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14.1 Work and Power 14-1 cornell p. 412 What is work Calculating work What is power? Calculating power? James Watt and Horsepower.

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Presentation on theme: "14.1 Work and Power 14-1 cornell p. 412 What is work Calculating work What is power? Calculating power? James Watt and Horsepower."— Presentation transcript:

1 14.1 Work and Power 14-1 cornell p. 412 What is work Calculating work What is power? Calculating power? James Watt and Horsepower.

2 14.1 Work and Power 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.

3 14.1 Work and Power I.Work and Power A.Work- the product of force and distance. 1.Work Requires motion a.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. b.Any part of a force that does not act in the direction of motion does no work on an object.

4 14.1 Work and Power Work is done when a force acts on an object in the direction the object moves. Work is done when the weightlifter exerts an upward force to raise the barbell. What Is Work?

5 14.1 Work and Power Work 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. What Is Work?

6 14.1 Work and Power 2.Work Depends on Direction a.If all of the force acts in the same direction as the motion, all of the forces do work. b.If part of the applied force acts in the direction of motion, that part of the force does work. c.If none of the force is applied in the direction of the motion, the force does no work.

7 14.1 Work and Power A. All of the force does work on the suitcase. What Is Work? Force Direction of motion Force and motion in the same direction

8 14.1 Work and Power A. All of the force does work on the suitcase. B. The horizontal part of the force does work. What Is Work? Force This force does work This force does no work Direction of motion Force and motion in the same direction Part of force in direction of motion

9 14.1 Work and Power 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. What Is Work? Force This force does work This force does no work Force Direction of motion Force and motion in the same direction Part of force in direction of motion Lifting force not in direction of motion

10 14.1 Work and Power B.Calculating Work 1.Formula: a.force is in newtons (N) b.distance is in meters (m) c.The joule (J) is the SI unit of work. 1) A joule is equal to 1 newton-meter.

11 14.1 Work and Power 2.Using the Work Formula a.A weight lifter raises a 1600-newton barbell to a height of 2.0 meters. Work = Force × Distance Work = 1600 N × 2.0 m Work = 3200 N·m = 3200 J

12 14.1 Work and Power C.Power is the rate of doing work. 1.Doing work at a faster rate requires more power. 2.To increase power you can: a.increase the amount of work done in a given time, b.you can do a given amount of work in less time.

13 14.1 Work and Power Work 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. What Is Power?

14 14.1 Work and Power Because the snow blower can remove more snow in less time, it requires more power than hand shoveling does. What Is Power?

15 14.1 Work and Power D.Calculating Power 1.Formula a.work is measured in joules (J) b.time is measured in seconds (s). c.The SI unit of power is the watt (W) 1)A watt is equal to one joule per second.

16 14.1 Work and Power 2.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? ? 72 N 1m 2 sec fxd (w) P t P= f= d= t=

17 14.1 Work and Power 3.A tow truck exerts a force of 11,000N to pull a car out of a ditch. It moves the car a distance of 5 m in 25 seconds. What is the power of the tow truck? ? 11,000N 5m 25s P= f= d= t= fxd (w) P t

18 14.1 Work and Power E.James Watt and Horsepower 1.Another common unit of power is the horsepower. a.One horsepower (hp) is equal to about 746 watts. 2.James Watt ( ) used horsepower to compare the power outputs of steam engines he had designed. a.Horses were a logical choice for comparison as they were the most commonly used source of power in the 1700s.

19 14.1 Work and Power The horse-drawn plow and the gasoline- powered engine are both capable of doing work at a rate of four horsepower. James Watt and Horsepower

20 14.1 Work and Power Assessment Questions 1.In which of the following cases is work being done on an object? a.pushing against a locked door b.suspending a heavy weight with a strong chain c.pulling a trailer up a hill d.carrying a box down a corridor

21 14.1 Work and Power Assessment Questions 1.In which of the following cases is work being done on an object? a.pushing against a locked door b.suspending a heavy weight with a strong chain c.pulling a trailer up a hill d.carrying a box down a corridor ANS:C

22 14.1 Work and Power Assessment Questions 2.A tractor exerts a force of 20,000 newtons to move a trailer 8 meters. How much work was done on the trailer? a.2,500 J b.4,000 J c.20,000 J d.160,000 J

23 14.1 Work and Power Assessment Questions 2.A tractor exerts a force of 20,000 newtons to move a trailer 8 meters. How much work was done on the trailer? a.2,500 J b.4,000 J c.20,000 J d.160,000 J ANS:D

24 14.1 Work and Power Assessment Questions 3.A car exerts a force of 500 newtons to pull a boat 100 meters in 10 seconds. How much power does the car use? a.5000 W b.6000 W c.50 W d.1000 W

25 14.1 Work and Power Assessment Questions 3.A car exerts a force of 500 newtons to pull a boat 100 meters in 10 seconds. How much power does the car use? a.5000 W b.6000 W c.50 W d.1000 W ANS:A

26 14.1 Work and Power Assessment Questions 4.One horsepower is a unit of power equal to a W. b.1.0 W. c.746 W. d.2,000 W.

27 14.1 Work and Power Assessment Questions 4.One horsepower is a unit of power equal to a W. b.1.0 W. c.746 W. d.2,000 W. ANS:C


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