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Chapter 8: Work, Power, and Simple Machines, Lecture notes of Physics

Université de LiègePhysics

Answer Key. Chapter 8: Work, Power, and Simple Machines. 8.1 Work ... The following website contains practice questions with answers on the topic of work.

Typology: Lecture notes

2021/2022

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CK-12 Physics Concepts - Intermediate
Answer Key
Chapter 8: Work, Power, and Simple Machines
8.1 Work
Practice
Questions
1. What definition is given in the video for energy?
2. What is the definition given in the video for work?
3. What unit is used in the video for work?
The following website contains practice questions with answers on the topic of work.
http://www.sparknotes.com/testprep/books/sat2/physics/chapter7section6.rhtml
Answers
1. Energy is the ability to do work.
2. Work is the energy transferred by a force, given by force times distance.
3. The units used in the video for work are Newton-meters or Joules.
Review
Questions
1. How much work is done by the force of gravity when a 45 N object falls to the ground
from a height of 4.6 m?
2. A workman carries some lumber up a staircase. The workman moves 9.6 m
vertically and 22 m horizontally. If the lumber weighs 45 N, how much work was done
by the workman?
3. A barge is pulled down a canal by a horse walking beside the canal. If the angle of
the rope is 60.0°, the force exerted is 400. N, and the barge is pulled 100. m, how much
work did the horse do?
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Download Chapter 8: Work, Power, and Simple Machines and more Lecture notes Physics in PDF only on Docsity!

CK-12 Physics Concepts - Intermediate

Answer Key

Chapter 8: Work, Power, and Simple Machines

8.1 Work

Practice

Questions

  1. What definition is given in the video for energy?
  2. What is the definition given in the video for work?
  3. What unit is used in the video for work? The following website contains practice questions with answers on the topic of work. http://www.sparknotes.com/testprep/books/sat2/physics/chapter7section6.rhtml

Answers

  1. Energy is the ability to do work.
  2. Work is the energy transferred by a force, given by force times distance.
  3. The units used in the video for work are Newton-meters or Joules.

Review

Questions

  1. How much work is done by the force of gravity when a 45 N object falls to the ground from a height of 4.6 m?
  2. A workman carries some lumber up a staircase. The workman moves 9.6 m vertically and 22 m horizontally. If the lumber weighs 45 N, how much work was done by the workman?
  3. A barge is pulled down a canal by a horse walking beside the canal. If the angle of the rope is 60.0°, the force exerted is 400. N, and the barge is pulled 100. m, how much work did the horse do?

Answers

  1. Using 𝑤 = 𝑓𝑑 ∶ 45𝑁 ∗ 4.6𝑚 → Gravity does 207 Nm of work.
  2. The only force the workman is working against is the force of gravity, so only the vertical change is relevant. Using 𝑤 = 𝑓𝑑 ∶ 9.6𝑚 ∗ 45𝑁 → The workman did 432 Nm of work.
  3. Using 𝑤 = 𝑓𝑑 ∶ (400𝑁 ∗ 𝑐𝑜𝑠60°)(100𝑚) → The horse does 20,000 Nm of work.

8.2 Power

Practice

Questions

  1. What is the difference between positive and negative work?
  2. What are the standard units for power?
  3. What is horsepower?
  4. How many grandfather clocks could you power with the same amount of power used by a single light bulb?

Answers

  1. Positive work is the energy you put into a system, while negative work is the power that comes out.
  2. The standard units for power are Watts, defined as Joules/sec (J/s).
  3. Horsepower was how James Watt described the power of his machines to other people.
  4. You could run almost 300,000 clocks on the same amount of power as a single light bulb.

Review

Questions

  1. If the circumference of an orbit for a toy on a string is 18 m and the centripetal force is 12 N, how much work does the centripetal force do on the toy when it follows its orbit for one cycle?
  2. A 50.0 kg woman climbs a flight of stairs 6.00 m high in 15.0 s. How much power does she use?
  3. Assuming no friction, what is the minimum work needed to push a 1000. kg car 45. m up a 12.5° incline?
  1. No. The work input always equals the work output in an ideal machine, and the input will exceed the output in real machines because of friction.
  2. a. The IMA is 33m/16.5m, or 2. b. The AMA is 225N / 129N = 1.74.
  3. 𝑑𝑒 = (𝐹𝑟𝑑𝑟) 𝐹𝑒^ ∶^ (𝑑𝑒) =^

1250𝑁∗0.13𝑚 225𝑁 ∶^ The boy moves his side of the lever 0.722m.

  1. You can increase the IMA of an inclined plane by decreasing the angle of the inclined plane, or by increasing the length in proportion to the vertical height.

a. 𝐹𝑒 = 𝐹𝑟𝑑𝑟 𝑑𝑒^ ∶^ 𝐹𝑒^ =^

1000𝑁∗5.00𝑚 20.0𝑚 ∶^ Diana applied a force of 250. N. b. 300 N – 250 N = 50. N to overcome the force of friction. c. The work output is 1000𝑁 ∗ 5𝑚 = 5000𝑁𝑚 = 5000𝐽 d. The IMA is 20𝑚 5𝑚 =^ 4. e. The AMA is 1000𝑁 300𝑁 ∶^ 3.33.

a. The worker does 1042 J of work. This is obtained by multiplying his force (496 N) by the distance (2.10 mm). b. 84 J of his work is overcoming friction. We can calculate the amount of work necessary to lift the refrigerator by multiplying the distance (0.85 m) by the acceleration (9.8 m/s^2 ) by the mass (115 kg) for a total of 958 J. Subtract this work done on the refrigerator from the total work to determine the work done overcoming friction.

  1. The question specifies the diameter of the screw, therefore the IMA of this screw

is 0.812𝜋 0.318 =^ 8.02.