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Chapter 5

Chapter 5. Energy. Forms of Energy. Mechanical focus for now chemical electromagnetic nuclear. Using Energy Considerations. Energy can be transformed from one form to another Essential to the study of physics, chemistry, biology, geology, astronomy

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Chapter 5

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  1. Chapter 5 Energy

  2. Forms of Energy • Mechanical • focus for now • chemical • electromagnetic • nuclear

  3. Using Energy Considerations • Energy can be transformed from one form to another • Essential to the study of physics, chemistry, biology, geology, astronomy • Can be used in place of Newton’s laws to solve certain problems more simply

  4. Work • Provides a link between force and energy • The work, W, done by a constant force on an object is defined as the product of the component of the force along the direction of displacement and the magnitude of the displacement

  5. Work, cont. • F cos θ is the component of the force in the direction of the displacement • Δ x is the displacement

  6. Work, cont. • This gives no information about • the time it took for the displacement to occur • the velocity or acceleration of the object

  7. Units of Work • SI • Newton • meter = Joule • N • m = J • US Customary • foot • pound • ft• lb • no special name

  8. More About Work • Scalar quantity • The work done by a force is zero when the force is perpendicular to the displacement • 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

  9. More About Work, cont. • Work can be positive or negative • Positive if the force and the displacement are in the same direction • Negative if the force and the displacement are in the opposite direction

  10. When Work is Zero • Displacement is horizontal • Force is vertical • cos 90° = 0

  11. Work Can Be Positive or Negative • Work is positive when lifting the box • Work would be negative if lowering the box

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

  13. Work-Kinetic Energy Theorem • When work is done by a net 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

  14. 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

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

  16. Gravitational Potential Energy • Gravitational Potential Energy is the energy associated with the relative position of an object in space near the Earth’s surface • Objects interact with the earth through the gravitational force • Actually the potential energy of the earth-object system

  17. Work and Gravitational Potential Energy • PE = mg∆y • PE = PEf – PEi • Units of Potential Energy are the same as those of Work and Kinetic Energy

  18. Reference Levels for Gravitational Potential Energy • A location where the gravitational potential energy is zero must be chosen for each problem • The choice is arbitrary • Choose a convenient location for the zero reference height • often the Earth’s surface • may be some other point suggested by the problem

  19. Another form of PE • Energy stored because the shape of object temporarily changes • Elastic potential energy (EPE) • Examples in real life

  20. What does the word elastic mean in terms of the objects that store EPE?

  21. Elastic • Compression or stretching of object results in temporary change • Must go back to original shape when forces are removed

  22. What factors determine EPE?

  23. Factors • Length object is extended or compressed (x) • A measure of how tough it is to stretch an object = elastic or spring constant (k) • Units are N/m

  24. What happens to the force within an elastic material as it is stretched?

  25. What is the relationship • Between distance stretched and force needed to pull it? Direct or inverse? • Is the relationship linear or not? • What determines the magnitude of the force needed to stretch object?

  26. Potential Energy Stored in a Spring • Involves the spring constant (or elastic constant), k • Hooke’s Law gives the relationship between force, change in length and k F = - k x • F is the restoring force • F is in the opposite direction of x • k depends on how the spring was formed, the material it is made from, thickness of the wire, etc.

  27. Potential Energy in a Spring • Elastic Potential Energy • related to the work required to compress a spring from its equilibrium position to some final, arbitrary, position x • Related to area under f-d graph

  28. Questions • Which has more potential energy ? • Object with spring constant of 1245 N/m that stretches 0.5 m • Object with spring constant of 3429 N/m that stretches 0.25 m

  29. Shock absorber • If the K value of a shock absorber was 53,000 N/m , how much force is needed to compress it 0.15 m? How much energy is stored in this compression?

  30. Another possible source of PE • Einstein • E = mc2

  31. Mechanical Energy • Energy of actions and objects on human scale • KE • KErot • GPE • EPE • Mechanical waves with human scale components< generally connect to an objects KE or PE

  32. Non-mechanical Energy • Energy associated with actions and objects on the atomic scale • Heat or thermal energy (KE) • Chemical energy (PE) • Mechanical waves (like sound) (KE) • Electromagnetic energy (KE) • Nuclear energy (PE)

  33. Transferring Energy • By Work • By applying a force • Produces a displacement of the system

  34. Transferring Energy • Heat • The process of transferring heat by collisions between molecules

  35. Transferring Energy • Mechanical Waves • a disturbance propagates through a medium • Examples include sound, water, seismic

  36. Transferring Energy • Electrical transmission • transfer by means of electrical current

  37. Transferring Energy • Electromagnetic radiation • any form of electromagnetic waves • Light, microwaves, radio waves

  38. Doing Work means… • Transferring energy • Transforming energy is….. • Transforming energy happens as energy is transferred

  39. Conservation of energy means • That the total amount of energy is constant at all times in the system • Energy may be transferred between parts of system or may be transformed, but not created or destroyed

  40. Conserving energy • True only if system in question can be considered closed and isolated. • A system is identified by its parts • A closed isolated system does not lose or gain significant energy from external sources

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

  42. Conservation of Energy including a EPE • The PE of the spring is added to both sides of the conservation of energy equation

  43. Examples

  44. Consider just the wheel • What energy types does it possess at the top and bottom of the ramp?

  45. Problem Solving with Conservation of Energy • Define the system • Define the types of energy involved at specific times , indicate if problem has several stages (times) that are important • If GPE is involved, define reference level • If possible, determine total energy at any point • Define all available information • Place information in equation • Solve for unknown values by setting the energy values at 2 times equal

  46. Class/homework • Pg 297 15-18 • Pg 308-309 73-81 odds

  47. Energy chains

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