Quick Question - Moments about a force

In summary, a conceptual question was asked about finding the resultant force and moments of a given force system. The resultant force can be calculated by adding the position vectors of each force about the point of interest. The moments of the resultant force can be found by using the transport couple theorem.
  • #1
jmcmillian
28
0
Hi there, quick conceptual question just to make sure I know what I am doing...

I am given two forces, F1 and F2, in vector form with given magnitudes.

(I) Find the resultant force about point P (given).
(II) I am also asked to find the Moments of the resultant force about point P.

(I) I establish position vectors from P to each point (I'll call it A for F1 and B for F2) and multiply by magnitudes. Then add to get the resultant force about P.

(II) Then for the second part, I cross my resultant force about P with the "r" for the resultant force, which would be the distance between F1 and F2 from P?

It's that last sentence where you can see I'm unclear at. Any help would be greatly appreciated!
 
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  • #2
Hello!,

The resultant force about any point will be calculated by vectorial addition of all the forces. The moment of the resultant force about a point will be equal to the sum of the moments by each force about the same point.

This exercise sounds like a force-couple system, where you use the transport couple theorem in order to move all the force about the point P, and reduce the previous system to a equivalent one with one resultant and one couple.
 
Last edited:
  • #3


Hi there, it looks like you have a good understanding of how to find the resultant force about point P. To find the Moments of the resultant force about point P, you will need to use the concept of torque. Torque is the measure of the turning effect of a force, and it is calculated by multiplying the force by the distance from the axis of rotation. In this case, the axis of rotation is point P. So, to find the Moments of the resultant force about point P, you will need to take the cross product of the resultant force with the position vector from point P to the point where the force is applied. This will give you the magnitude and direction of the torque. Remember to use the right-hand rule to determine the direction of the torque. I hope this helps! Let me know if you have any other questions.
 

1. What is a moment about a force?

A moment about a force is a measure of the tendency of a force to rotate an object around a specific point or axis. It is also known as torque and is calculated by multiplying the magnitude of the force by the distance from the pivot point.

2. How is a moment about a force different from a force?

A force is a physical quantity that can cause an object to accelerate, while a moment about a force describes the rotational effect of the force. In other words, a force can cause linear motion, while a moment causes rotational motion.

3. What factors affect the magnitude of a moment about a force?

The magnitude of a moment about a force depends on the magnitude of the force, the distance from the pivot point to the line of action of the force, and the angle between the force and the line of action.

4. How is the direction of a moment about a force determined?

The direction of a moment about a force is determined by the right-hand rule. If the fingers of your right hand curl in the direction of the force, then your thumb points in the direction of the moment.

5. What is the unit of measurement for a moment about a force?

The unit of measurement for a moment about a force is the Newton-meter (Nm) in the SI system. In the imperial system, it is measured in foot-pounds (ft-lb).

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