# [1st CIVIL ENGINEERING],. MECHANCIS

1. Oct 21, 2011

### kougou

Hello

I am in the first year civil engineering course, we are currently learning mechancis. However, I don't get the concept of plane symmetry and symmetrical of forces in 3d problem. so, what's the condition or requirement for symmetry? for the first photo, 1 and 2 are symmetrical about the point a, but then we can't solve it using 2d since force 3 is not on the same plane.

and for photo 2, now it's something I dont understand. The question asks, at what angle of the force does leg 1 and 2 have the same force? i know same force means symmetrical.The answer is zero, but then I don't know why. All 3 legs are equal distance from the point where they intersect, but it doesn't make sense to me that only two of them have the same magnitude of the force while the other one is not. I know when the force is acting along the z-axis only, then the 3 legs will have the same forces. but then, now this case, when angle is zero, the force is acting along the y-axis.
does this mean symmetryl really means symmetrical about a plane instead of a point? My Ta told me that symmetrical is equal distant from a point. And on this question, this doesn't apply.

Thank you

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2. Oct 22, 2011

### kougou

no answers? come on guys

3. Oct 23, 2011

### MATLABdude

You may be ascribing more mystical attributes to symmetry than it actually has. Symmetry is just the basic geometric concept that you grew up with. In engineering (and especially, engineering problems in first and second year classes), symmetry often helps to reduce the complexity of problems.
http://en.wikipedia.org/wiki/Symmetry

Frankly, your diagrams and wording are a little hard to interpret--you may wish to scan in the actual problem statement. For the first diagram (and given the emphasis on symmetry), I'd expect the x-components of 1 and 2 to cancel out, leaving only a y-component balanced out by 3.

I (and probably everybody else who's seen this post) don't understand your formulation of the second problem. However, from your text, I must say that equivalency does not necessarily imply symmetry. Do you know how to resolve vectors into their separate components? I believe that's required for both these problems.