# Linear expansion problem

1. Apr 6, 2010

### lew0049

Hey guys, the following is a HW question I believe I am doing correctly but I'm not getting the correct answer, any input would be appreciated!
Q
uestion: A rod made from a particular alloy is heated from 27.5 °C to 127 °C. Its length increases by 9.13 x 10-4 m. The rod is then cooled from 27.5 °C to 4.19 °C. By how much does the rod shrink?

After breaking down the basic thermal expansion equation I get: (9.13E-4 / 99.5)* (23.31) = change of L = 2.14E-4 m ... am I missing something because this doesn't seem too complicated

2. Apr 6, 2010

### Filip Larsen

If the thermal expansion is linear and the rate is constant, then this looks about right.

3. Apr 6, 2010

### lew0049

As far as I know, that is correct. It's slightly frustrating because I can't think of any reason my answer is wrong.

4. Apr 6, 2010

### Stonebridge

If your answer is not the same as that in the back of the book, it doesn't mean you are wrong.
When I was at school (many years ago), the answers in the back of my book were more often wrong than right. My old physics teacher quite liked the idea because, as he used to say, "it kept us on our toes".

5. Apr 6, 2010

### lew0049

yeah I completely agree but we have to submit our HW on a website (wiley). I still haven't found a problem with my answer though. Oh well, thanks anyways.

6. Apr 6, 2010

### lew0049

Wow after I submitted the answer for the 3rd time, it gave me a link to the solution. The answer was NEGATIVE 2.14E-4 which is wrong and a self-contradiction because something cannot shrink by a negative number - that would essentially mean it lengthened. And this is why HW done via websites can be extremely frustrating.