Static Equilibrium and reaction forces

In summary: This moment is the product of the unknown vertical reaction force at A, A_z, and the unknown distance from A to the line of action of the reaction force, which is the unknown distance from A to the center of the journal bearing. I will call this distance "d". So you should have a term in your equation that looks something like:(d)(A_z)(i)If you add that term to your equation (and then equate the i coefficient to zero to find A_z), you should get the correct result for T_DE.
  • #1
MAPgirl23
65
0
Static Equilibrium... Please help

Homework Statement


The J-shaped member shown in the figure is supported by a cable DE and a single journal bearing with a square shaft at A. Determine the reaction forces and at support A required to keep the system in equilibrium. The cylinder has a weight = 5.50 , and F = 1.60 is a vertical force applied to the member at C. The dimensions of the member are w = 1.50 , l = 6.00 , and h = 2.00.

Homework Equations


Fx = 0
Fy = 0
Fz = 0

The Attempt at a Solution


W = -5.5 k
F = -1.6 k
Coordinates:
A(0,0,0); B(0,6,0); C(-1.5,6,0); D(-3,6,0); E(0,0,2)

Take moment about A:
(AD X T_DE) + (AB X W) + (AC X F) = 0 ...eqn 1

Position vector:
DE = 3i - 6j + 2k magnitude DE = 7
AB = 6j
AC = -1.5i + 6j
AD = -3i + 6j

Unit vector:
U_DE (3i - 6j + 2k)/7 = .43i-.86j+.29k

T_DE = T_DE(U_DE) = T_DE(.43i-.86j+.29k)

...eqn 1:
[(-3i+6j) X T_DE(.43i-.86j+.29k)] +[(6j) X (-5.5k)] + [(-1.5i+6j) X (-1.6k)] = 0

[T_DE(2.56k+.87j-2.58k+1.74i)] +[(-33i)]+ [(-2.4j-9.6i)] = 0

[T_DE(.87j+1.74i)] + [(-42.6i-2.4j)] = 0

Equating i coefficient to zero:
1.74T_DE - 42.6 = 0
T_DE = 24.48

From Equilibrium Condition:
F_z = 0
A_z + T_DE(.29) + W + F = 0
A_z + 24.48(.29) - 5.5 - 1.6 = 0
A_z + 7.1 - 5.5 - 1.6 = 0
A_z = 0

F_y = 0
A_y + T_DE(-.86) =0
A_y + 24.48(-.86) =0
A_y = 21.05

But it was wrong.
 

Attachments

  • problem.jpg
    problem.jpg
    8.5 KB · Views: 677
Last edited:
Physics news on Phys.org
  • #2
MAPgirl23 said:
[T_DE(.87j+1.74i)] + [(-42.6i-2.4j)] = 0

Equating i coefficient to zero:
1.74T_DE - 42.6 = 0
T_DE = 24.48

But if you equate the j coefficient to zero you will get a very different result for T_DE. So, something's wrong.

I believe you left out the unknown moment about A that is produced by the "journal bearing" at A.
 

Related to Static Equilibrium and reaction forces

What is static equilibrium?

Static equilibrium refers to a state in which an object is at rest and all forces acting on it are balanced. This means that the object will not accelerate or move unless an external force is applied.

What are the conditions for static equilibrium?

The conditions for static equilibrium are that the net force acting on an object must be zero and the net torque (or rotational force) acting on the object must also be zero. This means that the sum of all forces and the sum of all torques must be equal to zero in order for an object to be in static equilibrium.

What is the difference between static equilibrium and dynamic equilibrium?

Static equilibrium refers to an object being at rest, while dynamic equilibrium refers to an object moving at a constant velocity. In static equilibrium, the net force and net torque are both zero, while in dynamic equilibrium, the net force is zero but the object may still be moving due to a balanced net torque.

How do you calculate reaction forces in a static equilibrium problem?

To calculate reaction forces in a static equilibrium problem, you must first draw a free body diagram of the object, indicating all the forces acting on it. Then, using the conditions for static equilibrium, you can set up equations to solve for the unknown reaction forces. These equations are typically based on the sum of forces and the sum of torques being equal to zero.

What are some real-world examples of static equilibrium?

Some real-world examples of static equilibrium include a book resting on a table, a person standing still on the ground, and a ladder leaning against a wall without sliding or falling. In each of these cases, the forces acting on the object are balanced, resulting in static equilibrium.

Similar threads

Back
Top