Unit 4 Work & Power

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Information: The Meaning of Work “I’m going to work.” “This job is hard work.” “I worked hard to study for this test.” In general, the word “work” refers to the effort it takes to get things done. Manual labor is seen as work and so is mental labor. Activities that may seem like work to you might NOT seem like work to a scientist. For example, if you sit quietly and study for a long time, a scientist would say that you’re not doing any work at all! And you could push against a car until you were exhausted, but if the car doesn’t move, the scientist would say you had done no work on the car. As you can see, there is a big difference between the everyday use and the scientific use of the word “work.” Two things must happen for a force to do work on an object. First, the force must push or pull on the object. Second, the object must move a distance. Both must happen; otherwise, no work is done. For example, if you pick up a book bag and put it on your desk, a scientist would say that you have done work on the book bag. Critical Thinking Questions 1. What two things does work depend on? 2. If an object is pushed on, but does not move, is there any work being done to the object? Why?

Teacher Initials

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Information: Calculating Work In the scientific community, work is only done if a force is applied to an object and that object moves a distance. To calculate the work done on an object, the force that pushes or pulls on the object is multiplied by the distance the object moves. Work involves both force and distance. So, how much work did you do on that book bag you lifted onto the desk? It’s not hard to figure out. Multiply the force needed to lift the bag by the distance the object as lifted. That’s it! In other words: Work = Force x Distance Force is measured in Newtons (N) and distance in measured in meters (m). When multiplying the two to find Work, we end up with a Newton-meter (N*m). A Newton-meter is also called a Joule (J). Work is measured in Joules (J), for James Joule, who made important discoveries about work and energy. Here’s an example of how to calculate work: Michael lifts his book bag, which weighs 25 N, from the floor to a desktop that is 0.80 m above the floor. How much work does Michael do on the bag? Work = Force x Distance Work = 25 N x 0.80 m Work = 20.0 J Michael does 20.0 J of work on the book bag. Critical Thinking Questions 1. What units is work measured in? 2. If a pencil drops from a desk, is work being done? Why?

3. What is doing the work on the pencil from question 2? 4. What is the equation for work? 5. What would the triangle for work look like? Teacher Initials

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Put it to Use: Work Mini Lab Problem: To investigate the scientific definition of work. Background Information: WORK is done when a force causes an object to move in the direction of force. For work to be done, two things must occur. First, you must apply a force to an object. Second, the object must move in the same direction as the force you apply. If there is no motion, there is no work. This is very different from the way we use the word work in everyday life. Work can be calculated with this formula: Work = Force x Distance W=F d The units of force are Newtons and the units of distance are meters. Therefore, work is measured in Newton-Meters. These units are referred to as Joules. Materials: 4 Books

Spring Scale

Meter Stick

5 different objects

Part A: 1. Stand with your arms out in front of you at waist level, palms up. 2. Have your partner put a book on each of your hands. 3. Lift the books to about shoulder level, then lower them. 4. From waist height, lift the books over your head, and then lower the books. Critical Thinking Questions: 1. When did you do more work: when lifting the books from waist to shoulder height or when lifting the books from waist height to over your head? Explain using what you’ve learned about work.

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Part B: 1. Stand with your arms out in front of you at waist level, palms up. 2. Have your partner put 2 books on each of your hands. 3. Lift the books to about shoulder level, then lower them. 4. From waist height, lift the books over your head, and then lower the books. Critical Thinking Questions: 1. Are you using more force when lifting 2 books than when you were holding only one book? Explain using what you know about force and weight.

Part C: 1. Stand with your arms stretched out to the sides (like you’re showing your wing span). 2. Have your partner put 2 books on each hand. 3. Hold the books at shoulder level until your arms get tired. Critical Thinking Questions: 1. Are you exerting a force on the books? 2. Are you doing work in this situation? Explain.

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Part D: 1. Attach one of the objects to the spring scale. 2. Record the object’s name in the data table below. 3. Hold the meter stick straight up in the air next to the object. 4. Slowly pull the object straight up along the meter stick. 5. Looking at the spring scale, determine how much force you used to pull the object (Newtons). 6. Record the force in the data table below. 7. Using the meter stick, measure the distance you moved the object. 8. Record the distance in meters in the data table below. 9. Calculate the work you did. 10. Record the work you did in the data table below. 11. Repeat steps 1-10 with the other objects. Data: Calculation of Work Object

Force (N)

Distance (m)

Work (J)

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Analysis Questions: 1. Using a force of 50 N, you push a cart 10m across a classroom floor. How much work did you do? Givens

Equation

Solving For

Substitution

Answer with Units

2. What is a Joule? ______________________________________________________________________________ 3. Were you doing work when you were holding the books? Explain your answer. ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 4. Were you doing work when you were lifting the objects with the spring scale? Explain your answer. ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 5. In Greek mythology, Atlas held the world on his shoulders. Did he do any work? Explain your answer. ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

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Work Problems Directions: Use your knowledge of power and work to answer the following problems. Make sure to show all work and include all units. 1. A person pushes a block 4 m with a force of 24 N. How much work was done? Givens Solving For

Equation

Substitution

Answer with Units

2. A person does 15 J of work moving a couch 1.3 m. How much force was used? Givens Solving For

Equation

Substitution

Answer with Units

3. Paul Konerko hit a 125 m grand slam in Game 2 of the World Series. He did 3000 J of work. With what force did he hit the ball? Givens Solving For

Equation

Substitution

Answer with Units

4. You lift a box that weighs 50 N to a height of 1.7 m. How much work did you do on the box? Givens Solving For

Equation

Substitution

Answer with Units

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5. A 750 N skydiver jumps out of an airplane that is flying at an altitude of 2800 m. By the time the skydiver reaches the ground, how much work was done on her by gravity? Givens Solving For

Equation

Substitution

Answer with Units

6. A bulldozer performs 75,000 J of work pushing dirt 18 m. What is the force exerted? Givens Solving For

Equation

Substitution

Answer with Units

7. Mr. Z holds 200 kg above his head for 5 seconds. What is the work done on the weights? Givens Solving For

Equation

Substitution

Answer with Units

8. If Ms. Oldham has a mass of 55 kilograms and climbs up stairs that are 30 meters tall then how much work was done? Givens Solving For

Equation

Substitution

Givens

Equation

Answer with Units

Solving For

Substitution

Answer with Units

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9. If 100 J of work are done by lifting a box 1.5 m, then how much mass was the box? Givens Solving For

Equation

Substitution

Givens

Equation

Answer with Units

Solving For

Substitution

Answer with Units

10. You hurry to your physics class after a yummy meal in the cafeteria. In your arms you carry books that include your highly valued EMP binder. All together the food and the binder weigh 22 N. How much work is done on the books when you walk 80 m along the hall, go up stairs 12 m high, and then turn right for an additional 15 m to class? Givens Solving For

Equation

Substitution

Answer with Units

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Information: The Meaning of Power “I have the power!” “She is in a powerful position.” “How much power do you have to make a change?” In general, the word “power” refers to strength, authority, control or influence. But in science, the word “power” like the word “work” has a very specific meaning. Power is the rate at which work is done or how quickly work is done. Power takes into consideration how much work is done on an object AND how quickly the work was done. Two things must happen for a force to do work on an object. First, the force must push or pull on the object. Second, the object must move a distance. Both must happen; otherwise, no work is done. Power also takes into consideration how much time it took for the force to move the object. For example, if you pick up a book bag and put it on your desk, a scientist would say that you have done work on the book bag. If you time how long it takes you to lift the book to the desk, then you can figure out how much power you used! Critical Thinking Questions 1. What two things does power depend on? 2. If an object is pushed on, but does not move, is there any power being used? Explain.

3. If an object is pushed on, moves 10 m and does so in 1 minute, is there any power used? Explain.

Teacher Initials

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Information: Calculating Power In the scientific community, power is only used if work is done on an object in a certain amount of time. To calculate the power used on an object, the work done on the object is divided by the time it took to do the work. Power involves both work and time. So, how much power did you do on that book bag you lifted onto the desk? It’s not hard to figure out. First find the work you did on the book bag. Multiply the force needed to lift the bag by the distance the object as lifted. Here’s an example of how to calculate power: Michael lifts his book bag, which weighs 25 N, from the floor to a desktop that is 0.80 m above the floor. It takes him 5 s to lift the book bag. How much power does Michael use? Work = Force x Distance Work = 25 N x 0.80 m Work = 20.0 J Michael does 20.0 J of work on the book bag. Then divide the work by the time it took to lift the book bag. Power =

Work time

Power =

20.0 J 5s

Power = 4 J/s or W Work is measured in Joules (J) and time is in measured in seconds (s). When dividing the two to find Power, we end up with Joules per second (J/s). A Joule per second (J/s) is also called a Watt (W). Power is measured in Watts (W). Critical Thinking Questions 1. What units is power measured in? 2. What is the equation for power? 3. What would the triangle for power look like? Teacher Initials

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Power Problems Directions: Use your knowledge of power and work to answer the following problems. Make sure to show all work and include all units. 1. Dante uses 14 J of work to lift a weight for 30 seconds. How much power did he use? Givens Solving For

Equation

Substitution

Answer with Units

2. A machine produces 4000 Joules of work in 5 seconds. How much power does the machine produce? Givens Solving For

Equation

Substitution

Answer with Units

3. Ms. Johnson can bench press 150 kg from 0.7 m from the ground to 1.5 m above the ground. a. How much weight (not mass) did Ms. Johnson lift? Givens Solving For

Equation

Substitution

Answer with Units

b. How much power did she use if she lifts the weights in 10s? Givens Solving For

Equation

Substitution

Answer with Units

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4. If it took Ms. Oldham 37 seconds to lift a 400 N student up 15 m, how much power did she use? Givens Solving For

Equation

Substitution

Answer with Units

5. Darth Vader unleashed the power of the dark side (1225 W) on the unsuspecting Jedi. If he did 727 J of work, how much time did it take? Givens Solving For

Equation

Substitution

Answer with Units

6. While playing 17 straight hours of the Wii, Mr. Wallaby successfully raised his arm to scratch his head 187 times. If he raised his hand 1 m and his arm had a mass of 2.3 kg, how much power was used? Givens Solving For

Equation

Substitution

Givens

Equation

Answer with Units

Solving For

Substitution

Answer with Units

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7. Superman moves a car 2700 N on a track of 500 m. If the car takes 32 seconds to move the entire distance, how much (super)power is exerted by Superman? Givens Solving For

Equation

Substitution

Givens

Equation

Answer with Units

Solving For

Substitution

Answer with Units

8. Superman is unhappy with his time in the above problem, so he attempts to lift the same car. This time, it takes 18.1 seconds. How much does his power increase? Givens Solving For

Equation

Substitution

Answer with Units

9. If a man slowly lifts a 20 kg bucket from a well and does 6.00 kilojoules of work (1000 Joules = 1 kilojoule), how deep is the well? Givens Solving For

Equation

Substitution

Givens

Equation

Answer with Units

Solving For

Substitution

Answer with Units

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10. Hulky and Bulky are two workers being considered for a job at the UPS loading dock. Hulky boasts that he can lift 100 kg box 2.0 m vertically in 3.0 seconds. Bulky counters with his claim of lifting a 200 kg box 5.0 m vertically in 20 seconds. Which worker has a greater power rating? __________________________________________ Hulky (3 steps) Givens Equation

Solving For Substitution

Givens Equation

Solving For Substitution

Givens Equation

Substitution

Substitution

Answer with Units

Solving For Substitution

Givens Equation

Answer with Units

Solving For

Givens Equation

Answer with Units

Solving For

Bulky (3 steps) Givens Equation

Answer with Units

Answer with Units

Solving For Substitution

Answer with Units

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11. You and a friend run up stairs that are 30 m high. Both of you reach the top in 12 seconds. You weigh 570 N and your friend weighs 620 N. Which of you has more power? _____________ Why? _______________________________________________ You Givens Equation

Solving For Substitution

Givens Equation

Solving For Substitution

Your Friend Givens Equation

Answer with Units

Solving For Substitution

Givens Equation

Answer with Units

Answer with Units

Solving For Substitution

Answer with Units

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How Many Horses? It was the late 1800’s, and engineer James Watt was stumped. He’d just figured out a way to make steam engines operate much more efficiently. He wanted to start manufacturing and selling his new invention. But how could he describe how powerful these amazing engines were? Watt’s answer? Compare the power of the steam engine with something that people were very familiar with: the power of a horse. In Watt’s day, ponies (small horses) were used to pull ropes attached to platforms that lifted coal to the surface of the earth from the mine below. Watt measured how much these loads weighed (force). Then he determined how far the ponies could raise them (distance) in one minute (time). Using these measurements, he calculated how much work a pony could do in a minute – he calculated the power of a pony – ponypower! At that time, the unit of work used by British scientists was the foot-pound (ft-lb). On the basis of his observations and calculations, Watt found that a pony could do 22,000 ft-lb of work a minute. Because he figured that the average horse was as powerful as 1.5 ponies, he multiplied the power of one pony (22,000 ft-lb of work per minute) by 1.5 and called it 1 horsepower (hp). In other words, 1 hp is equal to 33,000 ft-lb of work per minute, or 550 ft-lb of work per second. This means that an average horse can lift a 550-lb load a distance of 1 foot in 1 second. Horsepower can be translated into watts (W): 1 hp equals 750 W. A 350-hp engine, therefore, has the same power as a 262,500-W engine. But when numbers get as big as this, you can see that watts aren’t a convenient way of expressing the power of engines. So, the term “horsepower” stuck around. Using the word “horsepower” also probably makes drivers feel closer to the old days – when people were pioneers and mustangs were horses!! 1. Why do you think James Watt used a horse as a measure of a unit of power? ______________________________________________________________________________ ______________________________________________________________________________ 2. How did Watt decide the value of 1 horsepower? ______________________________________________________________________________ ______________________________________________________________________________ 3. Why is “horsepower” still a useful unit of power? ______________________________________________________________________________ ______________________________________________________________________________ 4. How many Watts make up 1 hp? ______________________________________________ 5. How long did it take a horse to lift 550 lb a distance of 1 ft, according to Watt? ___________ 19

Put it to Use: Power Up Background Information: • You are doing WORK when you use force to cause motion in the direction of force. • Work can be calculated mathematically. • The formula for work is: Work = force x distance • Time is not considered when calculating work • Force (or weight) is measured in Newtons. Distance is measured in meters. The unit for work is a Newton-meter. • A Newton-meter is called a Joule. • • •

POWER is the rate at which work is done. It is the amount to work per unit of time. The formula for Power is: Power = work / time Another way to calculate it is: Power = (force x distance) / time

Pre-lab Questions: 1. Do you do more work climbing stairs quickly or climbing stairs slowly? ________________ 2. Does it take more power to climb stairs quickly or climb stairs slowly? ________________ Materials: Calculator

Meter stick

Staircase

Bathroom Scale

Stopwatch

Procedure: 1. Calculate the DISTANCE you will move: •

Count the number of steps.

______________

•

Use the meter stick to measure the height of one step

______________ cm

•

Convert the height of one step from cm to m

______________m

•

Calculate the total height of the staircase and record in Data Table 1. (Hint: Height of the staircase = height of one step x number of steps.)

2. Calculate the FORCE you will use to climb the stairs •

Use the scale to find your mass

•

Convert your mass into weight in Newtons and record in Data Table 1.

______________kg ______________N

(This weight is your FORCE needed to climb the stairs.)

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3. One partner will now climb the stairs SLOWLY. 4. The other partner will use the stopwatch to time how long it takes to climb the stairs. 5. Record your data in Data Table 2. 6. Repeat two more times. 7. Calculate the average time to climb the stairs slowly. 8. The same partner will now climb the stairs QUICKLY. 9. The other partner will use the stopwatch to time how long it takes to climb the stairs. 10. Record your data in Data Table 3. 11. Repeat two more times. 12. Calculate the average time to climb the stairs quickly. 13. Calculate the amount of work and power that you did as you walked SLOWLY up the stairs. 14. Calculate the amount of work and power that you did as you walked QUICKLY up the stairs. 15. Record these values in Data Table 4.

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Data: Data Table 1: Background Data Height of Staircase (m) Force to climb stairs (N)

Data Table 2: Time to Climb the Stairs Slowly Time to Climb Slowly (s) Trial 1 Trial 2 Trial 3 Average

Data Table 3: Time to Climb the Stairs Quickly Time to Climb Stairs Quickly (s) Trial 1 Trial 2 Trial 3 Average

Data Table 4: Work & Power to Climb Stairs Work done climbing stairs (J) Slowly

Power to climb stairs (J/sec)

Quickly

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Analyze the Data: 1. Make a bar graph comparing the amount of WORK done when climbing the stairs slowly and quickly. Title: _______________________________________________________________

2. Make a bar graph comparing the amount of POWER done when climbing the stairs slowly and quickly. Title: _______________________________________________________________

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3. What do the graphs tell you about work and power? ______________________________________________________________________________ ______________________________________________________________________________ Post-lab Questions: 1. When did you do the most work, when you climbed the stairs slowly or quickly? ______________________________________________________________________________ 2. What two things could be changed to change the amount of work done? ______________________________________________________________________________ 3. When did you use the most power, when you climbed the stairs slowly or quickly? ______________________________________________________________________________ 4. If the amount of Work done stays the same, what must change in order to change the amount of power used? ______________________________________________________________________________

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WORK READING GUIDE Directions: Read pages 406 - 411 in your book, and answer the following questions. 1. What is work?

2. What two conditions need to exist in order to do work?

3. What is power?

4. What is the equation for Work?

5. What do work and power depend on? (What are the variables in the equation?)

6. What is the unit for work?

7. How are force and direction of motion related when talking about work?

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Bill Nye “Simple Machines” Directions: Answer the following questions as you watch Bill Nye. 1. Simple machines let us change the _________________________ and _______________________ of force. 2. What simple machine is in a catapult, and how does it work? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 3. What is the fulcrum of a lever? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 4. Which object shot by a catapult do you like best? Why? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 5. How are wheels and levers related? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 6. What parts of a bicycle are simple machines? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 7. How does a ramp make work easier? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

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8. Why doesn’t the rope break when a large book is pulled on a ramp? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 9. How is a screw related to a ramp? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 10. What does a pulley do to require less force? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ 11. What simple machines can be seen on a sailboat, and what do they do? ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________ ______________________________________________________________________________

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Simple Machines Internet Review Purpose: To review simple & compound machines Part 1: 1. Go to this website: http://edheads.org/activities/simple-machines 2. Click on the start button

3. A new window will open up. You can bounce between the two windows as necessary. 4. Choose your first activity from THE HOUSE (the second window) a. There are four locations in the house: the garage, the bedroom, the bathroom, and the kitchen.

5. Click on each location and do the activity as it is directed. MAKE SURE TO USE YOUR HEADPHONES!!!

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6. Go back to the original screen and click on the TOOLSHED.

6. Following the directions in the intro, click on each compound machine and figure out what simple machines make up the compound machine. 7. List the simple machines found in each compound machine.

Part 2 Go to the following website http://edheads.org/activities/odd_machine/index.htm Click on the start button and play THE ODD MACHINE GAME

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Going Up!! It was May 1854. The World’s Fair was being held in New York City. On display were the newest inventions from many countries. The crowds were amazed by the promise of technology. A crowd gathered around a tall, dignified man in a top hat. He mounted a platform. As people looked on, the platform was slowly raised by a rope that was wrapped around a motor-driven drum. When the platform had ascended well above the crowd, another figure standing on a landing above the platform suddenly reached out and slashed the heavy rope by which the platform was suspended. The crowd gasped. The platform dropped, but only by a few centimeters. Then it came to a stop. “All safe, ladies and gentlemen, all safe!” the man on the platform proclaimed. The man on the platform was Elisha Otis, and he’d just proudly demonstrated his invention – the safety elevator. His device would become the first public passenger elevator. Just three years after this dramatic demonstration, the first public passenger elevator was put into service at a New York City department store. By 1873, more than 2000 Otis elevators were being used in office buildings, hotels, and department stores. An Elevator Fit for a King The earliest elevators were little more than lifting platforms. More than 2000 years ago, the Romans described lifting platforms that featured pulleys and rotating drums. The power for these devices was supplied by humans or animals. In 1742, France’s King Louis XV had a private elevator built in his palace at Versailles. It was operated using human power. Servants pulled on ropes to lift and lower the king. Counterweights helped balance the weight of the king as he moved from floor to floor. These early elevators had a simple design. The car was suspended by a rope or cable that ran over a pulley at the top of the elevator shaft. At the other end of the cable was a counterweight that balanced with the weight of the car plus the average weight of the load the elevator carried. The car and the counterweight were guided between rails to keep from swinging freely. Putting on the Brakes Beginning in 1830 or so, freight elevators were in common use. But all these elevators, including the one used by King Louis XV, had a big drawback: if the rope from which they were suspended snapped, the elevator went crashing to the ground. There was nothing to cushion or stop its descent. That’s why Otis’s invention was so important. His safety elevator had something that none of the earlier models did – a brake. If the rope broke, a large spring forced two large latches to lock into ratchets on the guide rails. These latches kept the elevator from falling.

New Forms of Power 33

The earliest passenger elevators were powered by steam engines. As years passed, other power sources were used. Water pressure was tried. The invention of electric-powered elevators, like Otis’s safety device, was an important advance in elevator technology. The invention of the electric-powered elevator for passengers had a strong effect on city living. Before it came into use, most buildings were no more than four stories high. People just couldn’t huff and puff their way up any more flights of stairs! The lack of appropriate building materials was another drawback to the growth of tall buildings. Changing the Landscape – And People’s Lives By the beginning of the 20th century, the word “skyscraper” had entered the English language. Buildings were built taller and taller – and thanks to elevators, people could make their way easily to the top. Additional refinements included self-opening doors, an automatic leveling feature, and faster speeds. Modern elevators travel up to 600 meters a minute. Today, you can zoom to the top of the Washington Monument or the Empire State Building and back in minutes, thanks to electric-powered elevators. And, thanks to Elisha Otis, you can be assured of a safe trip in both directions.

1. How are pulleys used in elevators? ______________________________________________________________________________ ______________________________________________________________________________ 2. What is the purpose of the counterweight in an elevator? ______________________________________________________________________________ ______________________________________________________________________________ 3. What has been the impact of elevators on building design? ______________________________________________________________________________ ______________________________________________________________________________

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