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Chapter 7

Chapter 7. Work and Energy. Conservation Laws. Mass Electric Charge Conservation of Energy Sum of all forms of energy is conserved. Energy: ability to do work. Forms of Energy. Mechanical Focus for now May be kinetic (associated with motion) or potential (associated with position)

jessica-blanchard
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Chapter 7

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  1. Chapter 7 Work and Energy

  2. Conservation Laws • Mass • Electric Charge • Conservation of Energy Sum of all forms of energy is conserved Energy: ability to do work

  3. Forms of Energy • Mechanical • Focus for now • May be kinetic (associated with motion) or potential (associated with position) • Chemical • Electromagnetic • Nuclear

  4. Some Energy Considerations • Energy can be transformed from one form to another • Essential to the study of physics, chemistry, biology, geology, astronomy • From one body to another – Work! • Can be used in place of Newton’s laws to solve certain problems more simply

  5. Work • Provides a link between force and energy • The work, W, done by a constant force on an object is the product of the force times the distance through which the force acts.

  6. Units of Work • SI • Newton • meter = Joule • N • m = J • J = kg • m2 / s2 • US Customary • foot • pound • ft• lb • no special name

  7. Work, cont. In general • F is the magnitude of the force • s is the distance of the object moved •  is the angle between force and direction of motion

  8. Work, cont. • This gives no information about • the time it took for the motion to occur • the velocity or acceleration of the object • Work is a scalar quantity

  9. More About Work • The work done by a force is zero when the force is perpendicular to the displacement • F=0 • s=0 • cos 90° = 0 • If there are multiple forces acting on an object, the total work done is the algebraic sum of the amount of work done by each force

  10. More About Work, cont. • Work can be positive or negative • Positive if s is in the same direction as F • Negative if s is in opposite direction to F • Zero if s is perpendicular to F

  11. Work Can Be Positive or Negative • Work is positive when lifting the box • Work would be negative if lowering the box • The force would still be upward, but the displacement would be downward

  12. Example 50N force pulls a 20 kg object at 30° above horizontal and moves it 2m, friction f=15N. Acceleration along the ground? Work done? Work done by friction?

  13. Kinetic Energy • Energy associated with the motion of an object • Scalar quantity with the same units as work • Work is related to kinetic energy

  14. Work-Kinetic Energy Theorem • When work is done by the resultant external force on an object and the only change in the object is its speed, the work done is equal to the change in the object’s kinetic energy • Speed will increase if work is positive • Speed will decrease if work is negative

  15. Work and Kinetic Energy • An object’s kinetic energy can also be thought of as the amount of work the moving object could do in coming to rest • The moving hammer has kinetic energy and can do work on the nail

  16. Example 50N force pulls a 20 kg object at 30° above horizontal and moves it 2m, friction f=15N. Find vf if vi=4m/s.

  17. Power • Often also interested in the rate at which the energy transfer takes place • Power is defined as this rate of energy transfer • SI units are Watts (W)

  18. Power, cont. • US Customary units are generally hp • Need a conversion factor • Can define units of work or energy in terms of units of power: • kilowatt hours (kWh) are often used in electric bills 1kWh=3.6x10^6 J • This is a unit of energy, not power

  19. Example An 80hp outboard motor, operating at full speed, can drive at speed boat at 11 m/s. What is the forward thrust(force) of the motor?

  20. Potential Energy • Potential energy is associated with the shape or position of the object • Potential energy is a property of the system, not the object • A system is a collection of objects interacting via forces or processes that are internal to the system

  21. Elastic Potential Energy • Compression of a spring • Restoring force • Elastic potential energy • Energy is available to do work:

  22. Gravitational Potential Energy • Lift object vertically, work is done against the force of gravity of Earth and energy is stored in the object in the form of Gravitational Potential Energy (PE) • Falling object: PE is changed to KE • PE of water in reservoir is used to generate electricity

  23. Example A 1500kg pile driver lifted 20 m in the air have PE …

  24. Energy Conservation Energy is never created or destroyed. Energy can be transformed from one kind into another, but the total amount of energy remains constant. Example: Pendulum

  25. Conservation of Mechanical Energy • Conservation in general • To say a physical quantity is conserved is to say that the numerical value of the quantity remains constant throughout any physical process • In Conservation of Energy, the total mechanical energy remains constant

  26. Conservation of Energy, cont. • Total mechanical energy is the sum of the kinetic and potential energies in the system • Other types of potential energy functions can be added to modify this equation

  27. Work and Energy • If a force (other than gravity) acts on the system and does work • Need Work-Energy relation • or

  28. Example Cart on a roller-coaster…

  29. Example Two cars each with mass 2000kg and speed 80km/h collide.

  30. Example Child on a slide If a child of 25kg slides down and reaches only 3m/s. What work was done by the frictional force acting on the child?

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