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Lecture4

# Lecture4 - Physics 7A Lecture 4 Winter 2010...

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Physics 7A – Lecture 4 Winter 2010 Prof. Robin D. Erbacher 343 Phy/Geo Bldg [email protected]

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Join this Class Session with your PRS clicker! Quiz 2 being graded– will keep you informed. Quiz 3 is next Monday. Check Physics 7 website frequently for calendar & Announcements. Turn off cell phones during lecture.
Three New Energy Systems

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• Kinetic energy is simply E moving . For translational energy, the indicator is speed ; the faster an object moves, the more KE it has. There is a quantitative relationship between KE and speed. Also, it is proportional to the mass of the object: The direction of motion of the object is unimportant. KE trans = ½ m v 2 Baseba ll Work KE KE Speed Speed
Conservation of Energy says… PE grav = Work = F you on mass height= mg(h final - h initial ) m m v f =0 Pull v i =0 Work was done on the mass: Work = F || x Where did the energy go?? x PE grav Height Work What is the indicator of the object  change? Temperature?     Phase?    Speed? m m

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PE grav Height PE gravity = mg h Gravitational potential energy-system exists for each pair of objects interacting by the gravitational force ∆PE gravity depends on two quantities: the change in vertical distance that the object moved, and the mass of the object. Usually, we focus on the gravitational potential energy due to the interaction between an object and the Earth. Crumpled Paper KE Speed Note: we are neglecting friction
1) You throw a ball to the height of the first floor window.

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Initial Final (Still in motion) PE grav Height KE Speed
PE grav Height KE Speed Final Initial (In motion)

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PE grav Height KE Speed Initial Final (Still in motion)
Springs contain energy when you stretch or compress them. We will use them a lot in Physics 7. The indicator is how much the spring is stretched or compressed, x, from its equilibrium (rest) state. k is a measure of the “stiffness” of the spring, with units [k] = kg/s 2 . x: Much easier to stretch a spring a little bit than a lot!

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