How Do You Calculate the Compression of Aluminum and Brass Rods Under Force?

[Lalasushi]

"2 rods of equal length (0.45m) and diameter (1.5cm) are placed end to end. one rod is aluminium, the other is brass. if a compressive force of 8400N is applied to the rods, how much does their combined length decrease?" [ans: 0.55mm)

with this question, I am not sure whether its ok to find the compression of each of the rods separately first, then add the answers together to get the combined length decrease becuase i couldn't get the right answer that way...and i don't know if its ok to add the 2 young's modulus for the 2 materials together and then treat it as one material...can anyone help me out here?

 

[OlderDan]

Your first method should have worked. Did you find the areas? You cannot add the moduli of the two materials first. Show what you did and someone will find where you went wrong.

 

[Dr.Brain]

Hint: You need to calculate the decrease in length for two rods individually and then add the compressions of both.

BJ