Concept-Development 9-1 Practice Page

Name

Work and Energy

Class

Date

Concept-Development Practice Page

9-1

1. How much work (energy) is needed to lift an object that weighs 200 N to a height of 4 m? 800 J

2. How much power is needed to lift the 200-N object to a height of 4 m in 4 s? 200 W

3. What is the power output of an engine that does 60,000 J of work in 10 s? 6 kW

4. The block of ice weighs 500 newtons.

a. What is the mechanical advantage of the incline? 2:1

b. How much force is needed to push it up the incline (neglect friction)? 250 N

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5. All the ramps are 5 m high. We know that the KE of the block at the bottom of the ramp will be equal to the loss of PE (conservation of energy). Find the speed of the block at ground level in each case. [Hint: Do you recall from earlier chapters how long it takes something to fall a vertical distance of 5 m from a position of rest (assume g = 10 m/s2)? And how much speed a falling object acquires in this time? This gives you the answer to Case 1. Discuss with your classmates how energy conservation gives you the answers to Cases 2 and 3.]

Case 1: Speed = 10 m/s Case 2: Speed = 10 m/s Case 3: Speed = 10 m/s Block on A reaches bottom first; greater acceleration and less ramp distance. Although it will have the same speed at bottom, the time it takes to reach that speed is different!

CONCEPTUAL PHYSICS

Chapter 9 Energy 47

6. Which block gets to the bottom of the incline first? Assume no friction. (Be careful!) Explain your answer. Ball A gets to the bottom first due to a greater acceleration down a shorter ramp. (Note that SPEED at the bottom, not TIME, is the same for both.)

7. The KE and PE of a block freely sliding down a ramp are shown in only one place in the sketch. Fill in the missing values.

75 J

25 J

25 J

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8. A big metal bead slides due to gravity along an upright friction-free wire. It starts from rest at the top of the wire as shown in the sketch. How fast is it traveling as it passes

Point B? 10 m/s

Point D? 10 m/s

Point E? 10 m/s

At what point does it have the maximum

speed?

C

CONCEPTUAL PHYSICS 48 Chapter 9 Energy

9. Rows of wind-powered generators are used in various windy locations to generate electric power. Does the power generated affect the speed of the wind? Would locations behind the "windmills" be windier if they weren't there? Discuss this in terms of energy conservation with your classmates. Yes, by the conservation of energy, the energy gained

by the windmills is taken from the KE of the wind.

So strictly speaking, the wind must slow down and

locations behind would be a bit windier without the

windmills.

Name

Conservation of Energy

Class

Date

Concept-Development Practice Page

9-2

1. Fill in the blanks for the six systems shown.

30 J

30 J 20 J

30 J

3750 J

4 ? 106 J

9 ? 106 J

7500 J

50 J

104 J

25 J

11250 J 15000 J

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8 J CONCEPTUAL PHYSICS

10 J

10 J 0 J

Chapter 9 Energy 49

2. The woman supports a 100-N load with the friction-free pulley systems shown below. Fill in the spring-scale readings that show how much force she must exert.

100 N

50 N

50 N

3. A 600-N block is lifted by the friction-free pulley system shown.

a. How many strands of rope support the 600-N weight? 6

b. What is the tension in each strand? 100 N

c. What is the tension in the end held by the man? 100 N

d. If the man pulls his end down 60 cm, how many cm will the weight rise? 10 cm

e. What is the ideal mechanical advantage of the pulley system? 6:1

f. If the man exerts 60 joules of work, what will be the increase of PE of the 600-N weight? The same, 60 J

4. Why don't balls bounce as high during the second bounce as they do in the first? During each bounce, some of the ball's mechanical energy is transformed into heat (and even sound), so the PE decreases with each bounce.

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CONCEPTUAL PHYSICS 50 Chapter 9 Energy

Name ___________________________ Class __________________ Date ____________ Chapter 9 Energy

Exercises

9.1 Work (pages 145?146)

1. Circle the letter next to the correct mathematical equation for work.

a. work = force ? distance c. work = force ? distance

b. work = distance ? force d. work = force ? distance2

2. You can use the equation in Question 1 to calculate work when

the force is

constant

and the motion takes place in

a straight line in the direction of the force .

3. You do work if you lift a book one meter above the ground. How does the amount of work change in each of the following cases?

a. You lift the book twice as high. You do twice as much work.

b. You lift two identical books one meter above the ground.

You do twice as much work.

4. Complete the table by naming the two general categories of work and giving an example of each.

Category of Work

work done against another force

Example

Possible answer: When an archer stretches her bow, she is doing work against the elastic

forces of the bow.

work done to change the speed of an object Possible answer: An engine does work

increasing or decreasing the speed of a car.

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5. The unit of work is the

joule

.

6. Suppose that you apply a 50-N horizontal force to a 25-kg box, pushing

the box 6 meters across the floor. How much work do you do on the box?

300 J

9.2 Power (pages 146?147)

7. Power is the rate at which

work

is done.

8. Power equals

work done

divided by

time interval

.

9. The unit of power is the

watt

.

10. One megawatt (MW) equals

one million

watts.

11. In the United States, we customarily rate engines in units of

which is equivalent to

0.75

kilowatt.

horsepower

,

9.3 Mechanical Energy (page 147)

12. Define energy.

the property of an object or system that enables it to do work

13. What is the SI unit of energy?

joule

Conceptual Physics Reading and Study Workbook N Chapter 9 67

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