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Ch6_lecture_notes_P125 Work and Energy Oct 2009

# Ch6_lecture_notes_P125 Work and Energy Oct 2009 - 1 Ch6...

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Unformatted text preview: 1 Ch6: Work and Energy Recall from Ch4: Force: any agent of change. Energy, ~ E , is the capacity a body or system has to cause change. Work is Energy transferred to (+) or removed from (-) a system or body by means of a force acting on it. Work and Energy are scalars. 2 The Work done by the force F on the box is: Work = (the component of the force along the direction of motion) * (the distance thru which that component acts) W = F x ∆x (units??) W = Fcos θ ∆x = F ∆x ) x , F ( cos r r ∆ F Moving → F x = F cos θ b ∆ x B θ b ∆ x B 3 For a constant force F: W = F x ∆x but F = ma So W = ma ∆x use the 3 rd eq of motion… W = ½mv f 2- ½mv o 2 The quantity ½mv 2 is called the KINETIC ENERGY. K = ½mv 2 KE is the energy a body or system has due to its motion. UNITS?? Note: KE ≥ 0 why? F Moving → F v 0 v f b ∆ x B 4 So, W = ½mv f 2- ½mv o 2 becomes W = K f- K o or W net = ∆K Work-Energy Theorem Work-Energy Theorem : = The net work done by all external forces The change in Kinetic Energy 5 Or, more intuitively: K o = K f + W net Thus, when a force F does work on an object, that object will acquire a KE of ½ m v 2 . F Moving → 6 Recap: Energy: the capacity a body has to cause change. Work is Energy transferred to or from a body or system by means of a force acting on it. A B C 1. You did work to lift the ball to ‘A’ by applying a force F. 2. The ball has acquired some form of Energy. (Energy has been transferred to the ball.) 3. Now let go of the ball. 4. The ball speeds up from A -> B because gravity is doing positive work. 5. The ball slows down from B -> C because gravity is doing negative work. So we’ll see that Work can be both + or -. 4 Consider a system of objects: { Ball + ___?___ } Let’s slowly lift the ball from point A to point B. W net = ∆K W net = W gravity + W me = - mgh + mgh = 0 So W net = ∆K = 0. But WORK is the transfer of Energy. 5 W me = mgh, but in this case the Work done by me did not change the ball’s Kinetic Energy, rather the W me is now stored as Gravitational Potential Energy in the Ball- Earth system GPE: the energy a body has due to its...
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Ch6_lecture_notes_P125 Work and Energy Oct 2009 - 1 Ch6...

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