Lect11-Phys172s11-(6.7-6.11-Energy_Principle)

# Lect11-Phys172s11-(6.7-6.11-Energy_Principle) - PHYS 172...

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Spring 2011 PHYS 172: Modern Mechanics Lecture 11 – The Energy Principle Read 6.7 – 6.11 Work done by non-constant force 1 F 1 r Δ 2 F 2 r Δ 1, i i i N W F r = = ⋅Δ f i W F dr = ± 1 1 1 W F r = ⋅Δ 2 2 2 W F r = ⋅Δ Energy principle: 2 2 2 1 f i mc E F dr v c ² ³ ´ µ Δ = Δ = ´ µ - · ± W F r = ⋅Δ &

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Example: work done by a spring f i W F dr = x 0 x S F k x = - F ' 0 x S W k xdx = - x’ ' 2 2 0 ' 2 2 x S S k x k x W = - = - CLICKER : 1. What is the sign of work done by a spring when you compress it? A) Positive B) Negative 2. What is the sign of work done by a spring when you stretch it? equilibrium Interaction energy in a multiparticle system Interactions: have potential to perform work, there is energy associated with them 1, surr F 2, surr F 1 m 2 m 3 m 3, surr F 2,3 f 3,2 f 2,1 f 1,2 f 1,3 f 3,1 f ( ) 1 1,2 1,3 1, 1 1,int 1, surr surr E f f F r W W Δ = + + ⋅Δ = + ( ) 2 2,1 2,3 2,
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Lect11-Phys172s11-(6.7-6.11-Energy_Principle) - PHYS 172...

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