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HOMEWORK PACKET: WORK, POWER, ENERGY
Part 1: Review of Notes – Work & Power
1. Circle the numbers that represent force.
Draw a square around the numbers that represent distance.
Underline the numbers that represent work. Double underline the numbers that represent power.
17 m
23 N
23 W
13 N
81 W
14.5 m 88 J
11 s
21 m
124 N
17 J
5N
0.25 N
2.2 W
0J
2. How is power related to work?
3. If it takes 100 N of force to move an object 10 meters, how much work is done?
4. If you are holding a 1000-lb car engine over your head for 50 seconds, how much work is done?
5. How much power is produced when 500J of work is done in 2 seconds? When 750J of work is done in 3
second? 600J in 4 sec?
6. To produce more power, do you want to do work faster or slower? Why?
W
P
W
t
F
d
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Part 2: Work and Power Calculations
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Part 3: Calculations Work, Power
1. How many joules of work are done when a force of 1 N moves a book 2 m?
2. How much work is done when you push a crate horizontally with 100 N across a 10 m factory floor?
3. A car moving at 50 km/h skids 15 m with locked brakes. How far will the car skid with locked brakes at 150
km/h?
4. How many watts of power are expended when a force of 1 N moves a book 2 m in a time of 1 s?
5. A deflated hot-air balloon weighs a total of 8000 N. Filled with hot air, the balloon rises to a height of 1000
m. How much work is accomplished by the hot air?
6. A rope is thrown over a beam, and one end is tied to a 300 N bundle of lumber. You pull on the free end
of the rope with a force of 400 N and lift the lumber 2 m off the ground. How much work have you done?
Part 4: More Work and Power (more challenging!)
Directions: Explain if work is being done in each of the following scenarios and explain why.
1. A teacher applies a force to a wall and becomes exhausted.
2. A book falls off a table and free falls to the ground.
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3. A waiter carries a tray full of meals above his head by one arm straight across the room at constant speed.
(Careful! LOOK at the picture above!)
4. A rocket accelerates through space.
Directions: Use your understanding of work and power to answer the following questions.
5. Two physics students, Will N. Andable and Ben Pumpiniron, are in the weightlifting room. Will lifts the 100pound barbell over his head 10 times in one minute; Ben lifts the 100-pound barbell over his head 10 times
in 10 seconds. Which student does the most work? Which student delivers the most power? Explain your
answers.
6. During a physics lab, Jack and Jill ran up a hill. Jack is twice as massive as Jill; yet Jill ascends the same
distance in half the time. Who did the most work? Who delivered
the most power? Explain your answers.
7. A tired squirrel (mass of approximately 1 kg) does push-ups by applying a force to elevate its center-of-mass
by 5 cm in order to do a mere 0.50 Joule of work. If the tired squirrel does all this work in 2 seconds, then
determine its power.
8. When doing a chin-up, a physics student lifts her 42.0-kg body a distance of 0.25 meters in 2 seconds. What
is the power delivered by the student's biceps?
9. An escalator is used to move 20 passengers every minute from the first floor of a department store to the
second. The second floor is located 5.20 meters above the first floor. The average passenger's mass is 54.9
kg. Determine the power requirement of the escalator in order to move this number of passengers in this
amount of time.
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Part 5: KE and PE Changes (ME is mechanical energy)
Part 6: Potential and Kinetic Energy Calculations
1.
What is the kinetic energy of a 1000 kg rollercoaster moving at a speed of 20 m/s?
2. If the rollercoaster above was moving twice as fast, what is the new KE?
3. If the rollercoaster in number 1 had double the mass and the same speed, what would the kinetic energy be?
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4. Missy Divewater, the former platform diver for the Ringling Bros Circus, had a kinetic energy of 15,000 J
just before she hit the water. If her mass is 50 kg, what is her speed?
5. A 750 kg compact car travels at 100 km/hr and has 3,750,000 J of kinetic energy. What is the KE of the car
if it travels at 50 km/hr?
6. John has a 50 kg object suspended in air, 50 m above the ground. If the object is dropped, how much
work will be done?
7. Mrs. Jacobs dropped an object from a height of 10 m. What was the weight of the object if it did 50 J of
work?
8. A cart is loaded with a brick and pulled at a constant speed along an inclined plane to the height of a seat
top. If the mass of the brick and cart is 3.0 kg and the seat is 0.45 m high, what is the potential energy of
the brick and cart when they reach the top?
Part 7: More Kinetic and Potential Energy Calculations
1. A moving car has kinetic energy. If it speeds up until it is going 4 times the original speed, how much kinetic
energy does it have compared to the original?
2. When the mass of a moving object is doubled with no change in speed, by how much will the kinetic energy
change?
3. When the velocity of an object is doubled, by what factor is its kinetic energy changed?
4. Car A is lifted a certain distance in a service station and therefore has potential energy relative to the floor.
If car B were lifted twice as high, how much potential energy would Car B have compared to Car A?
5. Two cars are lifted to the same height in a service station. If car A is twice as massive as the Car B, which has
more potential energy? How much more? Why?
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6. How many joules of potential energy does a 1 N book gain when it is elevated 4 m? 8 m?
Part 8: Conservation of Energy
A bull drops a rock off the edge of a cliff. The rock is 2 kg and the cliff is 125 m tall.
1. What is the potential energy of the rock before it begins to fall?
2. What is the kinetic energy before it begins to fall?
3. 3 seconds after the rock is dropped, what is its new height off
the ground below?
Hint: Use this formula for distance fallen d = 4.9 (t) ²
4. So what is its new P.E.?
5. And how much P.E. has it “lost” since starting to fall?
6. What is the new velocity of the rock 3 seconds after it got dropped?
7. So what is its new kinetic energy?
8. And how much KE has it gained?
9. Now compare the amount of PE lost and KE gained. Explain
10. Now do the same calculations (3 through 8) but consider the rock 5 seconds after it was dropped.
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Part 9: Energy Crossword
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Part 10: Energy Transformations
Directions: For each event (1-8), identify the type of energy before the transformation and type of energy after
the transformation. Then, write a one sentence description of the energy transformation
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Part 11: Energy Efficiency – Read the following and then answer the questions.
We all use devices every day that use energy - or more accurately, transfer energy from one form to another.
Everything we use wastes energy - some of the energy transfers into forms that are not useful to us. For example
when driving a car, energy from burning fuel is transferred into kinetic energy of the car. However, more than
half the energy is lost as heat and sound. How effectively devices transfer energy - i.e. how much of the energy
used is useful - is called its efficiency. Efficiency is normally calculated as a percentage - something 90% efficient
is considered good at its job. Devices that transfer only 5% of the energy they use into something useful are
inefficient (very wasteful). We can calculate efficiency if we know the total energy used, and how much is
transferred into useful forms. To solve for efficiency think of it this way, the resulting work that a machine does
for you (work output) divided by the net work you do (work input). Example: If you do 80 Joules of work
(work input) on a something and get 40 Joules of work out of it (work output), you have 50 % efficiency. 40 J
/ 80 J = .50 or 50% IT WILL ALWAYS BE PERCENTAGE!!!!
1. A student pushes on a lever with a force 175 N through a height of 5 m. As a result a 450 N crate raises 1 m
upward. Calculate the efficiency of this machine.
2. What is the efficiency of a pulley that can lift a 250N treasure chest 13 m when 150 N of force is used to pull
the rope 25 m?
3. A wedge is calculated to be 82% efficient. If the wedge produces 210 J of work, how much work was put
into the wedge?
4. A box weighing 100 N is pushed up an inclined plane that is 5 meters long. It takes a force of 75 N to push it
to the top, which has a height of 3 meters. Calculate the efficiency of this machine.
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