) 45.0 cos50 75.0 2170 J =⋅ = o Yes Why don’t I see the mass term in the work at all then? It is reflected in the force. If an object has smaller mass, it would take less force to move it at the same acceleration than a heavier object. So it would take less work. Which makes perfect sense, doesn’t it? ( ) F d = J GJ G ( ) cos F d θ J G
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Wednesday, Mar. 12, 2008 PHYS 1441-002, Spring 2008 Dr. Jaehoon Yu 7 Scalar Product of Two Vectors Product of magnitude of the two vectors and the cosine of the angle between them A B J GJ G Operation is commutative cos AB AB θ == JG JG Operation follows the distribution law of multiplication A B J G How does scalar product look in terms of components?
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phys1441-spring08-031208-post - PHYS 1441 Section 002...

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