Devil physics The baddest class on campus Pre-IB Physics

1. Devil physicsThe baddest class on campusPre-IB Physics

2. Giancoli Lesson 9-6Elasticity: Stress and Strain

3. Objectives • Understand how elasticity is related to Hooke’s Law for springs. • Know that the change in length of an object is proportional to the force placed on the object, but only to the proportional limit. • Understand the meaning of the terms elastic region, proportional limit, elastic limit, plastic region, and breaking point.

4. Objectives • Know that the elastic modulus, or Young’s modulus, is a ratio of the stress on an object to the strain of the object. • Differentiate between tensile stress, compressive stress, and shear stress. • Solve problems using elastic modulus, shear modulus, and bulk modulus.

5. Introductory Video:Stress and Strain

6. Reading Activity Questions?

7. Hooke’s Law • The change in length of an object is directly proportional to the amount of force placed on it • Normally thought of in terms of springs where k is the spring constant • The same is true for any material – but only to a certain point

8. Elasticity • Hooke’s Law holds true for all materials up to the material’s proportional limit • Beyond the proportional limit, the change in length per force applied increases

9. Elasticity • The material will return to its original shape when the force is removed, provided the elastic limit is not exceeded • Once the elastic limit is exceeded, permanent deformation, or permanent set, occurs

10. Elasticity • Deformation continues until the breaking point • Breaking, or fracture, is the subject of Lesson 9-7 • The maximum force that can be applied to a material without breaking is its ultimate strength

11. Elasticity • When force is applied to a material, it is said to undergo stress • Stress is defined as force per unit cross-sectional area • The ratio of the change in length to the original length is called strain

12. Elasticity • Cross-Sectional Area • Cylindrical rod – πr2 • Rectangular rod – l x w • Triangular prism – ½ bh

13. Elasticity • Experimentation has found that the change in length of an object is directly proportional to the force placed on in, but inversely proportional to the object’s cross-sectional area • This value is specific to the material the object is made of

14. Elasticity • The elastic modulus, E, replaces the spring constant in Hooke’s Law • The elastic modulus, or Young’s modulus, is the ratio of stress to strain

15. Elasticity

16. Elasticity • Rearranging, we find a formula for the change in length, ΔL, of a material with an elastic modulus E, cross-sectional area A, original length L0, subjected to a force F

17. Elasticity • This formula applies equally to tensile stress and compressive stress • Values for elastic moduli are found in Table 9-1 on page 254 in your textbook and in the data guide • Notice the applied force is perpendicular to the cross-sectional area

18. Shear Modulus • A similar formula applies to shear stress using the shear modulus, G • In tensile or compressive stress, the applied force is perpendicular to the cross-sectional area • In shear stress, the applied force is parallel to the cross-sectional area

19. Bulk Modulus • A third situation arises when an object is subjected to forces from all sides • Like a submarine at the bottom of the ocean or a balloon when the air inside is expanding • This creates a change in volume instead of just length

20. Bulk Modulus • The proportionality constant for this situation is called bulk modulus, B • ΔP is the change in pressure • Pressure is defined as force per area and is thus the equivalent of stress • ΔV is the change in volume and V0is the original volume • The minus sign indicates volume decreases with applied force

21. Sample Problems?

22. Σary Review • Do you understand how elasticity is related to Hooke’s Law for springs? • Do you know that the change in length of an object is proportional to the force placed on the object, but only to the proportional limit? • Do you understand the meaning of the terms elastic region, proportional limit, elastic limit, plastic region, and breaking point?

23. Σary Review • Do you know that the elastic modulus, or Young’s modulus, is a ratio of the stress on an object to the strain of the object? • Can you differentiate between tensile stress, compressive stress, and shear stress? • Can you solve problems using elastic modulus, shear modulus, and bulk modulus?

24. QUESTIONs?

25. Homework #44-52