Understanding Couples, Moments & Forces

[ual8658]

I'm trying to understand couples, moments and forces. I know that a moment is a force causing a rotation about a certain point/axis. And I know a couple is a vector that represents the... and then that's where I'm trying to get clarification.
Is a couple vector a representation of the general tendency of the entire rigid body to rotate as a result of two forces that qualify, and hence why the couple vector can be moved anywhere in space without affecting its value.?
And in calculating moments about a certain point the couple is added up just like a regular moment at that point?
Also if let's say a system is acted on by a couple vector and 3 forces. If you calculate the moment about a certain point, you add that couple vector with the moments of the other 3 forces? But when calculating a resultant force of just the 3 forces given, the resultant force should cause the same moment about that point as the moment with the couple included?

 

[Simon Bridge]

A couple is any system of vectors that produces rotation but no translation.
It is an example of a pseudovector - so it has vector-like properties, and some other stuff that you are finding confusing.

See:
http://web.mit.edu/4.441/1_lectures/1_lecture12/1_lecture12.html
... also: https://en.wikipedia.org/wiki/Couple_(mechanics)

 

[ual8658]

A couple is any system of vectors that produces rotation but no translation.
It is an example of a pseudovector - so it has vector-like properties, and some other stuff that you are finding confusing.

See:
http://web.mit.edu/4.441/1_lectures/1_lecture12/1_lecture12.html
... also: https://en.wikipedia.org/wiki/Couple_(mechanics)

Ok I think I get it now. So the couple is basically a combination of vectors that are no longer "visible" in a force/couple diagram that creates rotation. But for example if I am given a system with one couple and let's say 3 forces. The resultant force must produce the same moment about a certain point as the moments of the 3 forces and the couple combined right because the couple is a pseudovector of force vectors?

 

[Simon Bridge]

The net moment is that produced by the couple, and each force, combined - yes.
If you get into trouble - just decompose the couple into a pair of forces and analyse normally.

 

[ual8658]

The net moment is that produced by the couple, and each force, combined - yes.
If you get into trouble - just decompose the couple into a pair of forces and analyse normally.

Thank you so much! It makes sense now.

 

[Simon Bridge]

That's actually what I do. Physics courses tend to ignore moments and couples completely in favour of torque. Engineering courses seem to like moments and couples.

 

[ual8658]

That's actually what I do. Physics courses tend to ignore moments and couples completely in favour of torque. Engineering courses seem to like moments and couples.

Even in my engineering class it seems the professor makes couple appear to be a moment when they're not entirely the same thing.

 

[Simon Bridge]

I think it is a distinction without a difference in most cases. I had a hard time working out why a moment was thought to be different from a torque.
It is common for people with lots of experience use terms interchangeably where context makes the matter clear - it only gets worse as you advance.

 

[ual8658]

I think it is a distinction without a difference in most cases. I had a hard time working out why a moment was thought to be different from a torque.
It is common for people with lots of experience use terms interchangeably where context makes the matter clear - it only gets worse as you advance.

Actually after thinking about this I had one more question. A couple can be moved freely but a moment can't right? Because it's relative to a point? But they both provide similar rotation.