Name: _________________________________________________ Period: 1 2 3 4 5 6 7

Work Questions and Problems

Part 1: Is there work done in this situation? Circle your answer.

1. Darryl carries an 8-kg cat across the road to his friend’s house, 50 m away. WORK / NO WORK

2. Jasmine pushes a 400-N wheelbarrow up a hill angled at 50 degrees for 4 m. WORK / NO WORK

3. Scott throws a ball 6 m into the air with a force of 4 N directly upward. WORK / NO WORK

4. Yeni kicks a soccer ball 10 m horizontally, overcoming a 4-N force of friction. WORK / NO WORK

5. Using a pulley system, Forrest raises a car 2 m, using 1200 N of force. WORK / NO WORK

6. Hunter pushes against a wheelbarrow with 40 N of force, but doesn’t move it. WORK / NO WORK

Part 2: True / False. Circle your answer. Rewrite any false statements in a way that makes them true. READ VERY CAREFULLY!

7. If an object does not move, there is no work done. TRUE / FALSE

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8. If an object moves in a direction perpendicular to the force applied, work is done. TRUE / FALSE

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9. If you push an object 5 m and exert 12 N of force, the work done is 60 kg. TRUE / FALSE

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10. The greater the angle between force and distance, the less work is done. TRUE / FALSE

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11. The maximum amount of work is done when force and distance are at 45o. TRUE / FALSE

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12. If one person lifts a 30-N box 2 m and one lifts a 40-N box 1.5 m, they do equal amounts of work. TRUE / FALSE

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Part 3: Word Problems. Solve in the space provided. SHOW YOUR WORK!

13. EQUATIONS (fill in from your notes):

weight = work = 14. Bud stands on a pogo stick. How much work is done if Bud’s mass is 130 kg and he compresses the

spring 0.50 m?

15. Dean pushes a 40-kg box up a ramp. The vertical distance he raises the box is 2 m. How much work did Dean do?

16. Sam lifts a similar 40-kg box directly upward 2 m. How much work did Sam do?

17. Angela pulls a wagon with 50 N of force and moves the wagon horizontally 8 m. How much work did she do?

18. Jack pushes a crate with 80 N of force and moves the crate horizontally 14 m. How much work did he do?

19. Kaylee lifts 3-kg box 1.5 m. How much work did she do?

20. Simon carries a 15-N box across a flat road for 11 m. How much work did he do?

Name: _________________________________________________ Period: 1 2 3 4 5 6 7

Power Problems Power is a rate - the amount of work done per unit of time: P=w/tPower = work/time work = joules time = seconds power = joules/s = watts

SHOW YOUR WORK! Place your answers in the boxes provided.

1. A set of pulleys is used to lift a piano weighing 1000.0 N. The piano is lifted 3 m in 60 seconds. How much power is used?

2. How much power is used if a force of 35 N is used to push a box a distance of 10 m in 5 seconds?

3. What is the power of a kitchen blender if it can perform 3750.0 Joules of work in 15 seconds?

4. How much work is done using a 500 Watt microwave oven for 5 minutes?

5. How much work is done using a 60 Watt light bulb for 1 hour?

Name: _________________________________________________ Period: 1 2 3 4 5 6 7

Identify the Following Simple Machines(inclined plane, wheel and axle, wedge, pulley, lever, screw)

How does the water drain?

Name: _________________________________________________ Period: 1 2 3 4 5 6 7

Mechanical Advantage of Simple MachinesAMA = FR/FEFR = resistance forceFE = effort force

IMA levers and ic planes = dE/dR dE = effort distancedR = resistance distance

IMA wheel and axle = R/r R = wheel radiusr = axle radius

IMApulley = # ropes supporting weight

5m 2.5m

Name: _________________________________________________ Period: 1 2 3 4 5 6 7

Calculating Efficiency

Calculate the efficiency and show your work! Place answers including units in boxes.1. A man expends 100 J of work to move a box up an inclined plane. The amount of work produced is

80 J.

The amount of work obtained from a machine is always less than the amount of work put into it. This is because some of the work is lost due to friction. The efficiency of a machine can be calculated using:

% Efficiency = work output/work input x 100

2. A box weighing 100 N is pushed up an inclined plane that is 5m long. It takes a force of 75 N to push it to the top, which has a height of 3 m.

3. Using a lever, a person applies 60 N of force and moves the lever 1 meter. This moves a 200 N rock at the other end by .2 m.

4. A person in a wheelchair exerts a force of 25 N to go up a ramp that is 10 m long. The weight of the person and wheelchair is 60 N and the height of the ramp is 3 m.

5. A boy pushes a lever down 2 m with a force of 75 N. The box at the other end with a weight of 50 N moves up 2.5 m.

6. A pulley system operates with 40% efficiency. If the work put in is 200J, how much useful work is produced?

Work and Power Problems and Questionsweight = mg F = ma W = F·d P = W/t

SHOW YOUR WORK. USE UNITS. PUT A BOX AROUND YOUR ANSWERS!

1. How much work is done when a 5-N force moves a block 4 m?

2. How much work is done when a 4.33 N force moves a block 4 m?

3. How much work is done when a 3.54 N force moves a block 4 m?

4. How much work is done when a 2.5 N force moves a block 4 m?

5. How much work is done when no force is used to move a block 4 m?

6. As the force applied increases, the amount of work done ( increases / decreases / remains the same ).

7. When the angle between force and distance is ( 0 / 45 / 90 ) degrees, the amount of work done will be the maximum value.

8. When the angle between force and distance is ( 0 / 45 / 90 ) degrees, there is no work done at all.

9. How much work is done when a 100-N force moves a block 25 m?

10. How much work is done when a 100-N force moves a block 50 m?

11. How much work is done when a 100-N force moves a block 75 m?

12. How much work is done when a 100-N force moves a block 100 m?

13. As the distance increases, the amount of work done (increases / decreases / remains the same).

14. How much power is used when a 25-N force moves a block 15 m in 8 s?

15. How much power is used when a 25-N force moves a block 15 m in 12 s?

16. How much power is used when a 25-N force moves a block 15 m in 16 s?

17. How much power is used when a 25-N force moves a block 15 m in 24 s?

18. As the time increases, the amount of power generated by a force ( increases / decreases / remains the same )

19. If you get work done faster than someone else, your power is ( greater / less ) than theirs.

20. How far will 350 J raise a block that weighs 5 N?

21. How far will 350 J raise a block with a mass of 20 kg?

22. How far will 350 J raise a block that weighs 100 N?

23. How far will 350 J raise a block with a mass of 100 kg?

24. How much power does it take to do 500 J of work in 10 s?

25. How much power does it take to do 100 J of work in 8 s?

26. How much work can be done by a 300-W motor in 60 s?

27. How much work can be done by a 100-W motor in 60 s?

28. How much power does it take to lift 30 N 10 m high in 5 s?

29. How much power does it take to lift 30 kg 10 m high in 5 s?

30. How much power does it take to lift a 250-N crate 40 m high in 2 s?

31. You expend 500 W of power to raise a box 5 m in 10 s. What force did you use?

32. Which is more work? ( pushing with 115 N of force over 15 m / lifting a 20-N object up 10 m )

33. Which is more work? ( pushing with 115 N of force over 15 m / lifting a 20-kg object up 10 m )

Energy

Energy is defined as the ability to do work. (Remember that work is defined as a change in energy caused by a force applied over a distance.) Energy and work have the same unit: the Joule (J). Energy comes in many forms, including chemical, electrical, nuclear, thermal, and mechanical. For this semester, we'll be focusing on mechanical energy: the energy of movement or position, which includes kinetic energy and potential energy.

Kinetic energy (KE) is the energy of motion. Like momentum, kinetic energy depends on the object's mass and velocity. Unlike momentum, velocity has a MUCH greater effect on kinetic energy than the mass does. If you double the mass of an object, the kinetic energy doubles. If you double the velocity of the object, the kinetic energy quadruples (becomes 4x as much). The equation for kinetic energy is:

KE = (1/2)mv2. Because you're dealing with velocity, sometimes you'll need to use kinematic equations to figure out an object's speed before you can figure out its kinetic energy. Be warned!

Potential energy (PE) is the amount of stored energy in an object. Compressed springs and stretched rubber bands have elastic potential energy; batteries, fossil fuels, and food all contain chemical potential energy. In this class, we'll be working with gravitational potential energy, or the energy stored when objects are elevated. The higher off the ground an object gets, the greater its gravitational potential energy. More massive objects have greater gravitational potential energies than less massive objects. The equation for gravitational potential energy is: PE = mgh (so basically, PE = weight x height).

Work is defined as a change in energy due to a force applied over a certain distance. Therefore, whenever work is done, the energy changes, and if energy changes, then work is done. Work is equal to the change in energy. This is called the work-energy theorem and can be stated in its short equation form as W = ΔE.

Energy cannot be created or destroyed: it can be converted from one form to another, but the total amount of energy doesn't change. This is the definition of the law of conservation of energy. What we mean when we say energy is "lost" is that energy has been converted to a form that we can't measure easily—usually heat. Whenever we deal with friction or air resistance, some of the energy is converted to heat, so it seems that energy is lost. In reality, it's still out there. We just can't harness it.

For mechanical energy, the law of conservation of energy states that total initial mechanical energy is equal to total final mechanical energy. In other words: PEo + KEo = PEf + KEf

Name: ________________________________________________ Period: 1 2 3 4 5 6 7

Potential Energy1. What is potential energy? _____________________________________________________________________________

2. What is gravitational potential energy? ________________________________________________________________

3. What is the equation for gravitational potential energy? _______________________________________________

4. Find the gravitational potential energy for the following situations:a. 20 kg, 6 m high b. 15 kg, 9 m high

c. 50 kg, 2 m high d. 8 kg, 16 m high

e. 3 kg, 20 m high f. 50 kg, 0 m high

g. 12 kg, 16 m high h. 150 kg, 4 m high

5. A 5-kg box is dropped from a building roof 60 m above the ground. a. What is the box's initial potential energy?

b. After 1 s, how far had the box fallen?

c. How high above the ground was the box after 1 s? (Original height – distance fallen)

d. What was the potential energy after 1 s?

e. After 2 s, how far had the box fallen?

f. How high above the ground was the box after 2 s? (Original height – distance fallen)

g. What was the potential energy after 2 s?

Name: _________________________________________________ Period: 1 2 3 4 5 6 7

Kinetic EnergyPart 1: Definitions/Concepts Review

1. What is energy? _______________________________________________________________________________________

2. What is mechanical energy? ___________________________________________________________________________

3. What are the 2 types of mechanical energy? __________________________________________________________

4. What is kinetic energy?_________________________________________________________________________________

5. What is the kinetic energy equation? __________________________________________________________________

Part 2: PROBLEMS. You will need the kinetic energy AND kinematics equations for this!

6. A 5-kg object is traveling at 12 m/s. What is its kinetic energy?

7. A 15-kg object is at rest. What is its kinetic energy?

8. A 25-kg object increases speed from 13 m/s to 20 m/s.a. What is the initial kinetic energy?

b. What is the final kinetic energy?

9. Mary pushes a 3-kg box 2 m in 4 seconds.a. What is the average speed of the box?

b. What is the box’s kinetic energy?

10. A 1200-kg car accelerates from rest at 2 m/s2 for 4 s. a. What is the object’s initial kinetic energy?

b. What is the object’s final velocity? (USE KINEMATIC EQUATIONS!)

c. What is the object’s final kinetic energy?

11. A 0.8-kg apple has been falling for 3 seconds. It was dropped from rest. a. What is the velocity of the apple after 3 seconds?

b. What is the kinetic energy of the apple after 3 seconds?

12. An 18-kg object accelerates at 4 m/s2 from 4 m/s over a distance of 9 m. a. What is the object’s final velocity?

b. What is the initial kinetic energy?

c. What is the final kinetic energy?

13. It takes 6 seconds for a 20-kg object to stop when its acceleration is -4 m/s2. a. What was the object’s initial velocity?

b. What was the final kinetic energy?

c. What was the initial kinetic energy?

14. A 2-kg book is dropped from a REALLY tall building.a. What is the book’s velocity after 4 seconds?

b. What is the book’s kinetic energy after 4 seconds?

c. What is the book’s velocity after 8 seconds?

d. What is the book’s kinetic energy after 8 seconds?

15. A 12-kg box is dropped from the top of a building, 50 m above the ground. a. What is the box’s final velocity (just before it hits the ground)?

b. What is the box’s final kinetic energy?

Name: ________________________________________________ Period: 1 2 3 4 5 6 7

Work-Energy Theorem

KE = ½·m·v2 W = F·d·cos(A) W = ΔKE

vf = vo + at vf2 = vo

2 + 2ad d = vot + ½ at2

1. Penny drops a 3-kg box off a bridge and it falls -60 m to the river below.a. What is the box’s initial velocity?

b. What is the box’s initial kinetic energy?

c. What is the box’s final velocity (just before it hits the ground)?

d. What is the box’s final kinetic energy?

e. How much work was done by gravity?

f. What is the box’s weight?

g. Verify that the work done by gravity using the equation W = F·d matches your answer to part e.

2. A 2.5-kg box is dropped from an airplane.a. What is the box’s initial velocity?

b. What is the box’s initial kinetic energy?

c. What is the box’s velocity after 6 seconds?

d. What is the box’s kinetic energy after 6 seconds?

e. How much work was done by gravity?

f. What is the box’s weight?

g. How far did the box fall during those 6 seconds?

3. Alfred pulls a 50-kg wagon from rest up to 4 m/s in 6 s. a. What is the wagon’s acceleration?

b. What force did he use to move the wagon?

c. How far did he move the wagon?

d. How much work did he do?

e. What was the wagon’s initial kinetic energy?

f. What was the wagon’s final kinetic energy?

g. According to the work-energy theorem, how much work was done?

h. Are your answers to d & g the same?

i. If friction was present, would your answers to d & g be the same? Explain.

4. According to the work-energy theorem, the amount of work done is equal to an object's

________________________________________________________.

Name: ________________________________________________ Period: 1 2 3 4 5 6 7

Conservation of Energy

KE = ½·m·v2 PE = m·g·h

1. A 1500-kg boulder sits at the top of an 80-m-tall cliff. Someone pushes it until it falls over the edge.a. What is the initial potential energy of the boulder?

b. What is the initial kinetic energy of the boulder?

c. What is the boulder’s speed after it has fallen for 1 s?

d. What is the boulder’s kinetic energy after 1 s?

e. What is the boulder’s potential energy after 1 s?

f. How high up is the boulder after 1 s?

g. What is the boulder’s speed after 2 s?

h. What is the boulder’s kinetic energy after 2 s?

i. What is the boulder’s potential energy after 2 s?

j. How high up is the boulder after 2 s?

k. What is the boulder’s final velocity (just before it hits the ground)?

l. What is the boulder’s final kinetic energy (just before it hits the ground)?

m. What is the boulder’s final potential energy (just before it hits the ground)?

2. A 0.5-kg ball is tossed from ground level straight up into the air with an initial velocity of 10 m/s. It rises to a height of 4 m.

a. What is the ball’s initial kinetic energy?

b. What is the ball’s kinetic energy at the top of the arc?

c. What is the ball’s initial potential energy?

d. What is the ball’s potential energy at the top of the arc?

e. What was the change in potential energy? (ΔPE = PE – PEo)

f. What was the change in kinetic energy? (ΔKE = KE – KEo)

g. What was the total change in energy? (ΔE = ΔKE + ΔPE)

h. How much energy was “lost” due to air resistance?

i. If there had been no air resistance, how high could the ball have gone?

3. The Law of Conservation of Energy says that ___________________________________________________________

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4. Energy that is “lost” is usually converted into _______________ energy.