A theoretical mechanics uses the node method to analyze the problem of scaffolding, please come in a

First of all, the positive directions of the x-axis and y-axis are different when analyzing different nodes in the process of problem solving. For example, when analyzing node e, the positive directions are left and down, respectively; When analyzing node B, it becomes right and down. Although there is no problem with this, it is still not recommended and is not standardized.

Then let's answer why feb is positive when analyzing node e, and negative when analyzing node b's equilibrium in the y direction.

The answer is to see if the direction of this force is the same as the positive direction specified by the author. The specific analysis is as follows:

1. FEB appears for the first time when solving node E, so the direction can be assumed to be oblique up or downward, and the coordinate axis is assumed here.

The positive direction is the same – oblique downward, so it is positive when the equation is listed.

2. When analyzing the force of node B, the direction of the force of the EB rod can no longer be assumed, because the direction of the FEB has been given, and this is a two-force rod.

The two forces must be in opposite directions, so the direction of the force on the EB rod at point B is diagonally upward. (Don't consider the actual direction of the force here, whether the actual value of the force is positive or negative naturally indicates whether the direction of the actual force is the same as the direction represented in the diagram.) ）

3. Since the direction of feb in node B is known (obtained by the second step analysis), then when the equation is listed, it is enough to see whether the direction of this force is the same as the positive direction specified by the author. It is the same as the x-axis forward (the x-axis is positive to the right of node b), so the first one is positive; It is the opposite of the y-axis, so the second one is negative.

After understanding the second step of the above analysis, all of them are understood, and the other nodes are similar.

It's very simple theoretical mechanics.

Knowledge, how can you not understand it.

Of course, the problem solver is also convinced, and it is not standardized at all, and the readability is very poor.

That's not a moment, it's just a force, the direction is different, so it's subtracted (positive and negative), and finally the direction of the force is the same as in the figure.

A rod with two forces plus a force couple cannot be regarded as a two-force rod, think about how the conclusion came about before using the conclusion, and what conditions are available,,, the empty formula is not good for learning physics.

The solution is solved using the node method or the cross-section method.

What is the node method or section method, you can take a good look at the book.

The characteristic of the truss is that the rods inside are all two-force rods.

Remember to adopt it.

Answer: c, d

Regardless of the weight of the rod, each rod is a two-force bar, and the direction of the force is along the direction of the rod, and the vector triangle of the force can be obtained from the internal force of the bar ab.

The ab internal force of the pole is the pressure.

Node method', take a

qcos30°-nab=0

nab = -qcos 30° (compression).

This is not included in the answer.

Solution: Find cc', using the cross-section method, cc'The internal force is f1 and takes 1-1'Cross-section, moment for point e.

me=0, f1·2l+p·l+p·2l=0, and f1=-30 [under pressure].

Ask for aa'，a'b'Internal force.

Seek A and A first'of support reactions.

Take 2-2 cross-sections, take the moment of point a, and you can find a'c'internal forces.

And then to a'The aa can be obtained by analyzing the force of the node'，a'b'Internal force.

1 3 is the zero rod and the axial force is 0

So choose D

13 is a zero rod, 2 is -f, this node method will come out at once.

This FB should refer to the force exerted by the rod on the right side of the hinge C on C vertically downward, which is just a code name for ease of use.

Because in the equilibrium equation at point C, Fb appears in the Y direction, the answer does not specify the specific meaning of Fb, but it can be seen that Fb is clearly a force experienced by C in the Y direction. When you solve the problem yourself, you can use other symbols to represent this force, and if you don't draw a picture, it may be easy to figure it out.