How to find Tb and Moments 3D equilibrium of rigid bodies

In summary: You can use the x and y axis to find the tension in the cable at B.Yes. As I indicated in post #2, it has reduced to being a 2D problem. You can use the x and y axis to find the tension in the cable at B.
  • #1
mcrooster
7
0
Homework Statement
How to find Tb and Moments 3D equilibrium of rigid bodies
Relevant Equations
Find Tb, F=4kn parallel to xy plane

Find Reaction at Ax Ay and Az
1599730756503.png

The boom is supported by a ball-and-socket joint at AA and a guy wire at BB

Hey guys, I am stuck with this question in find the Tension in B and the moments around A. I have done plenty of 2d Tension questions but not a 3D one.
 
Physics news on Phys.org
  • #2
mcrooster said:
not a 3D one.
Well, yes, it's not really a 3D problem.
What is the nett of the two Fs?
 
  • #3
haruspex said:
Well, yes, it's not really a 3D problem.
What is the nett of the two Fs?
The component has a x y z, does that not make it 3d with there being 3 axis.

Anyways, the question simply said
If loads F = 4 kN lie in a plane which is parallel to the xy plane, determine the tension in the cable at B
.
 
  • #4
Welcome, rcrooster :cool:

Have you done any work so far that we could see?

Because it has a ball-and-socket joint, which is unable to counteract any moment, any force that is not contained within plane y-z would make the system non-stable.
A single guy wire can only do so much.
 
  • #5
mcrooster said:
The component has a x y z, does that not make it 3d with there being 3 axis.

Anyways, the question simply said .
You did not answer my question. There are two applied forces F, same magnitude but different directions. What is the resultant of those two?
 
  • #6
haruspex said:
You did not answer my question. There are two applied forces F, same magnitude but different directions. What is the resultant of those two?
Sorry for the late reply. Would Fr not be the sum of all forces which is 4cos30+4sin30+4cos30+4sin30 = 11
 
  • #7
mcrooster said:
Sorry for the late reply. Would Fr not be the sum of all forces which is 4cos30+4sin30+4cos30+4sin30 = 11
4cos30 is the magnitude of the X component of one of the two Fs, and 4sin30 that of a Y component. And the two X components are in opposite directions along the X axis.
You cannot add magnitudes of vectors if the vectors are in different directions. Do you not know how to add vectors?
 
  • #8
haruspex said:
4cos30 is the magnitude of the X component of one of the two Fs, and 4sin30 that of a Y component. And the two X components are in opposite directions along the X axis.
You cannot add magnitudes of vectors if the vectors are in different directions. Do you not know how to add vectors?
My bad. Fr should be 5.65.

(4cos30)^2+(4sin30)^2 and then root the answer.
 
  • #9
mcrooster said:
My bad. Fr should be 5.65.

(4cos30)^2+(4sin30)^2 and then root the answer.
##(4\cos(30))^2+(4\sin(30))^2=4^2\cos^2(30)+4^2\sin^2(30)=4^2(\cos^2(30)+\sin^2(30))=4^2##.
But this is still not how to add vectors.

First, resolve the vectors into a common coordinate system, paying attention to signs.
Using unit vectors based on the coordinates you can write these two F forces as
##4\cos(30)\hat x-4\sin(30)\hat y## and ##-4\cos(30)\hat x-4\sin(30)\hat y##.
Do you understand why those are the appropriate signs?

The two ##\hat x## vectors are parallel so we can add them arithmetically, but we must still honour the signs:
##4\cos(30)\hat x-4\cos(30)\hat x=0##.
And for the y coordinate
##-4\sin(30)\hat y-4\sin(30)\hat y=-8\sin(30)\hat y##.
 
  • #10
I see it now. I was looking at it the wrong way. Completely overlooked the direction in which y was going so it should be a negative. As the X goes along both the positive x-axis and negative x-axis the sum will equal to zero.

So the resultant should be -8sin30y?
 
  • #11
mcrooster said:
I see it now. I was looking at it the wrong way. Completely overlooked the direction in which y was going so it should be a negative. As the X goes along both the positive x-axis and negative x-axis the sum will equal to zero.

So the resultant should be -8sin30y?
Yes.
 
  • Like
Likes mcrooster
  • #12
haruspex said:
Yes.
That gives me a y force of -4kN. This does not give me the answer to Tb. Do I now use the Z and Y axis to figure out my answer since Fx = 0
 
  • #13
mcrooster said:
That gives me a y force of -4kN. This does not give me the answer to Tb. Do I now use the Z and Y axis to figure out my answer since Fx = 0
Yes. As I indicated in post #2, it has reduced to being a 2D problem.
 

1. How do you determine the equilibrium of a rigid body in 3D?

To determine the equilibrium of a rigid body in 3D, you must first calculate the forces acting on the body. These forces include external forces, such as gravity or applied forces, and internal forces, such as tension or compression. Once all forces are identified, you can use the equations of equilibrium (sum of forces and sum of moments) to solve for the unknowns and determine if the body is in equilibrium.

2. What is the difference between static and dynamic equilibrium?

Static equilibrium refers to a system that is at rest or moving at a constant velocity, with no acceleration. In this state, the sum of all forces and the sum of all moments acting on the system must be equal to zero. Dynamic equilibrium, on the other hand, refers to a system that is in motion with a constant acceleration. In this state, the sum of all forces and the sum of all moments acting on the system must be equal to the mass of the system multiplied by its acceleration.

3. How do you calculate the torque required to maintain equilibrium?

To calculate the torque required to maintain equilibrium, you must first identify the pivot point or axis of rotation. Then, you can use the equation τ= r x F, where τ is the torque, r is the distance from the pivot point to the point of application of the force, and F is the magnitude of the force. This equation takes into account the direction of the force and the direction of rotation around the pivot point.

4. Can the center of mass be used to determine equilibrium in 3D rigid bodies?

Yes, the center of mass can be used to determine equilibrium in 3D rigid bodies. The center of mass is the point where the body's mass is concentrated, and it can be used as the reference point for calculating the sum of moments. If the sum of moments around the center of mass is equal to zero, then the body is in equilibrium.

5. What are some common examples of 3D equilibrium in everyday life?

Some common examples of 3D equilibrium in everyday life include a book resting on a table, a ladder leaning against a wall, and a seesaw with equal weights on either side. In all of these examples, the forces acting on the objects are balanced, and there is no net torque, resulting in 3D equilibrium.

Similar threads

  • Introductory Physics Homework Help
Replies
15
Views
3K
  • Introductory Physics Homework Help
Replies
11
Views
2K
  • Mechanical Engineering
Replies
3
Views
2K
  • Engineering and Comp Sci Homework Help
Replies
20
Views
3K
  • Introductory Physics Homework Help
Replies
10
Views
2K
  • Introductory Physics Homework Help
Replies
2
Views
5K
  • Mechanical Engineering
Replies
4
Views
1K
  • Introductory Physics Homework Help
Replies
4
Views
2K
  • Introductory Physics Homework Help
Replies
1
Views
2K
  • Engineering and Comp Sci Homework Help
Replies
1
Views
3K
Back
Top