Elastic Potential Energy ( Chapter 11 section 11.1)

1. Elastic Potential Energy ( Chapter 11 section 11.1) • Conservative & Non- Conservative Forces (Chap 10 Section 10.1)

2. Objectives • Define elastic potential energy in terms of the energy stored as a result of its change in shape • Indicate that the E.P.E is measured in joule (J) (SI) • 3.3.3 Explain the relationship between E.P.E, spring constant k and displacement • Use the expression for E.P.E to calculate its value

3. Objectives • Distinguish between conservative and non-conservative forces in terms of the work done by each • Indicate that the net work done in case of conservative and non-conservative forces is given by the expression: • Wc + Wnc = ∆K.E

4. Elastic Potential Energy • When a string on a bow is pulled, work is done on the bow, storing energy in it. • Thus, the energy of the system increases. • Identify the system as the bow, the arrow, and Earth. • When the string and arrow are released, energy is changed into kinetic energy. • The stored energy in the pulled string is called elastic potential energy,which is often stored in rubber balls, rubber bands and slingshots.

5. The work done on the bowing is positive because the force and displacement of the bowing are in the same direction.

6. Section 11.1 Elastic Potential Energy • Energy also can be stored in the bending of an object. • When the pole straightens, the elastic potential energy is converted to gravitational potential energy and kinetic energy as the pole-vaulter is lifted as high as 6 m above the ground.

7. Section 11.1 Elastic Potential Energy • A pole-vaulter runs with a flexible pole and plants its end into the socket in the ground. • When the pole-vaulter bends the pole, as shown in the figure, some of the pole-vaulter’s kinetic energy is converted to elastic potential energy.

8. Section 11.1 Elastic Potential Energy • When the pole straightens, the elastic potential energy is converted to gravitational potential energy and kinetic energy as the pole-vaulter is lifted as high as 6 m above the ground. • Unlike stiff metal poles or bamboo poles, fiberglass poles have an increased capacity for storing elastic potential energy. • Thus, pole-vaulters are able to clear bars that are set very high.

9. Conservative and Non Conservative Forces The gravitationalforce has an interesting property that when an object is moved from one place to another, the work done by the gravitational force does not depend on the choice of path. Forces like these are called conservative forces.

10. Definition Of A Conservative Force A force is conservative when the work it does on a moving object is independent of the path between the object's initial and final positions.

11. Examples of Conservative Forces • Examples of conservative forces • Gravitational Potential Energy • Elastic Spring Force

12. Work done by conservative forces • Work done by conservative forces, or path-independent forces, results in changes in the object's potential energy.

13. Definition Of A nonconservative Force A force is non conservativewhen the work it does on a moving object is dependent of the path between the object's initial and final positions.

14. Nonconservative Forces • Examples of Nonconservative forces • Friction • Air resistance • Tension • Each of these forces depends on the path

15. Both usually happen at once • Often both conservative and nonconservative forces act on an object at once. • We can write Work done by net external force as • W = Wc + Wnc

16. Non-conservative forces: Sliding a book on a table