Work & Energy

1. Work & Energy

2. The physics definition and the energy Definition of Work

3. Definition of Work Kinematics (mechanics) Dynamics (energy) This is the real definition of work Work is the process of transferring energy from one form to another Usually this means transferring kinetic energy to potential energy or vice versa • An oversimplified, but functional definition of work • Work is the product of the force exerted by the distance that it is exerted over • There is a correction factor for how F and d are aligned Force Distance

4. Definition of Work Kinematics (mechanics) Dynamics (energy) Work is the process of transferring energy from one form to another Force Distance

5. Definition of Work • Work is correctly thought of as both of these definitions. Forces cause energy to change. Force Distance

6. Definition of Work • Work carries a specific meaning in physics • Simple form: work = force  distance • Work is only done when the force and displacement are in parallel directions • Work can be done by you, as well as on you • Are you the pusher or the pushee? • Work is a measure of energy that has changed forms [some say “is expended”] • Work is measured in joules (same as energy) • Machines make work APPEAR easier • Apply less force over larger distance for same work

7. Energy is a concept that you can say “I understand it” better than you can define it. We will start by looking at five definitions and discussing them. Definition and kinds of Energy

8. Merriam-Webster on Energy

9. Wikipedia on Energy

10. ‘Simple English’ Wikipedia on Energy

11. 10.3 Energy and Conservation of Energy • Energy is the ability to make things change. • A system that has energy has the ability to do work. • Energy is measured in the same units as work because energy is transferred during the action of work.

13. So, what is energy? • Energy is a part of ‘things’ that we will treat like a property of matter • It is measured in Joules (J) which are N×m • Usually stated: “Energy is the capacity to do work” • Energy comes in many forms • Work is the process of transferring energy between forms (which is measurable with W=Fd) • Some forms are: potential, kinetic, thermal, electrical, light, and even mass (E=mc2) • Energy is always conserved • It cannot be created or destroyed, just change forms

15. A Tale of Two Energies… Gravitational Potential Energy Kinetic Energy Energy based on how something is moving • Energy based on location, and how far something can fall

17. The energy associated with the ability of gravity to do work on an object in the future. Gravitational Potential Energy

18. Gravitational Potential Energy • Gravitational potential energy comes from the distance that an object can fall in the earth’s gravitational field • Gravitational potential energy is fairly straightforward when you consider it near earth’s surface • It is much more challenging when you consider it in other scenarios (You need to use force-field math)

19. Gravitational Potential Energy • Ug=mgh • Gravitational potential energy comes from the distance that an object can fall in the earth’s gravitational field • Gravitaitionoal potential energy is fairly simple when you cinsider it near earth’s surface • It is much more challenging when you consider it in other scenarios (it falls under regular force field math Mass (kg) Potential Energy (joules) Height (m) Acceleration of gravity (m/sec2)

20. The energy of macroscopic motion Kinetic Energy

21. Kinetic Energy • Energy of motion is called kinetic energy. • The kinetic energy of a moving object depends on two things: mass and speed. • Kinetic energy is proportional to mass.

22. Kinetic Energy • Mathematically, kinetic energy increases as the square of speed. • If the speed of an object doubles, its kinetic energy increases four times. (mass is constant)

23. Kinetic Energy (cont: square of speed) • Kinetic energy is proportional to v2… • Watch out for fast things! • Damage to car in collision is proportional to v2 • Trauma to head from falling anvil is proportional to v2, or to mgh (how high it started from) • Hurricane with 120 m.p.h. packs four times the punch of gale with 60 m.p.h. winds

24. Kinetic Energy Mass (kg) Speed (m/sec) Kinetic Energy (joules)

25. Kinetic Energy • Kinetic energy becomes important in calculating braking distance.

26. Calculate Kinetic Energy • A car with a mass of 1,300 kg is going straight ahead at a speed of 30 m/sec (67 mph). • The brakes can supply a force of 9,500 N. • Calculate: a) The kinetic energy of the car. b) The distance it takes to stop.

27. Calculate Potential Energy • A cart with a mass of 102 kg is pushed up a ramp. • The top of the ramp is 4 meters higher than the bottom. • How much potential energy is gained by the cart? • If an average student can do 50 joules of work each second, how much time does it take to get up the ramp?

28. Energy is conserved

29. Law of Conservation of Energy • As energy takes different forms and changes things by doing work, nature keeps perfect track of the total • No new energy is created and no existing energy is destroyed • The total energy (in all forms) in a “closed” system remains constant

30. Power is a rating for how quickly work is done Power- work and energy use

31. Power • Power is simply energy exchanged per unit time, or how fast you get work done (Watts = Joules/sec) • One horsepower = 745 W • Run upstairs, raising your 70 kg (700 N) mass 3 m (2,100 J) in 3 seconds  700 W output! • Shuttle puts out a few GW (gigawatts, or 109 W) of power!

32. Power Example Problem • A person must exert a force of 250N to move this big blue block. If the person here has pulled it 12m in 120s, how much work have they done? • 3000J • What was the power rate for this work? • 25W