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Lecture10

# Lecture10 - PHYSICS 220 Lecture 10 Potential Energy and...

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PHYSICS 220 Lecture 10 otential Energy and Energy Conservation Potential Energy and Energy Conservation Textbook Sections 6.3 – 6.7 Lecture 10 Purdue University, Physics 220 1

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Overview • Last Lecture – Energy is Conserved – Work = Transfer of energy using force an be positive negative or zero • Can be positive, negative or zero •W = F d co s θ – Kinetic Energy (Motion) • KE = ½ m v 2 – Work = Change in Kinetic Energy Σ W = Δ KE • Today – Potential (stored) Energy – Power Lecture 10 Purdue University, Physics 220 2
Work Done by Gravity 1 • Example 1: Drop ball W g = (mg)(s)cos θ Y i = h s = h W = mghcos(0 0 ) = mgh S g S g Δ y = y f -y i = -h Y f = 0 mg y W g = -mg Δ y x Lecture 10 Purdue University, Physics 220 3

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Work Done by Gravity 2 • Example 2: Toss ball up Y i = h W g = (mg)(s)cos θ mg S s = h W = mghcos(180 0 ) = -mgh Y f = 0 y g Δ y = y f -y i = +h x W g = -mg Δ y Lecture 10 Purdue University, Physics 220 4
Work Done by Gravity 3 (mg)(s)cos • Example 3: Slide block down incline W g = (mg)(s)cos θ s = h/cos θ W g = mg(h/cos θ )cos θ h θ g S W g = mgh = y mg Δ y = y f -y i = -h W = -mg Δ y Lecture 10 Purdue University, Physics 220 5 g

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Work Done by Gravity • Depends only on initial and final height! hen an object of mass llows any path that moves When an object of mass m follows any path that moves through a vertical distance h , the work done by the gravitational force is always equal to mgh W g = -mg(y f -y i ) = -mg Δ y – Independent of path – If you end up where you began, W g = 0 – Note: can do work “against” gravity, then get gravity to do work ack back. – Define: Potential Energy…… – An object near the Earth’s surface has a potential energy (PE) that We call this a “Conservative Force” because we depends only on the object’s height, h Lecture 10 Purdue University, Physics 220 6 can define a “Potential Energy” to go with it.
Quiz 1) Is it possible to do work on an object that remains at rest (no change in temperature)? A) No ) Yes B) Yes Lecture 10 Purdue University, Physics 220 7

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Quiz 2) A pendulum bob swings back and forth along a circular ath Does the tension in the string do any work on the path. Does the tension in the string do any work on the bob? Does gravity do work on the bob? A) only tension does work B) both do work C) neither do work D) only gravity does work Lecture 10 Purdue University, Physics 220 8
Potential (stored) Energy • Work done by gravity is independent of path W g = -mg (y f -y i ) = - Δ PE g • Define PE g = mgy Only the difference in potential energy is physically meaningful, i.e., you have the freedom to choose the reference (or zero otential energy) point potential energy) point. Stored gravitational energy can be converted to kinetic energy • Works for any CONSERVATIVE force ± Gravity PE g = m g y Lecture 10 Purdue University, Physics 220 9 ± Spring PE s = 1/2 k x 2 ± NOT friction

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Conservative Forces • Conservative forces are forces that are associated with a potential energy function
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Lecture10 - PHYSICS 220 Lecture 10 Potential Energy and...

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