Why is the linear speed at point M used in calculating EMF in a magnetic field?

[nokia8650]

See question below:

http://img140.imageshack.us/img140/479/30680788kr3.th.jpg

In order to calculate the EMF, the markscheme using the formula "E=Blv", the markscheme uses the linear speed of point M. Can someone please explain why this is the case - I understand the derivation of E=Blv, just not why the linear speed at M is used.

I used a different method (I calculated the area/second and multiplied it by B), however I would be interested to know how the method above manifests itself.

Thanks

 

[Dadface]

Work out what v is by using by using speed equals circumference of circle divided by t.It gives the same answer as your method but your method is better.

 

[nokia8650]

So why is the midpoint, M used?

Thanks

 

[Dadface]

Think of one of the radial lines of length l which is rotating.The velocity of this(and all the other lines)varies from zero at the centre to a maximum at the end.When we use Blv we should take the average velocity which is at the midpoint M.I still prefer your method.