HCI 05 WEP Lecture Notes

Hwa Chong Institution (College) H2 Physics C1 2023 1 Chapter 5 Work, Energy and Power Pictures (from top left and bottom left to right): credits to www.knxradio.com/guardian.co.uk/fluidmechanic.co.uk/aafaq.genistra.com “There is a fact, or if you wish, a law governing all natural phenomena that are known to date. There is no known exception to this law – it is exact so far as we know. The law is called the conservation of energy. It states that there is a certain q uantity, which we call “energy” that does not change in the manifold ch anges that nature undergoes. That is a most abstract idea, because it is a mathematical principle; it says there is a numerical quantity which does not change when something happens. It is not a description of a mechanism, or anything concrete; it is a strange fact that when we calculate some number and when we finish watching nature go through her tricks and calculate the number again, it is the same. It is important to realize that in physics today, we have no knowledge of what energy “is.” We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. It is an abstract thing in that it does not tell us the mechanism or the reason for the various formulas.” Richard Feynman The Feynman Lectures on Physics (Volume 1) A B C D

Hwa Chong Institution (College) H2 Physics C1 2023 2 Contents H2 Physics Syllabus 9749 ................................ ................................ ................................ ................................ ....... 3 5.1 Work ................................ ................................ ................................ ................................ ............................ 4 5.1.1 Work done by a constant force ................................ ................................ ................................ ................. 4 5.1.2 Work done by a variable force ................................ ................................ ................................ .................. 6 5.2 Energy ................................ ................................ ................................ ................................ ......................... 7 5.2.1 Kinetic energy, Ek................................ ................................ ................................ ................................ ..... 7 5.3 Potential energy ................................ ................................ ................................ ................................ ........... 9 5.3.1 Gravitational Potential Energy (GPE), Ep or U ................................ ................................ ........................ 10 5.3.2 Relationship between Gravitational Force and Gravitational Potential Energy ................................ ......... 12 5.3.3 Elastic Potential Energy ................................ ................................ ................................ ......................... 13 5.3.4 Work done by External Force on a Deformed Material ................................ ................................ ............ 14 5.4 Principle of Conservation of Energy ................................ ................................ ................................ ........... 15 5.5 Power ................................ ................................ ................................ ................................ ........................ 17 5.5.1 Relationship between Power, Force and Velocity ................................ ................................ ................... 18 5.6 Efficiency (of energy conversion),  ................................ ................................ ................................ ........... 19 Appendix 1 Potential Energy (U) and Conservative Force (Good to know)................................ ......................... 20 Self Review Questions ................................ ................................ ................................ ................................ ........... 22 Discussion Questions ................................ ................................ ................................ ................................ ............ 24 Videos of Lecture Examples can be found at https://youtube.com/playlist?list=PL_b5cjrUKDlYIoMzYi383KRNFmeWtS5Mh

Hwa Chong Institution (College) H2 Physics C1 2023 3 H2 Physics Syllabus 9749 Topic 5: Work, Energy and Power Content  Work  Energy conversion and conservation  Potential energy and kinetic energy  Power Learning Outcomes Candidates should be able to: (a) define and use work done by a force as the product of the force and displacement in the direction of the force. (b) calculate the work done in a number of situations including the work done by a gas which is expanding against a constant external pressure: W = pV (*will be covered in Thermal Physics) (c) give examples of energy in different forms, its conversion and conservation, and apply the principle of energy conservation. (d) show an appreciation for the implications of energy losses in practical devices and use the concept of efficiency to solve problems. (e) derive, from the equations for uniformly accelerated motion in a straight line, the equation Ek = ½ mv2. (f) recall and use the equation Ek = ½ mv2. (g) distinguish between gravitational potential energy, electric potential energy and elastic potential energy. (h) deduce that the elastic potential energy in a deformed material is related to the area under the force - extension graph. (i) show an understanding of and use the relationship between force and potential energy in a uniform field to solve problems. (j) derive, from the definition of work done by a force, the equation Ep = mgh for gravitational potential energy changes near the Earth’s surface. (k) recall and use the equation Ep = mgh for gravitational potential energy changes near the Earth’s surface. (l) define power as work done per unit time and derive power as the product of a force and velocity in the direction of the force.

Hwa Chong Institution (College) H2 Physics C1 2023 4 5.1 Work The meaning of ‘work’ in physics differs from everyday usage. Work done by a force acting on a body refers to the amount of energy that was transferred to the body by the agent that exerts the force. A person holding a heavy book at rest may feel that he is doing work as he feels tired after a while but , although he is exerting a force on the book, there is no energy transferred to the book as a result and hence the work done on the book by him is zero. 5.1.1 Work done by a constant force Consider a constant force F acting on a body over a period of time , during which the object has a displacement s in the direction of the force. The work done BY the force ON the body is defined as the product of the magnitude of the force F and the displacement s in the direction of the force. For the general case when the displacement is not in the direction of the constant force, the work done by the force is the product of the displacement and the component of the force in the direction of the displacement, or, equivalently, the product of the force and the component of the displacement that is in the direction of the force. As can be seen below, both have the same mathematical expression. W = F s// = F (s cos θ) Alternatively W = F// s = (F cos θ) s where F is the magnitude of constant force s is the displacement  is the angle between the force and displacement * subscript // refers to its vector component parallel with respect to some other vectors Notes:  Work