FLT : Work and Energy

1. FLT : Work and Energy • Develop and use models to illustrate energy at a macroscopic scale that can be accounted for as a combination of energy associated with the motion of objects and the transformation of potential energy to kinetic energy  and  kinetic energy to thermal energy.

2. WORK and ENERGY Work Kinetic Energy Work Energy Theorem Potential Energy Conservation of Energy Power

3. WORK • Work is a transfer of energy • Force causing an object to have a displacement • Maximum work: F and d are parallel • Minimum work: F and d are perpendicular • W = Fd • Units: N∙m SI Units: Joules (J) • Scalar • Independent of pathway http://www.nu.ac.za/physics/1M2002/Energy%20work%20and%20power.htm

4. WORK • Importance of sign • + W: F and d are in the same direction • - W: F and d are in the opposite direction http://www.physics.upenn.edu/courses/gladney/phys150

5. WORK Hewitt Physics

6. WORK • Graphing Work • Area under the curve of the F vs. d graph.

7. WORK Independent of pathway

8. ENERGY • Ability to do work • Two types • Kinetic Energy • Potential Energy • Heat measures the transfer of energy.

9. KINETIC ENERGY • Energy of motion • KE = ½ m v2 • Units: kg m2 / s2 = Joule • Scalar • Work-Energy Theorem • Net work is equal to the change in energy • W =  KE • Fd = KEf - KE° • Fd = ½ m vf2 - ½ m v°2 = ½ m (vf2 - v°2)

10. POTENTIAL ENERGY • Store energy • Able to do work later • Units: kgm2/s2 = Joules • Scalar • Two main types • Gravitational potential energy • Elastic potential energy

11. GRAVITATION POTENTIAL ENERGY • Energy possessed by object because of its position in a gravitational field. • W = Fd • PE = mgh • Zero Gravitation Potential Energy is the point of reference

12. ELASTIC POTENTIAL ENERGY • Potential energy stored in the deformation (compression or stretched) of an elastic object. • Hooke’s Law • Restoring force • F = -kx • W = Fd • PE = ½ k x2 • Units: kg m2 / s2 = Joule

13. CONSERVATION OF ENERGY • First Law of Thermodynamics • Energy is never created or loss; it is just transfer from one form to another. • Energy before = energy after in an isolated system. • Second Law of Thermodynamics • Transfer of energy • Mechanical Energy is the total energy • TE = KE + PE (conserved) • TE = KE + PE + Wf (not conserved)

14. CONSERVATION OF ENERGY • Energy is never created or loss; it is just transfer from one form to another. • Energy before = energy after in an isolated system. • Mechanical Energy is the total energy • TE = KE + PE (conserved) • TE = KE + PE + Wf (not conserved)

15. CONSERVATION OF ENERGY Hewitt Physics

16. CONSERVATION OF ENERGY Hewitt Physics

17. Class work on WORK, Energy and Power P 120 -121 Conceptual Physics #s 21-27 #s 33, 42,43, 44 P 171-173 College Physics by Buffa #s 1,2,3,5,6,8,9,10,11,12,15, 25, 27, 30,32,36,42,44,48,53,57

18. NEXT-TIME QUESTION • Three baseballs are thrown from the top of the cliff along paths A, B and C. If their initial speeds are the same and there is no air resistance, the ball that strikes the ground below with the greatest speed will follow path 

19. Two smooth tracks of equal length have, "bumps" - A up, and B down, both of the same curvature. NEXT-TIME QUESTION If two balls start simultaneously with the some initial speed, the ball to complete the journey first is along If the initial speed equals 2 m/sec, and the speed of the ball at the bottom of the curve on Track B is 3 m/sec, then the speed of the ball at the top of the curve on Track A is

20. POWER • Rate work is done or which energy is transferred • P = W / t = Fd / t = F v • Units: J/s = Watts (W)

21. CONCEPTS • As a consultant to the soft-drink industry, Dr. J is given the task of conducting the ultimate Pepsi taste test. This is Dr. J's tenth taste test, which puts him seven up on his nearest consultant, who had only done three. Of course Dr. J is very qualified, having been hooked on soft drinks (especially orange soda) since he was Nehi to a pop bottle. Dr. J mounts a rather large container of Pepsi on a ledge some 3 meters above the ground. A bullet of mass 5 grams is then fired into the container, thus killing the taste. Not only that, but the Pepsi falls through the bullet hole onto the ground below (causing the taste to go flat). The wall of the container is 2 cm thick. The velocity of the bullet changes from an initial value of 500 m/sec just before striking the container wall to 5 m/sec upon leaving the container wall and entering the Pepsi. It finally fizzles out at a point 25 cm from the container wall.        • A. How much work does the container wall do on the bullet? • How much work does the Pepsi do on the bullet? • At what velocity does the Pepsi hit the floor?

22. Chapter 10: R pg 199 • 1) A force of 825 N is needed to push a car across a lot. Two students push the car 35m. a) How much work is done? b) After a rainstorm, the force needed to push the car doubled because the ground became soft. By what amount does the work done by the students change? • 29000J; work doubles

23. Chapter 10: R pg 199 • 2) A delivery clerk carries a 34 N package from the ground to the fifth floor of an office building, a total height of 15 m. How much work is done by the clerk? • 510 J • 3) What work is done by a forklift raising a 583 kg box 1.2 m? • 6900 J

24. Chapter 10: R pg 199-202 • 4) You and a friend each carry identical boxes to a room one floor above you and down the hall. You choose to carry it first up the stairs, then down the hall. Your friend carries it down the hall, then up another stairwell. Who does more work? • Same amount of work • 5) How much work does the force of gravity do when a 25 N object falls a distance of 3.5 m? • 88 J

25. Chapter 10: R pg 202 • 6) An airplane passenger carries a 215 N suitcase up stairs, a displacement of 4.20 m vertically and 4.60 m horizontally. a) How much work does the passenger do? b) The same passenger carries the same suitcase back down the same stairs. How much work does the passenger do now? • 903 J; -903 J • 7) A rope is used to pull a metal box 15.0 m across the floor. The rope is held at an angle of 46.0 ° with the floor and a force of 628 N is used. How much work does the force on the rope do? • 6540 J

26. Chapter 10: R pg 202-203 • 8) A worker pushes a crate weighing 93 N up an inclined plane, pushing horizontally, parallel to the ground in the figure. a) The worker exerts a force of 85 N. How much work does he do? b) How much work is done by gravity? c) The coefficient of friction is = 0.20. How much work is done by friction? • 340 J; -279 J; 130 J

27. Answers: Chapter 10 • 1) 800 J • 2) 12000 J • 3) 59.9 kg • 4) 1.86 x 105 J • 5) 0.80 J • 6) 25 N/m; 0.50 J • 7) 600 J • 8) 826 J; 1.13 x 10 4 J; - 1.13 x 10 4 J • 9) 1.20 x 10 4 J • 10) 58.7 degrees • 11) 1.8 x 10 4 J • 12) no work • 13) 7.7 J • 14) 518 J

28. Chapter 10: R pg 202-203 • 9) A box that weighs 575 N is lifted a distance of 20.0 m straight up by a rope. The job is done in 10.0 s. What power is developed in watts and kilowatts? • 1150 W; 1.15 kW

29. Chapter 10: R pg 203 • 10) A rock climber wears a 7.50 kg knapsack while scaling a cliff. After 30.0 min, the climber is 8.2 m above the starting point. a) How much work does the climber do on the knapsack? b) If the climber weighs 645 N, how much work does she do lifting herself and the knapsack? c) What is the average power developed by the climber? • 600 J; 5900 J; 3.3 W

30. Chapter 10: R pg 203 • 11) An electric motor develops 65 kW of power as it lifts a loaded elevator 17.5 m in 35.0s. How much force does the motor exert? • 1.3 x 105 N

31. Chapter 10: R pg 203 • 12) Two cars travel the same speed, so that they move 105 km in 1 h. One car, a sleek sports car, has a motor that delivers only 35kW of power at this speed. The other car needs its motor to produce 65 kW to move the car this fast. Forces exerted by friction from the air resistance cause the difference. a) For each car, list the external horizontal forces exerted in it, and give the cause of each force. Compare their magnitudes. b) By Newton’s third law, the car exerts forces. What are their directions? c) Calculate the magnitude of the forward frictional force exerted by each car? d) The car engines did work. Where did the energy they transferred come from? • Road on car; air on car; 1200 N; 2200N; chemical energy

32. Answer Chapter 10: pg214 • 15) 7400J • 16) 800 J; 600 J • 17) -5.53 x 10 3 J; no work; 5.53 x 10 3 J; no; 2.2 kW • 18) 9000 J; 3.00 kW • 19) 348 W; 696 W • 20) 220 J; 110W • 21) 110 kJ; 3.14 kW • 22) 1.8 x 10 4 J; 2.3 kW • 23) 160 W

33. Answer Chapter 10: pg214 • 24) 54.7 m • 25) 368 W • 26) 90 kW • 27) 2890 N • 28) 2300 N

34. Chapter 10: R pg 203 • 12) Two cars travel the same speed, so that they move 105 km in 1 h. One car, a sleek sports car, has a motor that delivers only 35kW of power at this speed. The other car needs its motor to produce 65 kW to move the car this fast. Forces exerted by friction from the air resistance cause the difference. a) For each car, list the external horizontal forces exerted in it, and give the cause of each force. Compare their magnitudes. b) By Newton’s third law, the car exerts forces. What are their directions? c) Calculate the magnitude of the forward frictional force exerted by each car? d) The car engines did work. Where did the energy they transferred come from? • Road on car; air on car; 1200 N; 2200N; chemical energy