# Can you do sum of all moments on a point that's not a rotational axis?

• Femme_physics
In summary, the conversation discusses taking the sum of all moments on a point that is not a rotational axis. It is mentioned that moments can be taken about any point in a plane or in 3D, with some being more useful than others. The conversation also mentions the use of wires for carrying tension loads and their tendency to kink when subjected to moments. The final part of the conversation discusses the application of static analysis to determine the tension in the wires of a structure and the use of point A as a reference point for moment calculations. It is concluded that the sum of moments can be taken on a wire, but it may not always be a useful approach.
Femme_physics
Gold Member
Can you do "sum of all moments" on a point that's not a rotational axis?

For instance, in this structure, with the two wires holding the beam

http://img228.imageshack.us/img228/3687/twowiresj.jpg

Can I do sum of all moments on B? Or is it not allowed because it's not a pivot point?

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You can take moments about any point in a plane or any line in 3D.

Some are more useful than others. We usually try to eliminate the effects of one or more external load by taking moments about a point or line that the line of external load passes through.

Can I even take a sum of all moments on a wire?

It depends on what you are looking for. Wires are good at carrying tension loads, but they tend to kink when subjected to moments or torques.

Although you have not stated a full problem, it appears that you have been asked to do a static analysis of this structure, to determine the tension in the wires given the weight of the rod and the magnitude of the applied load F. In addition to the tension, there will be a reaction at A, the nature of which will depend on whether A is fixed or pinned in some manner.

Hint: When writing your equations of static equilibrium, if you use point A as the reference point for moment calculations, then the reaction force at point A will not contribute to the sum of the moments, because the moment arm from A to A equals 0. You will, however, obtain a moment expression involving T, W, and F, with W and F being known. This makes it easier to substitute for T in the other equation of static equilibrium involving just the summation of forces.

Femme_physics said:
Can I even take a sum of all moments on a wire?

As Studiot already said: "You can take moments about any point in a plane or any line in 3D."

So yes, you can take the sum of moments on a wire.
You can even take the sum of moments somewhere in thin air, although that will usually not be very useful.

You may also note his careful distinction of a plane and of 3D.
In 3D you're actually always taking the sum of moments with respect to a line.

You can even take the sum of moments somewhere in thin air, although that will usually not be very useful.

When you are finding the centroid of an odd shaped object such as an L shape or the letter O you will find that the centroid actually is in thin air.

## 1. What is the definition of "moment" in physics?

Moment in physics refers to the turning effect produced by a force around a fixed point, also known as a rotational axis.

## 2. Can you explain the concept of "sum of all moments" in more detail?

The sum of all moments refers to the total sum of all the turning effects produced by different forces acting on a body around a fixed point. It is also known as the net torque.

## 3. Why is it important to calculate the sum of all moments on a point?

Calculating the sum of all moments is important in understanding the rotational motion of an object. It helps us determine the net torque acting on the object, which is necessary for predicting its rotational acceleration and behavior.

## 4. How do you calculate the sum of all moments on a point?

The sum of all moments can be calculated by multiplying the magnitude of each force by its perpendicular distance from the rotational axis, and then adding all these values together. This can be expressed mathematically as ΣM = r1F1 + r2F2 + ... + rnFn, where ΣM is the sum of all moments, r is the distance from the axis, and F is the force.

## 5. Can the sum of all moments be calculated for a point that is not a rotational axis?

Yes, the sum of all moments can be calculated for any point in space, regardless of whether it is a rotational axis or not. However, it is most commonly calculated for the axis of rotation in order to understand the rotational motion of an object.

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