# What causes rotation, a coupe or a moment?

1. May 12, 2012

### Sarin

Does a moment have any real (physical) significance or is it just a definition/ an aid to understand and calculate a couple. Is it that the rotation is actually caused by a couple!

I know we can resolve a force causing a moment about a point into a couple and an equal force acting at that point, but what i want to know is how does this actually happen in a real world scenario.

Also,
Say if there was a body (an extended bar or rod) in space so that no force would act on its centre of mass, then if a force F was applied on one of its ends what would happen and why?

forgive me if the question sounds stupid, but this concept has ben bugging me for a very long time now!

2. May 15, 2012

### Dr Bwts

A couple is a special case of a moment when 2 equal moments act in opposite direction about a pivot.

The resolving of forces is (in my opinion anyway) a useful mathematical tool but in real life such things as the material constituent behaviour come into play and things get much more complicated, especially if the material deforms.

It would move in the direction of the applied force.