Physics131_L20WI10

Physics131_L20WI10 - Physics 131 Mechanics Lectur e 19 Cha pter 11.4-7 WORK M ar ch 3 2010 Homeyra Sadaghiani [email protected] Schedul e

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Physics 131 - Mechanics Lecture 19 Chapter 11.4-7 WORK March 3 , 2010 Homeyra Sadaghiani [email protected]
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9 1-Mar 19 Work, force, potential energy 11.4-6 (12 P)   3-Mar 20 Conservation of energy, power HW#8   5-Mar 21 Rotational motion, center of mass 10 8-Mar 22 Moment of inertia, Torque, static equilibrium 12.4-8 (7 P)   10-Mar 23 Rotational dynamics, final assessment 12.5-11 (20 P) HW#9   12-Mar R Review   19-Mar FINAL EXAM 11:30 A.M.-1:30 P.M. 3-2137 - Comprehensive Schedule 131.03 11:00-12:00 We are here! October 24, 2010 2 Physics 131 - Lecture 20
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9 1-Mar 19 Work, force, potential energy 11.4-6 (12 P)   3-Mar 20 Conservation of energy, power   5-Mar 21 Rotational motion, center of mass 12.1-3 (8 P) 10 8-Mar 22 Moment of inertia, Torque, static equilibrium 12.4-8 (7 P)   10-Mar 23 Rotational dynamics, final assessment 12.5-11 (20 P) HW#9   12-Mar R Review   17-Mar FINAL EXAM 11:30 A.M.-1:30 P.M. 3-2137 - Comprehensive Schedule 131.04 12:00-1:00 We are here! October 24, 2010 3 Physics 131 - Lecture 20
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Physics 131 - Lecture 20 4 Work Done by a Spring that Obeys Hooke’s Law (Hooke's Law) x F kx = - 2 2 1 1 2 1 by spring 2 1 1 2 2 1 2 2 2 2 x x x x x x x W F dx k xdx x k k x = = - = - = - 1 1 2 2 by spring 2 2 i f W k x k x = - Equilibrium October 24, 2010
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Physics 131 - Lecture 20 5 Example: Work Done on a Block by a Spring A 4.0 kg block on a frictionless is attached to a horizontal spring with k = 400 N/ m. The spring is initially compressed to 5.0 cm. (a) Find the work done on the block by the spring as the block moves from x = x 1 = -5.0 cm to its equilibrium position of x = x 2 = 0 cm. (b) Find the speed of the block at x 2 = 0 cm. 1 1 2 2 2 2 2 2 2 f i f i W W mv v v m = - = + 2 2(0.50 J) 0.50 m/s (4.0 kg) f W v m = = = 2 2 1 1 1 1 2 2 1 2 2 2 1 2 2 2 (400 N/m)[( 0.05 m) (0 m) ] 0.50 J x x x x x W F dx k xdx kx k x = = - = - = - - = October 24, 2010
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Physics 131 - Lecture 20 6/23 Power and Velocity so v v t t = ∆ = ∆ r r r r l l W F F v t = ⋅∆ = ⋅ ∆ r r r r l SI Units for power: 1 watt = 1 W = 1 J/s 3 4 1 hp = 550 ft lb/s = 746 W kW 6 1 kW h = (1000 W)(3600 s) = 3.6 10 W h = 3.6 MJ × W P F v t = r r Power is the rate of energy flow. October 24, 2010
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Physics 131 - Lecture 20 7/23 Power Power is a measure of the rate at which work is done: SI power unit: 1 J/ s = 1 watt = 1 W 1 horsepower = 1 hp = 746 W October 24, 2010
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Physics 131 - Lecture 20 8/23 Power October 24, 2010
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Physics 131 - Lecture 20 9/23 Example : The Power of a Motor A small motor is used to operate a lift that raises a load of bricks weighing 500 N to a height of 10 m in 20 s at constant speed. The lift weighs 300 N. What is the power output of the motor? ( 29 cos cos 0 P F v Fv Fv Fv φ = ⋅ = = = r r (500 N 300 N)(10 m/20 s) 400 W 0.54 hp P = + = = October 24, 2010
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Physics 131 - Lecture 20 10 Thermal Energy Dissipative Forces: Forces such as friction and drag cause the macroscopic kinetic energy of a system to be “dissipated” as thermal energy. E th = K micro + U micro Macroscopic perspective: object of mass M moves with velocity V obj: K macro c = 1 2 MV obj 2 Microscopic perspective: there is a system of particle with microscopic potential and kinetic energy is associated with the motion of atoms.
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This note was uploaded on 10/24/2010 for the course PHY PHYSICS 13 taught by Professor Mylander during the Spring '09 term at Cal Poly Pomona.

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Physics131_L20WI10 - Physics 131 Mechanics Lectur e 19 Cha pter 11.4-7 WORK M ar ch 3 2010 Homeyra Sadaghiani [email protected] Schedul e

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