Lecture18 - PHYSICS 149: Lecture 18 Chapter 7: Linear...

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PHYSICS 149: Lecture 18 • Chapter 7: Linear Momentum – 7.2 Momentum – 7.3 The Impulse-Momentum Theorem – 7.4 Conservation of Momentum Lecture 18 Purdue University, Physics 149 1

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ILQ 1 Two objects are known to have the same kinetic energy. Do these two objects necessarily have the same momentum? A) Yes The example of the ice skaters starting from rest and pushing off f each other. ..the total momentum B) No of each other. is zero because they travel in opposite directions but the kinetic nergy is not zero (they start with energy is not zero (they start with none and gain a bunch) Lecture 18 Purdue University, Physics 149 2
ILQ 2 An astronaut is taking a space walk when his ther breaks e has a big drill How can he tether breaks. He has a big drill. How can he get back to the spaceship? A) Face away from the ship and throw the drill forward. B) Face the ship and throw the drill forward. C) Throw the drill towards the tether. Lecture 18 3 Purdue University, Physics 149

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Momentum omentum a ector uantity Momentum is a vector quantity. • Units: kg m/s, N s – Unit conversion: 1 kg m/s = 1 N s omentum is denoted by Momentum is denoted by p . • Momentum is sometimes referred to as “linear momentum” to distinguish it from angular momentum. Lecture 18 4 Purdue University, Physics 149
Momentum is Conserved • Momentum is “Conserved” meaning it can not be reated nor destroyed created nor destroyed – Can be transferred • Total Momentum does not change with time • Momentum is a VECTOR Conservation Laws in one! 3 Conservation Laws in one! Lecture 18 Purdue University, Physics 149 5

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Momentum is Conserved • Define Momentum p = m v –m 1 Δ v 1 = -m 2 Δ v 2 omentum is conserved Δ p 1 = - Δ p 2 Δ p 1 + Δ p 2 = 0 Momentum is conserved p 1f -p 1i +p 2f -p 2i =0 p Σ p initial = Σ p final • Example: Jane pushes Fred so he is going 2.0 m/s. Fred is twice as heavy as Jane how fast does p 1f +p 2f =p 1i +p 2i If Fred is twice as heavy as Jane, how fast does Jane end up moving? Σ p initial = Σ p final –0 = m Fred v Fred + m Jane v Jane – v ane = -m red v red / m ane = -4 m/s Lecture 18 Purdue University, Physics 149 6 Jane Fred Fred Jane
Momentum Transfer • During an interaction, momentum is transferred from one body to another, but the total momentum of the two is unchanged. f f i i p p p p or p p 2 1 2 1 1 2 G G G G G G + = + Δ = Δ • Consider a collision between two spaceships. i v 2 G f v 1 G f v 2 G i v 1 G t F v m p t m F t a v Δ = Δ = Δ Δ = Δ = Δ 12 1 1 1 1 12 1 1 G G G G G G t F v m p t m F t a v Δ = Δ = Δ Δ = Δ = Δ 21 2 2 2 2 21 2 2 G G G G G G G G G G G G G G 1 1 1 12 21 2 2 2 12 21 ) ( p v m t F t F v m p F F Δ = Δ = Δ = Δ = Δ = Δ = 1 2 , p p Thus G G Δ = Δ Lecture 18 7 Purdue University, Physics 149

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Impulse • Impulse is the product of (average) force and time interval during which the force acts.
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This note was uploaded on 04/23/2011 for the course PHYS 149 taught by Professor Staff during the Spring '08 term at Purdue University-West Lafayette.

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Lecture18 - PHYSICS 149: Lecture 18 Chapter 7: Linear...

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