3 6A Objectives Describe the relationship between work and energy. Display an ability to calculate work done by a force.Identify the force that does work.Differentiate between work and power and correctly calculate power used.
4 Concept Development Map What is it?ApplicationsDefinitionsThe property of an object that allows it to produce change in itself or its environment.Latin energia =en (in) + ergon (work)= activeChemical EnergyThermal EnergyInternal orInherent PowerEnergyNuclear EnergyEnergetic, energizeMotion Energy(momentum)ExamplesPotential EnergyI ran out of energy.I need to re-energize.I don’t feel very energetic.
5 Concept Development Map What is it?ApplicationsDefinitionsThe process of changing energyof a system by means of forces.EnginesMiddle Europeanwerk, wirk, work = to doSprings; PulleysExertion of StrengthWorkThe matter on which someone labors on.Human EffortsExamplesShe’s a realpiece of work!TorqueThis is a work of art!I need to get to work.
10 Work Clarified Be careful! Only the force in the same direction as the motion counts towards the work.
11 Work ClarifiedCase 1. A man pushes against a car stuck in a snow bank while his date sits nervously behind the steering wheel trying not to make the tires spin. However, the car does not move. How much work did he do on the car?Answer: The man did no work on the car since d=0. He may have burned calories, converting chemical energy into heat, but still, the car did not move.
12 Work ClarifiedCase 2. Sally carries a 0.5 kg textbook under her arm along a horizontal path. How much work was done on the text book?Answer: None, since both gravity and the force Sally exerted against gravity are perpendicular to the distance the book moved. (cosø = 0 so W = 0).
13 Work ClarifiedCase 3. An asteroid traveling at constant velocity out of reach of gravitational fields [etc.]... How much work is done on the satellite?Answer: None, since F = 0, W = 0.
14 PowerPower - the rate of doing work. Work per unit time. 1 Watt = 1 Joule/sec.James WattsCompare:Units:Units:
18 Mechanical Energy Defined Mechanical Energy (ME): Energy due to the position or the movement of something; potential energy or kinetic energy or a combination of both.But just what is potential energy and kinetic energy?
20 Potential Energy Defined Potential Energy (PE): “Height” energy, position energy. It is usually related to the relative position of two things, such as a stone and the earth (gravitational PE), or an electron and a nucleus (Electric PE).h – relative to reference level; hground = 0.
21 What does this have to do with potential Energy?Work?
22 Kinetic Energy Defined Kinetic Energy (KE): Motion energy. Equal to half the mass multiplied by the speed (scalar!) squared.
28 6B ConclusionsPotential Energy – energy due to position = height energy = mghKinetic Energy - energy of an object due to its motion. Units of 1 kgm2/s2 = 1 Joule.Energy Transfer – Potential energy can be turned into kinetic energy and visa versa. Roller coasters and pendulums are examples of this.
30 6C Objectives Solve problems using the work-energy theorem. Solve problems using the law of conservation of energy.
31 Work versus Impulse Starting with the Impulse-Momentum Theorem: Multiplying both sides by d/t:
32 Work versus Impulse (cont’d) This simplifies to:Substitution of vavg = d/t = (v2+v1)/2:
33 Work - Energy Theorem This simplifies to: This is the Work-Energy Theorem:
34 Work - Impulse Comparison Let’s compare the two theorems:Impulse-Momentum Theorem:Work-Energy Theorem:
35 Kinetic Energy vs. Momentum Another conclusion:Kinetic Energy is the derivative of Momentum
36 Work - Impulse Comparison Let’s compare the two graphs:Impulse (Force versus Time)Work (Force versus Distance)ForceTime
37 Conservation of Mechanical Energy Mechanical Energy is the sum of kinetic energy and gravitational energy. It cannot change in an ideal system.The decrease in potential energyis equal to the increase in kinetic energy.The decrease in kinetic energyis equal to the increase in potential energy.
38 Conservation in a Pendulum Simple Harmonic Motion conserves energy on each swing.The decrease in potentialenergy is equal to theincrease in kinetic energy.The decrease in kineticenergy is equal to theincrease in potential energy.
44 6D Objectives Demonstrate Knowledge of why simple machines are useful. Communicate an understanding of mechanical advantage in ideal and real machines.Analyze compound machines and describe them in terms of simple machines.Calculate efficiencies for simple and compound machines.
45 6D VocabularyMachine - A device that changes the magnitude or the direction of the force needed to do work, making the task easier to accomplish.Simple Machine - A lever, pulley, gear, wheel and axle, inclined plane, wedge, or screw.Compound Machine - A device that consists of two or more simple machines linked so that the resistance force of one machine becomes the effort force of the second machine.
48 Mechanical AdvantageMechanical Advantage - The ratio of the resistance (r) force to the effort (e) force.Ideal Mechanical Advantage - The ratio of the resistance distance to the effort distance.Torque Balance - The resistance torque equals the effort torque.
49 EfficiencyPercent Efficiency - The ratio of the output work to the input work times 100%.
51 Baseball-Basketball Bounce Demo:Place a baseball on top of a basketball. Drop both at the same time on the floor andsee what happens. What do you think will happen? Why?
52 Baseball-Basketball Bounce What does all this have to do with baseball or sports in general?When you bounce a baseball off a basketball, you are transferring energy from the deformation of the basketball to the baseball. When you bounce a baseball off a bat, you are transferring energy from the bat to the baseball. How well a ball bounces off the basketball has to do with timing. When the basketball hits the floor, it squashes the bottom a bit. When it springs back to its original shape, it pushes off the floor -- it bounces. The baseball indents into the basketball on the top. When the basketball returns to its round shape all the energy is transferred to the baseball. The effect is similar to a man on a trampoline.
53 Conservation of Mechanical Energy The Amazing Oscillating Spring Thing