Elasticity

**1. State Hooke’s Law. If a spring extends by 0.02 m when a force of 25 N is applied to it what is the spring constant?**

**2. Draw a graph to show the forces between two adjacent molecules in a solid. Mark on your graph:**

**(a) the region where Hooke’s Law is obeyed**

**(b) the breaking point**

**3. A wire extends by 1% of its original length when a certain load is applied. What is the elastic strain of the wire?**

**4. If a wire has a diameter of 0.25 mm and has a force is 10 N applied to it what is the elastic stress on the wire in Pa?**

**5. A wire of length 1.25 m, diameter 0.25 mm and Young modulus 1.2x10**

^{11}Pa is stretched by a force of 25 N. What is the new length of the wire?**6. Calculate the elastic energy stored in a wire of original length 1.5 m and diameter 0.75 mm that extends by 0.2 mm when a certain load is applied. The Young modulus for the wire is 2x10**

^{11}Pa.**7. A lift and its passengers has a weight of 20 000 N. It is suspended from four steel cables of equal diameter. Calculate the minimum diameter of each cable allowing for a 50% safety margin. Breaking stress for steel = 500 MPa.**

**8. Write down the four types of elastic material and give one example of each.**

**9. What is meant by a dislocation in a crystal?**

**10. Calculate the force in a 25 km long section of railway line due to temperature increase of 20**

^{o}C. Take the cross sectional area of the line to be 200 cm^{2}, the Young modulus for the material to be 2x10^{11}Pa and the coefficient of linear expansion of the metal of the lines to be 1.2x10^{-5}^{o}C^{-1}.