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ENERGY & ENERGY TRANSFORMATIONS

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Energy (E) The capacity to do work (more to come in a couple slides)

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All forms of energy can be classified into one of two broad categories: Kinetic Energy (energy of motion) Potential Energy (stored energy)

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Forms of Energy Use the internet to prepare a definition for the following types of energy and classify them as kinetic or potential: Chemical Gravitational Kinetic Sound Elastic Electrical Nuclear Thermal Radiant

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Energy Transformation Equations Energy cannot be created or destroyed, only transformed from one type to another Energy transformation equations show how energy changes throughout a process Energy transformation equation for a microwave: electrical radiant thermal Arrows indicate a transformation

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Create Energy Transformation Equations for: (simple) A child jumps on a trampoline A portable CD player operates with a battery An incandescent light bulb is switched on A rollercoaster climbs and descends the first hill A person playing an electric organ presses a key and we hear the note played

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Create Energy Transformation Equations for: (complicated) A nuclear core heats up water to the boiling point which turns a generator Spring with a mass attached is pulled down and then released (moves up and down until it comes to a rest on it’s own) A match is struck against a matchbox and ignites

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Work (W) The energy transferred to an object by an applied force over a measured distance WORK is ENERGY TRANSFERRED

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You can think of energy as … The ability to do work Work waiting to happen

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The Work-Energy Equivalence In general, doing work gives an object energy AND an object that has energy can do work. Both Energy and Work have the unit Joule (with a capital “J” = N. m)

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The following conditions must be met in order for work to be done: 1.) A force must be exerted on an object 2.) The object must be displaced (moved) by the force 3.) At least part of the force must be in the same direction as the displacement

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Examples A person lifting a box from the floor to the table A person pushing a box along the floor from one spot to another

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What is Work? In Physics, work has a very specific definition Is this work? ▫Video

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Work Energy transferred to an object over a distance WORK IS ENERGY GAINED OR LOST W=FΔd Where: W – Work (J) F – Magnitude of Force (N) d – distance (m)

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W = F ∆d Important Notes: Work is scalar – directions are ignored for F and d This equation is used for 1D problems only Work is measured in N∙m = J (Joule)

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Example A worker pulls a heavy cart with a force of 40N [E] for 5.0m [E]. What is the work done by the person?

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W = F ∆d = (40N)(5.0m) = 2.0 x 10 2 J

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Positive Work When the force is in the same direction as the displacement "+" work = force tends to increase the energy of the object

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Negative Work If the force is opposite to the direction of the displacement (i.e. friction) "-" work = force tends to decrease the energy of the object

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Example A 42 kg child runs and then slides 7.8m along an iced over pond. The coefficient of kinetic friction between the child's boots and the ice is a.) What is the force of kinetic friction acting on the child? b.) Calculate the work done by the kinetic friction

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Solution a.) F K = μ K F N F N = F g = mg =(42kg)(9.8m/s 2 ) = 4.1 x 10 2 N F K = (0.0050)(4.1 x 10 2 N) = 2.1N

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b.) W = F ∆d = (2.1N)(7.8m) = 16 J

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Work Done Against Gravity Lifting an object to a higher position means you are working against gravity which is exerting a downwards force on that object If the velocity of the object being lifted is constant, than F net acting on the object is 0. Therefore, F A = F g

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Example A 23kg box is lifted 1.2m from the floor to a desk with no acceleration. Determine the work done on the box.

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F A = F g = mg = (23kg)(9.8m/s 2 ) = 225N W = F ∆d = (225N)(1.2m) = 2.7 x 10 2 J

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Who thinks they can do zero work? Hold a textbook at arms length

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Zero Work ( ZERO Energy Transferred ) Exerting a force but 0 displacement ▫a student holding another student on their shoulders Force is 0 but displacement occurs ▫a puck on an air table (no friction) Displacement is perpendicular to the force ▫a javelin thrower runs with the javelin above their head FaFa

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Challenge Question Who will do more work? A.) 2 people lifting a piano into the back of a truck OR B.) 2 people pushing the same piano up a ramp to the back of the same truck

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Homework! Create two questions with full solutions

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