# [1st CIVIL ENGINEERING],. MECHANCIS

• kougou

#### 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 don't 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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