2What is Energy? Ability to do WORK Ch 8 Energy NotesAbility to do WORKMeasured in Joules (calories or kilocalories)What is Energy?
3What is Work? Work depends on: Ch 8 Energy NotesWork depends on:The amount of force applied to the object.The distance that the object moves while the force is applied.The direction of the force with respect to the direction the object moves.What is Work?
4Work:Work is done on an object whenever a force is exerted on an object through some distance.If the force is constant and parallel to the direction of movement then:W = F dW = 0, if d = 0No work done if object doesn’t move.W = 0, if F = 0No work done, whenever there is no force.
5Ch 8 Energy NotesIf the force on the object is in the direction the object moves, the work done is:W = FxFx
6Ch 8 Energy NotesIf the direction of the force is opposite the direction the object moves, work is:W = -FxFx
7Ch 8 Energy NotesIf the force is perpendicular to the direction the object moves, the work done is 0.If the object doesn’t move, the work done is 0.FW = 0xForce is NOT Work
8How is all energy divided? PotentialEnergyKineticEnergyGravitationPotentialEnergyElasticChemical
9What is Potential Energy? Energy that is stored and waiting to be used later
10What is Gravitational Potential Energy? Potential energy due to an object’s positionP.E. (gravitational) = mass xheight x gravityDon’t look down, Rover!Good boy!
11What is Elastic Potential Energy? Potential energy due compression or expansion of an elastic object.Notice the ball compressingand expanding
13What is Chemical Potential Energy? Potential energy stored within the chemical bonds of an object
14K.E. = ½ mv2 What is Kinetic Energy? Ch 8 Energy NotesAn object’s kinetic energy depends on:the object’s mass.Kinetic energy is directly proportional to mass.the object’s speed.Kinetic energy is directly proportional to the square of the object’s speed.Energy an object has due to its motionK.E. = ½ mv2What is Kinetic Energy?
15Kinetic energy is a scalar quantity. Ch 8 Energy NotesKinetic energy is a scalar quantity.When the kinetic energy of an object changes, work has been done on the object.Work is a scalar quantity.What is the kinetic energy of an object with mass of 1 kg, moving at 1 m/s?0.5 Joule.
16Work and Kinetic Energy Ch 8 Energy NotesThe work done on an object by the net force equals the object’s change in kinetic energy.Wnet = DKEWork and Kinetic Energy
17Law of Conservation of Energy If no external forces act on a system, the total energy of the system will remain constant.Mechanical Energy = PE + KE
18What are different Forms of Energy/Energy Transformations?
19Your work = Fx = (10 N)(1 m) = 10 Joules Suppose you have to exert a force of 10 Newtons to push a book 1 meter across a horizontal table at constant velocity (as shown above). Since your force is in the direction the book moves, the work you do on the book is given by:Your work = Fx = (10 N)(1 m) = 10 JoulesWork done by Friction
20Work done by friction = -Fx = -(10 N)(1 m) = -10 Joules The work-energy equation would lead you to think that doing 10 Joules of work on the book should increase the kinetic energy of the book by 10 Joules - but that doesn't happen (the speed of the book is constant). What's going on?A friction force opposes the motion of the book. This force must also be 10 Newtons (Since the book moves at constant velocity, the net force on it must be zero.).The friction force pulls in the opposite direction from the direction the book moves, the work done by friction is given by:Work done by friction = -Fx = -(10 N)(1 m) = -10 JoulesThis means that the friction force removes 10 Joules of energy from the book. So, while you were adding 10 Joules of energy to the book, friction was busy taking the 10 Joules of energy away from the book. This is why the kinetic energy of the book does not change!
21DWork Power = time W P t Power is the rate work is done. Ch 8 Energy NotesPower is the rate work is done.Units of power: 1 Joule/sec = 1 Watt1000 Watts = 1 kilowattPower is a scalar quantity.DWorkPower =WtimePt
22What are (Simple) Machines? Ch 8 Energy NotesA machine is a mechanical device used to do work.Examples of simple machines:Inclined planeLeverpulleyWhat are (Simple) Machines?
23Workout = Workin (Simple) Machines Ch 8 Energy NotesA machine can never output more work (energy) than is put into it.At best,Workout = WorkinMachineWorkinWorkout(Simple) Machines
24What is Mechanical Advantage? Ch 8 Energy NotesMachines can’t multiply work or energy, but they can multiply force.Mechanical advantage measures how much a machine multiplies force.Force machine exertsMA =Force you exertWhat is Mechanical Advantage?
25How do we Measure Efficiency? Ch 8 Energy NotesThe efficiency of a machine tells how much of the energy (work) that goes into the machine actually does useful work.It is usually expressed as a percent.Useful work doneEfficiency =x 100%Energy inputHow do we Measure Efficiency?