1. ICNS 132 : Work, Energy and Power Weerachai Siripunvaraporn Department of Physics, Faculty of Science Mahidol University email&msn : wsiripun2004@hotmail.com

2. What is in this chapter? Work & PowerEnergy : KE & PE Conservation of Energy

3. Introduction to Energy A variety of problems can be solved with Newton’s Laws and associated principles. Some problems that could theoretically be solved with Newton’s Laws are very difficult in practice. –These problems can be made easier with other techniques. The concept of energy is one of the most important topics in science and engineering. Every physical process that occurs in the Universe involves energy and energy transfers or transformations. Energy is not easily defined. Introduction CH7

4. System & Environments

5. Work To understand what work means to the physicist, a force is applied to a chalkboard eraser, and the eraser slides along the tray. If we want to know how effective the force is in moving the object, we must consider not only the magnitude of the force but also its direction. Which is showing the most effective way in moving the eraser?

6. Work So, work is the quantity to measure how effective the force is in moving the object.

10. Positive F and r are in the same direction, i.e. cos  > 0 Negative F and r are in the opposite direction, i.e. cos  < 0 Zero F = 0 r = 0 F·r = 0, i.e cos  = 0 or,  = 90˚ F rr F rr F rr

11. Positive F and r are in the same direction, i.e. cos  > 0 EXAMPLES: F rr Negative F and r are in the opposite direction, i.e. cos  < 0 mg rr F·r = 0, i.e cos  = 0 or,  = 90˚ mg  r --> W mg = 0 n  r --> W n = 0

17. F1F1 x1x1 F2F2 x2x2 F3F3 x3x3 W = F 1 x 1 + F 2 x 2 + F 3 x 3

18. i.e. Work done by varying force is equal to the area under the curve of F and x

22. Energy

28. F Two external forces: applying force F and friction f k Applying force F  positive work Friction force f  negative work  W = Fd - f k d

30. Potential energy is the energy associated with the configuration of a system of objects that exert forces on each other. It is present in the Universe in various forms, including gravitational, electromagnetic, chemical, and nuclear. Here, we consider two types of potential energy: Gravitational potential energy and Elastic potential energy.

32. Reference level --- ------------ H h

33. Elastic Potential Energy: Hooke’s law

34. x is the position of the block relative to its equilibrium (x=0) position k is a positive constant called the force constant or the spring constant “the force required to stretch or compress a spring is proportional to the amount of stretch or compression x”  Hooke’s law. The value of k is a measure of the stiffness of the spring. Stiff springs have large k values, and soft springs have small k values. The units of k are N/m. The negative sign signifies that the force exerted by the spring is always directed opposite to the displacement from equilibrium. Hooke’s law

37. Chemical energy is stored in your body.

38. K.E. of runner -> P.E.

39. Closed system No energy can get in nor get out. Energy is therefore conserved.

40. h P.E. K.E. referen ce Durian is dropped from the top of a building. Mechanical Energy = Kinetic Energy + Potential Energy.

41. Example A ball is dropped from a height of 2 m. Find its speed half way down. A B C

47. Conservation of Energy : Conservative and Non-conservative forces

51. Huuu-raayy !!! Friction (non-conservative)

52. Example A stone is thrown with an initial velocity of u at an angle . Using the conservation of energy, show that the maximum height reached is  H