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PART I: Lesson Objectives What is meant by “work done”? How can you tell when work is done and when work is not done? How is work done measured and calculated? A Product of Force & Distance What is WORK??? However , in physics, work is done ONLY IF : In normal usage, work is about doing school homework or assignments, riding a bicycle, sweeping the floor, etc. - a force is applied on an object & - the object moves in the direction of force. The force and the motion must be in the same direction! Situations where Work is Done 1 Situations where Work is NOT Done Reading a Book No Work Done: Pushing against a wall Calculating Work Done It is true that a force is exerted simply to hold a book, but the book does not move through a distance. Therefore the distance moved is 0, and the work accomplished is also 0. Calculating Work Done Work Done = Force x Distance moved in the direction of that force W=F•d Units for Work The SI unit for work is the newton-meter (N • m) which is also called a Joule (J) 1 newton-meter = 1 Joule 1 N•m = 1 J 2 Try this: Bob the Builder moves a 40.0 Newton window up 9.00 meters to the second floor. 360 Newton-meters = 360 Joules His assistant moves uses a truck to slide a 120.0 Newton concrete slab 30.0 centimeters across the floor. 3600 Newton-centimeters = 36.0 Joules Units for Work (Con’t) newton-meters (N • m) • Are used to measure large amounts of work newton-centimeters (N • cm) • Can be used to measure small amounts of work How much work did each person do? We calculate work done by a force using: W=F•D work done = force x distance • the units of work: the Newton-meter ( N•m ) or the Newton-centimeter ( N•cm ) The amount of work done depends on two factors, namely 1. the force applied More work is done when a bigger force is applied. 2. the distance moved in the direction of the applied force More work is done when the object moves through a greater distance. 1 N•m = 1 Joule Work (Practice Problems) If I am holding a box that weighs 35 N, and walk from where I am standing to a point 2.0 m away, how much work have I done on the box? W=F•D work done = force x distance 3 If I lift a box that weighs 35 N up 2.0 m, how much work have I done on the box? If I lift a box that weighs 35 N up 2.0 m and then carry it forward to a point 20.0 m away, how much work have I done on the box? If I slide a box across a counter by pushing on its side with 12 N of force, and the box slides 3.0 m across the counter, how much work have I done on the box? If I slide a box that weighs 60 N across the floor by pushing on its side with 12 N of force, and the box slides 3.0 m across the floor, and then I pick it up 2.0 m above the floor, how much work have I done on the box? What is the work done in carrying a 25 N box up a flight of stairs of to a floor 4.5 m higher than the floor you started at? What is the work done in climbing a flight of stairs of height h? W=F•d W=F•d d d 4 What is the work done in climbing a flight of stairs of height h? POWER W=F•d Do you get more tired when you run or when you walk up the stairs? What is the difference? h TIME POWER Power = Work / Time POWER POWER IS: Electric Power = Current × Voltage • the rate at which work is done • how fast work is done • how much work you can do per second POWER What are the units for Power? Other units for Power: Metric system units for Power are: Watts (W) 1 Watt = 1 Joule per second 1 W = 1 J/s Electrical Power is sold in kilowatt-hours (kWh): 1 kW = 3,600,000 J/hr Cars and Machines are rated in Horsepower (hp) 1 hp = 746 W (a very strong horse! ) 5 POWER POWER W P MEASURED IN JOULES (J) MEASURED IN WATTS (W) Watt Demonstration: –Lift a 1-N mass a distance of 1 meter, and you have done 1 Joule of Work. –Do this once each second, and your power output is 1 Watt. t MEASURED IN SECONDS (s) 1 W = 1 J/s Electric Power = Current × Voltage What is the work done in climbing a flight of stairs of height h? What is the work done in climbing a flight of stairs of height h? W=F•d W=F•d Do you get more tired when you run or when you walk up the stairs? What is the difference? d h TIME POWER Power = Work / Time • next… POWER IS: • the rate at which work is done • how fast work is being done • how much work you can do per second 6 Example: What is the power output of a 100 N person who runs up a 10 m high flight of stairs in 2.5 s? Power, Energy, Work … all together now. THE RATE OF DOING WORK OR CONVERTING ENERGY. W = Fd = (100 N)x(10 m) = 10,000 J Power = Work / Time P=W/t = (10,000 J) / (2.5 s) = 4000 W h Power = Energy / Time UNIT- WATT = JOULE PER SECOND 7