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Unit 3 – Energy & Society.

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1 Unit 3 – Energy & Society

2 Work, Energy, & Power In this chapter we will explore;
The physical concepts of work, energy, power, and the law of conservation of energy. Describe various forms and transformations of energy, and some of their common uses. Distinguish between renewable and non-renewable energy resources. Problem solving involving work, energy, power, and the conservation of energy.

3 5.1 Work - Learning Goals By the end of this lesson I should be able to: Explain the concept of work using my own words. Identify the SI units used to measure work. Describe the conditions required for work to be done. Solve problems relating to work.

4 5.1 Work In physics, mechanical work (W) is done when a force is applied to an object and displaces it in the same direction of the force, or a component of the force. Mathematically, the mechanical work done by a force on an object is the product of the magnitude of the force, and the magnitude of the object’s displacement. Work is a scalar quantity; there is no direction associated with work. SI units ⤍ Newton⋅metre (N⋅m) ⤍ Joule (J); 1 J = 1 N⋅m SP #1,2 p

5 5.1 Work Work has a different meaning in physics than it does in everyday life. In physics, work is only done when a force displaces an object, but if the object does not move, no work is done. When describing work, you should always consider the object that does work and the object that the work is done on. We do not consider the amount of time the force acts on the object in the calculation of work.

6 5.1 Work Sometimes an object experiences a force acting in a different direction than the object moves. This occurs when you pull on a suitcase handle with wheels attached at the bottom. The FBD shows all the forces acting on the suitcase, including the applied force ( 𝑭 𝒂 ) broken into horizontal ( 𝑭 𝒉 ) and vertical ( 𝑭 𝒗 ) components. Since 𝑭 𝒉 is the only force in the direction of the suitcase’s displacement, it is the only force that causes the suitcase to move; does work on the suitcase. In general: SP #1 p.224

7 5.1 Zero Work If a force fails to displace an object, such as pushing on a wall, then the force does no work on the object; 𝜟 𝒅 = 0. If object moves on a frictionless surface at a constant velocity with no horizontal forces acting on it, such as an air hockey puck hovering over an air hockey table; 𝑭 𝒂 = 0. If the force acting on an object and its displacement are perpendicular, such as carrying a backpack; θ = 90° ⤍ cos 90° = 0. No work is done.

8 5.1 Negative Work If the force acting on an object and its displacement are in the opposite direction, such as friction slowing down a rolling ball; θ = 180° ⤍ cos 180° = -1. Negative work is done. An object experiences several forces at the same time. The total work (Wnet) done on the object is the algebraic sum of the work done by all the forces acting on the object. We assume that the forces act parallel to the object’s displacement; either in the same direction or the opposite direction (θ = 0° or θ = 180°). SP #1 p.226

9 5.1 Graphing Work Done Positive and negative work may also be represented using a force-position (F-d) graph, with the magnitude of force (F) on the y-axis and position (d) on the x-axis. The work done, W, is determined finding the area under the F-d graph. The work is positive if the area is above the position (x-axis) ⤍ blue rectangle. The work is negative if the area is below the position (x-axis) ⤍ red rectangle.

10 5.1 Homework Practice # 1, 2 p.225 Practice # 1 p.227
Questions # 3, 5-9, 11 p.229


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