What mistake did I make in finding the reaction at hinge A?

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Homework Help Overview

The discussion revolves around finding the reaction force at hinge A in a mechanics problem. The original poster presents their calculations and compares them to an answer provided in a textbook, expressing confusion over a significant discrepancy in both magnitude and direction.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • The original poster attempts to resolve the reaction force into its components and calculate its magnitude and direction. They question their reasoning after finding a large difference between their result and the textbook answer. Other participants suggest that the textbook may contain an error, prompting a discussion on the interpretation of directionality in relation to the calculated angle.

Discussion Status

Some participants express support for the original poster's calculations, indicating that the textbook's answer may be incorrect. There is an ongoing exploration of the implications of directionality in the context of the problem, with no explicit consensus reached on the correct interpretation.

Contextual Notes

Participants note the potential for misunderstanding regarding the definition of direction in relation to the calculated angle, as well as the possibility of a simple error in the textbook's answer.

gnits
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Homework Statement
To find forces in a framework
Relevant Equations
Equating of forces
Moments
A very simple (I thought!) question:

img096.jpg


I'm just looking at the first part, finding the reaction at the hinge A.

Here is my annotated diagram, with the reaction and A resolved into it's X and Y components, the force at E labelled as Fe and the length of ED labelled as L.

img096_ann.jpg


Considering the body as a whole:

Resolving vertically:

Ya = 500

Taking moments about E:

Xa * L = 2 * L * 500

which gives Xa = 1000

So the resultant at A will have size sqrt(1000^2 + 500^2) = 1118 (approx.)

Answer given in book is 112 N at 26.6⁰ below the horizontal.

My answer is very different and also above the horizontal.

What simple mistake have I made in my reasoning?

Thanks.
 
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The answer given in the book is not correct, yours is.
 
Thanks, that's a relief. The book has rarely been wrong and so I was all too ready to doubt myself. Much appreciated.
 
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gnits said:
Thanks, that's a relief. The book has rarely been wrong and so I was all too ready to doubt myself. Much appreciated.
The book's error in the magnitude is clearly just a decimal place blunder.
Above or below horizontal is inadequate to define the direction. Given the angle it makes to the horizontal, there are four possible directions. Up and to the left can be thought of as above horizontal, viewing it as a motion in that direction from A. But it can be viewed as below horizontal when considering the diagonal line through A that it lies on: first and third quarters = above horizontal, second and fourth quarters = below horizontal.
 

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