Chapter 14 Learning Objectives-Study this for TEST. Chapter 14 Work and Power

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1 Chapter 14 ork and Power GOAL: Students will be able to compare and contrast work and power qualitatively and quantitatively. Standard: SC.912.P.10.3 Students will: Level Scale 4 design and conduct experiments that demonstrate work, power, and simple machines. 3 compare and contrast work and power qualitatively and quantitatively. 2 Identify the formula involved in calculating work and power problems 1 describe workand power Chapter 14 Learning Objectives-Study this for TEST 1. Describe the conditions that must exist for a force to do work on an object. 2. Calculate the work done on an object. 3. Describe and calculate power. 4. Compare the units of watts and horsepower as they relate to power. 5. Describe what a machine is and how it makes work easier to do. 6. Relate the work input to a machine to the work output of the machine. The weight lifter applies a large force to hold the barbell over his head. Because the barbell is motionless, no work is done on the barbell. hat Is ork? ork is done when a force acts on an object in the direction the object moves. ork is done when the weightlifter exerts an upward force to raise the barbell. hat Is ork? ork Requires Motion The weight lifter does no work on the barbell as he holds it over his head. The force applied to the barbell does not cause it to move. hat Is ork? hen does a force do work? In science, work is the product of force and distance. 1

2 hat Is ork? For a force to do work on an object, some of the force must act in the same direction as the object moves. If there is no movement, no work is done. Any part of a force that does not act in the direction of motion does no work on an object. hat Is ork? ork Depends on Direction If all of the force acts in the same direction as the motion, all of the force does work. If part of the applied force acts in the direction of motion, that part of the force does work. If none of the force is applied in the direction of the motion, the force does no work. hat Is ork? A. All of the force does work on the suitcase. hat Is ork? A. All of the force does work on the suitcase. B. The horizontal part of the force does work. Force Force This f orce does work This f orce does no work Force and motion in the same direction Force and motion in the same direction Part of force in direction of motion hat Is ork? A. All of the force does work on the suitcase. B. The horizontal part of the force does work. C. The force does no work on the suitcase. Calculating ork Force This f orce does work This f orce does no work Force Force and motion in the same direction Part of force in direction of motion Lifting force not in direction of motion 2

3 Calculating ork Units of ork hen using SI units in the work formula, the force is in newtons, and distance is in meters. The joule (J) is the SI unit of work. A joule is equal to 1 newton-meter. Calculating ork Using the ork Formula A weight lifter raises a 1600-newton barbell to a height of 2.0 meters. ork = Force Distance ork = 1600 N 2.0 m ork = 3200 N m = 3200 J hat Is Power? How are work and power related? Power is the rate of doing work. Doing work at a faster rate requires more power. To increase power, you can increase the amount of work done in a given time, or you can do a given amount of work in less time. hat Is Power? ork is required to move snow from one location to another. A person using a shovel and a person using a snow blower can both do the work needed to remove the snow. The snow blower can do the job much faster because it has more power. hat Is Power? Because the snow blower can remove more snow in less time, it requires more power than hand shoveling does. 3

4 hen using SI units in the power formula, work is measured in joules (J), and time is measured in seconds (s). The SI unit of power is the watt (), which is equal to one joule per second. You exert a vertical force of 72 newtons to lift a box to a height of 1.0 meter in a time of 2.0 seconds. How much power is used to lift the box? Read and Understand hat information are you given? Read and Understand hat information are you given? Plan and Solve hat formula contains the given quantities and the unknown? Plan and Solve hat formula contains the given quantities and the unknown? 4

5 Plan and Solve Replace each variable with its known value and solve. Plan and Solve Replace each variable with its known value and solve. Look Back and Check Is your answer reasonable? Look Back and Check Is your answer reasonable? 36 watts is not a lot of power, which seems reasonable considering the box was lifted slowly, through a height of only 1 meter. 1. Your family is moving to a new apartment. hile lifting a box 1.5 m straight up to put it on a truck, you exert an upward force of 200 N for 1.0 s. How much power is required to do this? 1. Your family is moving to a new apartment. hile lifting a box 1.5 m straight up to put it on a truck, you exert an upward force of 200 N for 1.0 s. How much power is required to do this? Answer: ork = Force Distance = 200 N 1.5 m = 300 J Power = ork/time = 300 J/1.0 s = 300 5

6 2. You lift a book from the floor to a bookshelf 1.0 m above the ground. How much power is used if the upward force is 15.0 N and you do the work in 2.0 s? 2. You lift a book from the floor to a bookshelf 1.0 m above the ground. How much power is used if the upward force is 15.0 N and you do the work in 2.0 s? Answer: ork = Force Distance = 15 N 1.0 m = 15 J Power = ork/time = 15 J/2.0 s = You apply a horizontal force of 10.0 N to pull a wheeled suitcase at a constant speed of 0.5 m/s across flat ground. How much power is used? (Hint: The suitcase moves 0.5 m/s. Consider how much work the force does each second and how work is related to power.) 3. You apply a horizontal force of 10.0 N to pull a wheeled suitcase at a constant speed of 0.5 m/s across flat ground. How much power is used? (Hint: The suitcase moves 0.5 m/s. Consider how much work the force does each second and how work is related to power.) Answer: ork = Force Distance = 10.0 N 0.5 m = 5 J Power = ork/time = 5 J/1.0 s = 5 James att and Horsepower Another common unit of power is the horsepower. One horsepower (hp) is equal to about 746 watts. James att ( ) was looking for a way to compare the power outputs of steam engines he had designed. Horses were a logical choice for comparison as they were the most commonly used source of power in the 1700s. James att and Horsepower The horse-drawn plow and the gasoline-powered engine are both capable of doing work at a rate of four horsepower. 6

7 Formula Triangle: ORK, FORCE, DISTANCE Practice Problems #1. To help rearrange the furniture in your classroom, you exert a force of 20 N to push a desk 10 m. How much work did you do? F D ork = Force X Distance ork = 20 N X 10 m F D ork = 200 J #2. A hydraulic lift exerts a force of 12,000N to lift a car 2 m. How much work is done on the car? #3. It took 500 Joules to push a chair 10 meters across the floor. ith what force was the chair pushed? ork = Force X Distance Force = ork Distance ork = 12,000 N X 2 m Force = 500 J 10 m ork = 24,000 J Force = 50 N F D F D Formula Triangle- ORK, POER, TIME Practice Problems #1. A crane lifts an 8,000 N beam 75 m to the top of a building in 30 seconds. How much power does the crane use? ork = Force X Distance Power = ork Time P T ork = 8,000 N X 75 m ork = 600,000 J Power = 600,000 J 30 s Power = 20,000 F D ork Triangle P T Power Triangle 7

8 Practice Problems #2. A motor exerts a force of 10,000 N to lift an elevator 6 m in 5 seconds. hat is the power produced by the motor? Practice Problems #3. A tow truck exerts a force of 9,000 N to pull a car out of a ditch. It moves the car a distance of 6 m in 25 s. hat is the power of the tow truck? ork = Force X Distance Power = ork Time ork = Force X Distance Power = ork Time ork = 10,000 N X 6 m ork = 60,000 J Power = 60,000 J 5 s ork = 9,000 N X 6 m ork = 54,000 J Power = 54,000 J 25 s Power = 12,000 Power = 2,160 F D ork Triangle P T Power Triangle F D ork Triangle P T Power Triangle 8