# Couples and forces

1. Oct 22, 2016

### 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?

2. Oct 23, 2016

### Simon Bridge

3. Oct 23, 2016

### ual8658

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?

4. Oct 23, 2016

### 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.

5. Oct 23, 2016

### ual8658

Thank you so much! It makes sense now.

6. Oct 23, 2016

### 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.

7. Oct 24, 2016

### ual8658

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

8. Oct 25, 2016

### 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.

9. Oct 25, 2016

### ual8658

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.