Lect07-Phys172s10-(4.1-7,10.Model_of_solid)_(5.1-5-curved_motion)

# Lect07-Phys172s10-(4.1-7,10.Model_of_solid)_(5.1-5-curved_motion)

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Spring 2010 PHYS 172: Modern Mechanics Lecture 7 – Changing momentum, Curved Motion Read 4.1-4.7, 4.10, 5.1– 5.5 EXAM 1 is next week!!! Time: 8:00-10:00 pm Wed Feb 10th Place: Elliott Hall Material: lectures 1-8, HW 1-8 (chapters 1-5 in the book) Problems: multiple choice, 14 questions (70 points) write-up part, hand graded (30 points) Equation sheet: provided with exam Practice exam + equation sheet: will be posted at the end of this week Note: no lecture on Feb 11! Model of solid: chemical bonds If atoms don’t move too far away from equilibrium, the force between atoms is similar to a spring force.

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A ball-spring model of a solid Ball-spring model of a solid To model need to know: - spring length s - spring stiffness - mass of an atom s s F k s = ° s dp F dt = ° ° Initial conditions for circular motion
Length of a bond: diameter of copper atom N A molecules 1. Number of atoms in one cm 3 density ρ = 8.94 g/cm 3 : molecular weight = 63.55 g/mole 3 23 22 3 8.94 g/cm atoms atoms 6.022 10 8.47 10 63.55 g/mole mole cm N = × = × 2. Volume per one atom: 3 23 22 3 1 cm 1.18 10 8.47 10 atoms/cm atom Cu V - = = × × 3. Diameter of one molecule: 3 23 3 8 10 8.47 10 cm 2.27 10 cm=2.27 10 m=2.27 Å Cu d - - - = × = × × Compare: Estimating interatomic “spring” stiffness strain L L Δ = stress T F A = tension strain stress Y = Y -

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