Canada is one of the leading producers of energy in the world, and is also one of its biggest users. We use energy for many purposes, including

Transcription

1 Unit 3

2 Canada is one of the leading producers of energy in the world, and is also one of its biggest users. We use energy for many purposes, including transportation, heating, cooling, industrial production and leisure. Most of the energy produced in Canada is from coal, crude oil (including gasoline and diesel), natural gas, and radioactive elements such as uranium **not only does the extraction of these resources from the ground create problems when they are used to generate electricity, gases are released into the atmosphere, contributing to climate change

3 As a result, we are constantly searching for cleaner and greener ways of producing electricity One promising solution is to use wind to produce electricity Wind turbines provide an environmentally friendlier way of generating electricity Large groups of turbines, called wind farms, may provide electricity for an entire community **as you know, wind turbines are not without controversy.their location as well as health effects have come under scrutiny in many communities that have been identified as suitable locations

4 Wind turbines are only one of many exciting technologies being designed to reduce or replace our reliance on non-renewable resources Other alternative energy technologies include solar cells, geothermal systems, tidal turbines and biofuels **with increasing energy costs, companies must use energy effectively to be productive

5 By the end of this unit, students will: Analyse technologies that apply principles and concepts related to energy transformations, and assess the technologies social and environmental impact Investigate energy transformations and the law of conservation of energy, and solve related problems Demonstrate an understanding of work, efficiency, power, gravitational potential energy, kinetic energy, nuclear energy, and thermal energy and its transfer (heat)

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7 The term work has a specific meaning in physics Mechanical work (W) is done on an object when a force displaces the object in the direction of the force or a component of the force For example, mechanical work is done when a crane lifts a steal beam for a new building However, if you are holding a heavy box, you may feel pain and even break into a sweat, but you are not doing any work on the box because you are not moving it

8 Mechanical Work (W) - Occurs when a force displaces an object in the direction of the force or a component of the force

9 To determine what factors affect the amount of mechanical work done in a moving object, consider an employee at a grocery store pulling a string of empty carts at a constant velocity with a horizontal force 1. By fixing the distance that the carts are moved and double the applied force required to pull the carts (by adding twice as many carts), the amount of mechanical work is doubled.. In other work is proportional to force

10 2. Similarly, with a constant applied force, if the distant the carts are pulled is doubled, then again the mechanical work done is doubled distance is proportional to work

11 Combining these proportionalities we find While F and d are both vector quantities, work is a scalar quantity The SI unit for work is the newton metre (Nm) or the joule (J)

12 The equation W = Fd has important limitations It applies only when the applied force and the displacement are in the same direction In more complex situations where twodimensions are analyzed, the component of the applied force causing the displacement is required For example, consider a person pulling on a suitcase with wheels and a handle

13 In this case, the component of the applied force causing the displacement is As such, the equation for the work done becomes where is the angle between the applied force and the displacement

14 Work (W) Is a scalar quantity

15 1. An airport terminal employee is pushing a line of carts at a constant velocity with a horizontal force of magnitude 95 N. How much mechanical work is done in pushing the carts 16 m in the direction of the applied force? Express your answer in kilojoules. (W = 1500 J or 1.5 kj)

16 2. A curler applies a force of 15.0 N on a curling stone and accelerates it from rest to a speed of 8.00 m/s in 3.50 s. Assuming that the ice surface is level and frictionless, how much mechanical work does the curler do on the stone? (W = 210 J)

17 In the examples presented so far, the work has been positive, which is the case when the force is in the same direction as the displacement Positive work indicates that the force tends to increase the speed of the object However, if the force is opposite to the direction of the displacement, negative work is done Negative work means that the force tends to decrease the speed of the object (a force of kinetic friction does negative work on an object)

18 Positive work: The force and displacement are in the same direction The speed of the object tends to increase Negative work: The force and displacement are in opposite directions The speed of the object tends to decrease

19 3. A toboggan carrying several children (total mass = 100 kg) reaches its maximum speed at the bottom of a hill, and then glides 20 m along a horizontal surface before coming to a stop. The coefficient of kinetic friction between the toboggan and the snowy surface is 0.10 a) draw a FBD of the toboggan when it is gliding to a stop b) determine the kinetic friction acting on the toboggan (98 N) c) calculate the mechanical work done by the kinetic friction (W = J) d) why is the work negative?

20 In order to lift an object to a higher position, a force must be applied upward against the downward force of gravity If the force applied and the displacement are both vertically upward and no acceleration occurs, the mechanical work done by the force against gravity is positive

21 The applied force in this case is equal in magnitude to the weight of the object or the force of gravity on the object, i.e. Fapplied = Fgravity

22 4. A bag of groceries of mass 8.1 kg is raised vertically upwards, without acceleration, a distance of 92 cm. Determine: a) the force needed to raise the bag without acceleration (79 N) b) the mechanical work done on the bag of groceries against the force of gravity (73 J)

23 5. A forklift truck does 4.0 x 10 5 J of mechanical work on a 4.5 x 10 3 kg load. To what height does the truck lift the load? (9.1 m)

24 Situations exist in which an object experiences a force, or a displacement, or both, yet no work is done on the object. Consider the following: 1. If you are holding a toolbox on your shoulder, you may be exerting an upward force on the toolbox, but the toolbox is not moving, so the displacement is zero, and the mechanical work done is also zero

25 2. If a puck on an air table is moving, it experiences negligible friction while moving for a certain displacement. The force in the direction of the displacement is zero, so the mechanical work done on the puck is also zero

26 3. Consider a skater who glides along the ice while holding his partner above his head. There are both a force on the partner and a horizontal displacement. However, the displacement is perpendicular to the force, so no mechanical work is done on the girl. (of course, work was done in lifting the girl vertically to the height shown)

27 Situations exist in which an object experiences a force, or a displacement, or both, yet no work is done on the object.

28 6. A student pushes against a large maple tree with a force of magnitude of 250 N. How much work does the student do on the tree? Zero since the displacement of the tree is zero

29 7. A 500 kg meteoroid is travelling through space far away from any measurable force of gravity. If she travels at 100 m/s for 100 years, how much work is done on the meteoroid? 8. Zero since the force acting on the meteoroid is zero

30 8. A nurse holding a newborn 3.0 kg baby at a height of 1.2 m off the floor carries the baby 15 m at a constant velocity along a hospital corridor. How much work has the force of gravity done on the baby? 9. Zero since the force and the displacement are pependicular

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34 Work may be determined from a F-d graph by calculating the area between the graph and the x-axis Positive work = area is above the x-axis Negative work = area is below the x-axis

35 9. a) Calculate the work done in sections A, B and C b) Calculate the total work done c) Explain why the work done in section C is negative

36 9.

37 Pg. 229, #1, 2, 4 6