## Saturday, 6 September 2014

### ELASTICITY STRESS , STRAIN, YOUNG'S MODULUS PROBLEMS

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 1.2x1011 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 2x1011 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 oC. Take the cross sectional area of the line to be 200 cm2, the Young modulus for the material to be 2x1011 Pa and the coefficient of linear expansion of the metal of the lines to be 1.2x10-5 oC-1.