Why do I have to take the moment at the cut to get the right answer?

In summary, the conversation discusses taking moments to solve for unknown values and how choosing a point can affect the results. The speaker mentions getting the wrong answer when taking moments from a specific point and not understanding why. The correct answer is found by taking moments from a hypothetical cut. It is also mentioned that neglecting the force at the cut can lead to errors in the results. Additionally, it is stated that there is zero internal moment at the left end.
  • #1
Svelte1
9
2
Homework Statement
Need to set the derivative of m(x) to 0.
Relevant Equations
taking moments..
Every time I have taken moments before I have been able to solve for the required unknown regardless of what point I choose, unless there is more than 1 unknown! However I tried taking moments from the far left point load of 97.5 this time and I get the wrong answer. I don't understand why? Thankyou! This image details the correct answer taking the moment from the hypothetical cut so as to find the internal moment values.

Capture.PNG
 
Last edited:
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  • #2
If you take the moment around any other point than x then also the force at the cut will influence the result because it will get a non-zero lever. You likely neglected this but it is impossible to know unless you show your work.
 

1. Why is taking the moment at the cut important?

Taking the moment at the cut is important because it allows us to accurately determine the forces acting on an object or structure. By taking the moment at the cut, we can isolate and analyze the specific forces that are causing the object to rotate or remain in equilibrium.

2. What is the moment at the cut?

The moment at the cut is the sum of all the forces acting on an object, multiplied by their respective distances from a chosen point. This point is known as the pivot or axis of rotation. It is a crucial concept in mechanics and is used to analyze the stability and equilibrium of structures.

3. How do you calculate the moment at the cut?

To calculate the moment at the cut, you need to first identify the pivot or axis of rotation. Then, you need to determine all the forces acting on the object and their respective distances from the pivot. Finally, you can use the equation M = F x d, where M is the moment, F is the force, and d is the distance from the pivot, to calculate the moment at the cut.

4. Can you explain the concept of equilibrium in relation to taking the moment at the cut?

Equilibrium refers to a state where an object is not moving or rotating. When taking the moment at the cut, we are analyzing the forces acting on an object to determine if it is in equilibrium. If the sum of the moments is equal to zero, the object is in rotational equilibrium. This means that it is not rotating and all the forces acting on it are balanced.

5. What are some real-life applications of taking the moment at the cut?

Taking the moment at the cut is used in various fields, such as engineering, architecture, and physics. It is essential in designing and analyzing structures, such as bridges and buildings, to ensure they can withstand external forces without collapsing. It is also used in understanding the stability of objects, such as cranes and seesaws, and in calculating the torque of machines and engines.

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