DHS 05 Work, Energy & Power (Lecture Notes & Tutorial)

Dunman High School (Senior High Physics) Topic 5-1 | Topic 5 Work, Energy, Power Guiding Questions: • What is energy and how does it relate to work done? • How does energy get transferred and transformed? Content • Work • Energy conversion and conservation • Efficiency • 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 = pΔV (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 E k = 21 2 mv (f) recall and use the equation Ek = 21 2 mv (g) distinguish between gravitational potential energy, electric potential energy and elastic potential energy (h) deduce 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 E p = mgh for 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.

Dunman High School (Senior High Physics) Topic 5-2 | (a) define and use work done by a force as the product of the force and displacement in the direction of the force Work is a concept that provides a link between force and energy. Do not be misled by the many different meanings of the work in ordinary conversation – doing homework, going to work or having too much work to do. Not everything we call work in conversation is work i n the physics sense of the word. We will now define work – in the physics sense. Work done (by a constant force) Work done by a constant force on an object is defined as the product of the force and its displacement in the direction of the force. W = (F cos θ)s or W = F (s cos θ) where F is the magnitude of the force, s is the magnitude of the displacement and θ is the angle between the force and the displacement. Although force and displacement are vectors, work is a scalar quantity and the SI unit of work is the joule (J). θ = 0o W = F.s θ = 180o W = θ = 90o W = W = 0 also when (i) F = 0 or (ii) s = 0 0o<θ < 90o W = 90o<θ <