i am convinced i am doing this right, but the computer marks it wrong. my work is at the end of the question.?

Question

Concrete sidewalks are always laid in sections, with gaps between each section. For example, the drawing shows three identical 2.4-m sections, the outer two of which are against immovable walls. The two identical gaps between the sections are provided so that thermal expansion will not create the thermal stress that could lead to cracks. What is the minimum gap width necessary to account for an increase in temperature of 38° C?

Change in L=(12e-6)(2.4m)(38°c)

=.0010944 m

(3/2)(.0010944m)=.0016416m

Change in L=(12e-6)(2.4m)(38°c)
=.0010944 m
(3/2)(.0010944m)=.0016416m

Steve4Physics · Answer

Your working is fine.  Possiblilities are:

1) You have used the wrong value for the coefficient of expansion of concrete.  Where do you get 12x10⁻⁶ ⁻C° ? If it is not the value supplied in the question, a slightly different value could have been used to work out the official answer.

2) Sometimes the answer is checked for the correct number of significant figures.  Since the data are given to 2 significant figures, you should give the answer to to 2 significant figures. So try 0.0016m.

3) The program may be expecting the answer in mm, so it may be expecting 1.6 (mm).

4) There could be an error in the official answer - it sometimes happens.

RealPro · Answer

Well, with the assumption that 12e-6 is the thermal expansion coefficient...
I'm getting the same result. Maybe it's the significant figures.

Best wishes!

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i am convinced i am doing this right, but the computer marks it wrong. my work is at the end of the question.?

2 Answers