NJC 2023 Work, Energy, Power Notes

National Junior College 2023 Work, Energy and Power - 1 - Work, Energy and Power Contents ▪ Work ▪ Kinetic Energy & Potential Energy ▪ Energy conversion and conservation ▪ Power and Efficiency 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= 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 212kE mv= . f) recall and use the equation 212kE 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 an d 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 equation pE mgh= for 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.

National Junior College 2023 Work, Energy and Power - 2 - 1 Work 1.1 Work done by a constant force For a constant force F, where F is the magnitude of the constant force x is the displacement  is the angle between the force and displacement You can either view this as resolving F along the displacement ( F cos θ) or resolving x along the force (x cos θ). The S.I. unit for work done is joule (J). Example 1(a) Find the amount of work done by the tension of the cable when the crane pulls a load 20 m horizontally with a tension of 2000 N, making an angle of 30 o with the horizontal. Solution cos (2000)(20) cos30 26000 J W Fx = = = Example 1(b) Find the amount of work done by the tension of the cable when the crane lifts a load 20 m vertically with a tension of 2000 N. Solution cos (2000)(20) cos 0 40000 J W Fx = = = Definition Work done by a force on the body is defined as the product of the force and the displacement in the direction of the force. cosW Fx = F x θ Load v F a) define and use work done by a force as the product of the force and displacement in the direction of the force

National Junior College 2023 Work, Energy and Po