Understanding a stress equation

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In summary, the book presents an equation for calculating the force and moment on a surface, taking into account the lever arm length and cancelling out terms for clarity.
  • #1
EGill
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I really need help understanding how the book created the equation from the book. I would really appreciate help understanding it. The attachment shows an image and the equation.
 

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  • #2
They've divided by dz and canceled out some terms; that might be why it's a little confusing. The force (pressure times area) on the top surface is

[tex]\left[\sigma_y+\left(\frac{\partial \sigma_y}{\partial y}\right)dy\right]dx\,dz[/tex]

and the distance (lever arm length) is [itex]dx/2[/itex]. But the force on the bottom surface is

[tex]\sigma_y\,dx\,dz[/tex]

which creates a moment in the opposite direction that cancels out part of the moment from the force on the top surface. Starting to make sense now?
 
  • #3
Wow, thanks so much. The cancelling of dz was part of what was confusing me. Also, I was just looking at the diagram incorrectly for no particular reason, but since I saw it wrong initially I could not get it out of my head.

Also, how do you type formulas into this forum?
 
  • #4
Just click on the formula to see the markup language.
 
  • #5
EGill said:
I really need help understanding how the book created the equation from the book. I would really appreciate help understanding it. The attachment shows an image and the equation.

tanks
 

1. What is a stress equation?

A stress equation is a mathematical representation of the relationship between the force applied to an object and the resulting stress or strain on that object. It is often used in physics and engineering to analyze how materials respond to external forces.

2. How is a stress equation calculated?

A stress equation is typically calculated using the formula stress = force / area. This means that the stress on an object is equal to the force applied to it divided by the surface area over which the force is applied. It is important to note that there are different stress equations for different types of stress, such as tension, compression, and shear.

3. What factors affect a stress equation?

The factors that affect a stress equation include the type of material being stressed, the magnitude and direction of the force applied, and the geometry of the object. Different materials have different stress-strain relationships, and the shape and size of an object can also impact how it responds to stress.

4. How is a stress equation used in real-world applications?

A stress equation is used in many real-world applications, such as in the design of buildings, bridges, and other structures. Engineers use stress equations to determine the maximum load a structure can handle and ensure it is strong enough to withstand external forces. Stress equations are also used in material testing to determine the strength and durability of different materials.

5. What are some common misconceptions about stress equations?

One common misconception about stress equations is that they only apply to solid objects. In reality, stress equations can also be used for fluids and gases. Another misconception is that stress equations only apply to linear relationships between stress and strain. However, in some cases, materials may exhibit non-linear stress-strain relationships, requiring more complex equations to accurately model their behavior.

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