Temperature and stress question

[pinky2468]

Last question for the semester, but Physics II is right around the corner!!
This is an easy question but I am not sure how to answer it!
A steel ruler is accurate when the temperature is 25 degrees celsius. When the temperature drops to -15 degrees celsisus the ruler no longer reads correctly, but it can be made to read correctly if a stress is applied to each end of the ruler . Should the stress be compression or a tension? Why?

I think the answer is tension, but I really have no idea why! Thanks!

[dextercioby]

Last question for the semester, but Physics II is right around the corner!!
This is an easy question but I am not sure how to answer it!
A steel ruler is accurate when the temperature is 25 degrees celsius. When the temperature drops to -15 degrees celsisus the ruler no longer reads correctly, but it can be made to read correctly if a stress is applied to each end of the ruler . Should the stress be compression or a tension? Why?

I think the answer is tension, but I really have no idea why! Thanks!

The answer is right,and the explanation is simple.Cooling the ruler makes it shorter (experiments show that dilation/contraction is linearly dependent on the temperature,but for reasonably small chages in temperature (i guess your 40 would fit the linear approx.)),so you'd have to pull (very hard actually) both ends to lengthen it back again.That "pulling" is force which tensions the ruler.

Daniel;