Skip to main content

Introduction to Exercise Science for Fitness Professionals: Doing Work

Introduction to Exercise Science for Fitness Professionals
Doing Work
    • Notifications
    • Privacy
  • Project HomeNatural Sciences Collection: Anatomy, Biology, and Chemistry
  • Projects
  • Learn more about Manifold

Notes

Show the following:

  • Annotations
  • Resources
Search within:

Adjust appearance:

  • font
    Font style
  • color scheme
  • Margins
table of contents
  1. Cover
  2. Title Page
  3. Copyright
  4. Table Of Contents
  5. Attribution and OER Revision Statement
  6. Chapter 1: Body Systems Review
    1. The Cardiovascular System
    2. The Nervous System
    3. Reflexes
    4. The Skeletal System
    5. Divisions of the Skeletal System
    6. Skeletal Muscle
    7. Divisions of the Skeletal Muscles
    8. Describing Motion and Movements
    9. Identify Anatomical Locations
  7. Chapter 2: Biomechanics and Human Movement
    1. The Basics of Biomechanics
    2. Inertia and Momentum
    3. Force
    4. Doing Work
    5. Body Levers
    6. Nervous System Control of Muscle Tension
    7. Muscle Tissue and Motion
  8. Chapter 3: Exercise Metabolism
    1. Introduction to Bioenergetics and Metabolism
    2. Overview of Metabolic Reactions
    3. Metabolic States of the Body
    4. The Cardiorespiratory System and Energy Production
    5. ATP in Living Systems
    6. Types of Muscle Fibers
    7. Exercise and Muscle Performance
    8. Nutrition, Performance, and Recovery
    9. Carbohydrate Metabolism
    10. Protein Metabolism
    11. Lipid Metabolism
  9. Chapter 4: Fitness Principles
    1. What are Physical Activity and Exercise?
    2. The Physical Activity Guidelines for Americans
    3. Components of Health-Related Fitness
    4. Principles of Adaptation and Stress
    5. FITT Principle
    6. Rest, Recovery, and Periodization
    7. Reversibility
    8. Training Volume
    9. Individual Differences
    10. Creating a Successful Fitness Plan
    11. Additional Safety Concerns
    12. Test Your Knowledge
  10. Chapter 5: Flexibility Training Principles
    1. What is Flexibility?
    2. Benefits of Flexibility and Stretching
    3. Improving Range of Motion
    4. Improving Flexibility
    5. Creating an Effective Stretching Program
    6. Assessing Your Flexibility
    7. Test Your Knowledge
  11. Chapter 6: Cardiorespiratory Training Principles
    1. What are the Cardiovascular and Respiratory Systems?
    2. Introduction: The Cardiovascular System
    3. Introduction: The Respiratory System
    4. The Process of Breathing and Respiratory Function
    5. Modifications to Breathing
    6. Changes in the CR System
    7. Measuring Heart Rate
    8. Measuring Intensity
    9. Cardiorespiratory Fitness Assessment
    10. Test Your Knowledge
  12. Chapter 7: Core and Balance Training Principles
    1. Lumbar Spine
    2. Abdomen
    3. The Pelvic Girdle
    4. Creating Movement at the Hip
    5. Balance
    6. Center of Gravity
    7. Supporting the Body
    8. Friction in Joints
    9. Human Stability
    10. Guidelines for Core and Balance Training
  13. Chapter 8: Plyometrics, Speed, Agility, and Quickness Training Principles
    1. Plyometric Exercises
    2. Variables of Plyometric Training
    3. Progressing a Plyometric Program
    4. Speed, Agility, and Quickness
    5. Speed
    6. Agility
    7. Quickness
  14. Chapter 9: Resistance Training Principles
    1. Resistance Exercise Programming
    2. Exercise Order
    3. Types of Resistance Training
    4. Basics of Form during Resistance Training
    5. Resistance Training Systems
    6. Resistance Training Conclusion
    7. Test Your Knowledge
  15. References
  16. Glossary
  17. MARC Record

13

Doing Work

Lawrence Davis

We started the previous unit with a discussion of Jolene’s motion during a shift on the medical floor of a hospital, including all the starts and stops that she makes. When Jolene is standing still she has zero kinetic energy. As she takes a step to begin walking she now has kinetic energy. Jolene had to supply that energy from within herself. When Jolene comes to a stop her kinetic energy is transferred to thermal energy by friction. When she begins walking again she will need to supply the new kinetic energy all over again. Even if Jolene walks continuously, every step she takes involves two inelastic collisions (the push-off and the landing) so kinetic energy is constantly being transferred to thermal energy. To stay in motion Jolene has to re-supply that kinetic energy. Walking around all shift uses up Jolene’s stored energy and that is why she gets tired.

Work

The amount of energy transferred from one form to another and/or one object to another is called the work.  Doing work is the act of transferring that energy. Doing work requires applying a force over some distance. The sign of the work done on an object determines if energy is transferred in or out of the object. For example, the athlete on the right is doing positive work on the pole because he is applying a force in the same direction as the pole’s motion. That will tend to speed up the pole and increase the kinetic energy of the pole. The athlete on the left is doing negative work on the pole because the force he applies tends to decrease the energy of the pole.

Two people push inwards on opposite ends of a pole, from the right and from the left. The direction of motion is indicated to the left. Therefore the person on the right, applying a leftward force, is doing positive work. The person on the right is doing negative work.
Insuknawr, or Rod Pushing Sport is an indigenous game of Mizoram, one of the North Eastern States of India. A force applied in the same direction as an objects motion does positive work. A force applied in the opposite direction to motion does negative work. Image adapted from from Insuknawr (Rod Pushing Sport) by H. Thangchungnunga via Wikimedia Commons

[1]

The positive or negative sign of the work refers to energy transferring in or out of an object rather than to opposite directions in space so work is not a vector and we will not make it bold in equations.

Calculating Work

The actual amount of work done is calculated from a combination of the average force and the distance over which it is applied, and the angle between the two:

\begin{equation} W = Fdcos\theta \end{equation}

Everyday Example: Lifting a Patient

Jolene works with two other nurses to lift a patient that weighs 867  N (190 lbs) a distance of 0.5 m straight up. How much work did she do? Assuming Jolene lifted 1/3 of the patient weight, she had to supply an upward force of 289 N. The patient also moved upward, so the angle between force and motion was 0°.  Entering these values in the work equation:

\begin{equation*} W = Fdcos\theta = (289{N})(0.5{m})cos(0^{\circ}) = 144{Nm} \end{equation*}

We see that work has units of Nm, which are called a Joules (J). Work and all other forms of energy have the same units because work is an amount of energy, but work is not a type of energy.  When calculating work the costheta accounts for the  force direction so we only use the size of the force (F) in the equation, which is why we have not made force bold in the work equation.

The cos\theta in the work equation automatically tells us whether the work is transferring energy into or out of a particular object:

  1. A force applied to an object in the opposite direction to its motion will tend to slow it down, and thus would transfer kinetic energy out of the object. With energy leaving the object, the work done on the object should be negative. The angle between the object’s motion and the force in such a case is 180° and cos(180^{\circ}) = -1, so that checks out.
  2. A force applied to an object in the same direction to its motion will tend to cause it to speed up, and thus would transfer kinetic energy in to the object. With energy entering the object, the work done on the object should be positive. The angle between the object’s motion and the force in such a case is 0° and cos(0^{\circ}) = 1 so that also checks out.
  3. Finally, if a force acts perpendicular to an objects motion it can only change its direction of motion, but won’t cause it to speed up or slow down, so the kinetic energy doesn’t change. That type of force should do zero work. The angle between the object’s motion and the force in such a case is 90° and cos (90^{\circ}) = 0 so once again, the cos\theta in the work equation gives the required result. For more on this particular type of situation read the chapter on weightlessness at the end of this unit.

The work equation gives the correct work done by a force, no matter the angle between the direction of force and the direction of motion, even if the force points off at some angle other than 0°, 90°, or 180°. In such a case, some part of the force will be doing work and some part won’t, but the cos\theta tells us just how much of the force vector is contributing to work.

Reinforcement Exercises

An interactive H5P element has been excluded from this version of the text. You can view it online here:
https://mhcc.pressbooks.pub/hpe172/?p=478#h5p-1

Reinforcement Exercises

An interactive H5P element has been excluded from this version of the text. You can view it online here:
https://mhcc.pressbooks.pub/hpe172/?p=478#h5p-2

[2]

Davis, Lawrence. Body Physics: Motion to Metabolism. Open Oregon Educational Resources. https://openoregon.pressbooks.pub/bodyphysics 

  1. Adapted from Insuknawr (Rod Pushing Sport by H. Thangchungnunga [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0) or GFDL (http://www.gnu.org/copyleft/fdl.html)], from Wikimedia Commons ↵
  2. Image and associated practice problem were adapted from "This work" and by BC Open Textbooks is licensed under CC BY 4.0 ↵

Annotate

Next Chapter
Body Levers
PreviousNext
Kinesiology, Nutrition, and Fitness

Copyright © 2021

                                by Amanda Shelton

            Introduction to Exercise Science for Fitness Professionals by Amanda Shelton is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.
Powered by Manifold Scholarship. Learn more at
Opens in new tab or windowmanifoldapp.org