Multi Concept Physics Problem?

Question

A cylindrical brass rod (cross-sectional area = 1.20 x 10-5 m2) hangs vertically straight down from a ceiling. When a 790-N block is hung from the lower end of the rod, the rod stretches. The rod is then cooled such that it contracts to its original length. By how many degrees must the temperature be lowered?

Answer 1

To solve this problem, you need to know the modulus of elasticity of brass. The stress in the rod is the force divided by the cross-sectional area. The modulus of elasticity tells you by what fraction the material will stretch (otherwise known as strain) for a given amount of stress. Then you must look up the thermal expansion coefficient of brass, which tells you the fractional length change caused by a temperature change. Since you now know the fraction the rod stretched, you should be able to find out much temperature change is required to undo the length change.