# Homework Help: Relative displacement

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1. May 20, 2017

### EastWindBreaks

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
I am a little confused by the wording of the text book solution.
from the solution, the displacement at the end C of the rod = the displacement of C relative to the fixed end A,
why not the displacement of C relative to the fixed end B? is it because the end B is not fixed? from the figure it looks like its fixed by a nut though.

my attempt on the explanation is that the displacement of C relative to end B is just referring to the rod from C to B without the nut and the tube, so the displacement of C relative to the fixed end A is equivalent to the displacement of C relative to the fixed end B. (if we can assume B is fixed from the given figure,but from reading the text version of the problem given, we can not tell if B is fixed or not, therefor, we choose to call it "the displacement of C relative to the fixed end A)

also, the displacement of end B with respect to the fixed end A would be equivalent as saying the displacement of end B with respect to end A, correct? because at this part, we only want the displacement of the tube, so we dont care if A is fixed or not right here.

and just to confirm, this problem has nothing to do with superposition, correct?

2. May 21, 2017

### Staff: Mentor

My understanding: the only contact rod C makes with other metal in the illustration is at point B. So when tension is applied the entire rod stretches and the entire tube undergoes contraction. Is that how you see it?

When the rod is placed under tension, everything in the picture moves except the wall and what is fixed onto it, viz., the bracket at A.

3. May 21, 2017

### EastWindBreaks

rod C also makes contact with the tube at points from B to A though?

so you saying point B is not fixed in terms of displacement, but point A is, i think now I understand, thank you! when we want a displacement of a point, it means we want the displacement of that point relative to a fixed point as a reference point.