What are the tensions in BC and BD?

In summary, the problem involves determining the tensions in BC and BD using the given equations and structure. The sum of moments about point A from all the forces is 0. To solve for the tensions, the position vectors of all the points should be used and vector algebra can be applied. The structure is in static equilibrium, so the sum of forces in the x, y, and z directions is 0. By setting up the equations and using the position vectors, the tensions can be solved for.
  • #1
Uninspired
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mass of e is 100 kg
mass of bar is 20 kg and acts at midpoint
sume of moments about point a due to weight of bar and forces on b by bc, bd, and be, are 0, determine tensions in bc and bd

so i continued like this.
The moment about point A from all those forces is 0.
0 = (Raf x Wab) + (Rab x Wbe) -(Rab x Tbc) - (Rab x Tbd)

I can figure out the first two terms of this eq, but i have to find magnitude of the vectors of Tbc and Tbd. But i only have this equation and these two equations:

Tbc= |Tbc|*Ebc where Ebc is a unit vector in BC direction
Tbd= |Tbd|*Ebd where Ebd is a unit vector in the BD direction

Did i set this up incorrectly or how do i proceed?

a picture has been included.

Thanks for your help

Pt A is at the origin
Pt D is (0,5,5)
Pt C is (0,4,-3)
Pt B is (4,3,1)
Pt E is where the 100 kg weight is
AB is the bar where 20 kg acts down the midpoint
any help is APPRECIATED!
 

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  • #2
bump, anyone?
 
  • #3
You know the position vectors of all the points.
Use vector algebra to work out the vectors AB, DB, CD.
Thes structure is in static equilibrium, therefore
[tex]\sum F_i = 0[/tex] [tex]\sum F_j = 0[/tex] [tex]\sum F_k = 0[/tex]

[tex]\mbox{If}\ \bf{AB} = (x_1,y_1,z_1)\ \mbox{then}\ F_{AB} = \lambda(x_1,y_1,z_1)[/tex]

You should be able to use the above two lines to solve the problem.
 
Last edited:

What are moments about a point?

Moments about a point, also known as moments of force, are physical quantities that describe the rotational effect of a force around a specific point. They are calculated by multiplying the magnitude of the force by the perpendicular distance from the point to the line of action of the force.

How are moments about a point different from moments about a line?

Moments about a point and moments about a line are both measures of rotational effect, but they differ in the point or axis they are taken about. Moments about a point are taken about a specific point, while moments about a line are taken about an imaginary line that is perpendicular to the force vector.

What are some real-world applications of moments about a point?

Moments about a point have many practical applications in fields such as engineering, physics, and biomechanics. For example, understanding moments about a point is crucial in designing structures that can withstand forces and torques, analyzing the movement of objects in sports and exercise, and studying the mechanics of human joints and muscles.

How do you calculate moments about a point?

To calculate moments about a point, you need to know the magnitude and direction of the force acting on an object, as well as the perpendicular distance from the point to the line of action of the force. The moment is then calculated by multiplying the force by the distance. The unit of measurement for moments about a point is Newton-meters (Nm).

What is the significance of the direction of a moment about a point?

The direction of the moment about a point is important because it indicates the direction of the rotational effect of the force. A moment can be either clockwise or counterclockwise, depending on the direction of the force and the direction of the perpendicular distance. This can determine the stability of an object and how it will move under the influence of the force.

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