H2_Work__Energy_and_Power_Lecture_Notes

9749 H2 PHYSICS; 8867 H1 PHYSICS Lecture Notes Nanyang Junior College 1 Chapter 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) show an understanding of the concept of work in terms of the product of a 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 . □ □ □ (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 21 2 kE mv . □ □ □ (f) recall and use the equation 21 2 kE mv . □ □ □ (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 pE mgh for gravitational potential energy changes near the Earth’s surface. □ □ □ (k) recall and use the formula pE 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 force and velocity in the direction of the force. □ □ □

9749 H2 PHYSICS; 8867 H1 PHYSICS Lecture Notes Nanyang Junior College 2 5.1 WORK The way we use the term “work” in physics is different from the way it is used in everyday life. Work is done by a force on a body when the force causes a displacement of the body in the direction of the force.  Force exerted  Force exerted  Force exerted  Weights displaced by force X Wall not displaced by force  Wall displaced  Work is done X No work done  Work is done 5.1.1 WORK DONE BY A CONSTANT FORCE Symbol: W Unit: Joules (J) Calculating work done If a body is displaced by a constant force F at an angle θ from the displacement s, then the work done by the force, W, is cosW Fs  Definition of work done Work done by a force on a body is the product of the force and the displacement in the direction of the force. Graphical representation Graphically, the work done is given by the area under the Fs graph. Note: The component of the force perpendicular to displacement, sinF  , does not do any work.

9749 H2 PHYSICS; 8867 H1 PHYSICS Lecture Notes Nanyang Junior College 3 Example 1