If the magnetic pole has a strength ‘m’ then,

M = m $\times$ $l$

When the bar magnet is bent in the form of an arc as shown in figure then

l = ${\pi \over 3} \times r = {{\pi r} \over 3}$

$\Rightarrow$ r = ${{3l} \over \pi }$

New magnetic dipole moment

M' = m $\times$ 2rsin 30^o

= m $\times$ 2 $\times$ ${{3l} \over \pi } \times {1 \over 2}$ = ${{3ml} \over \pi } = {{3M} \over \pi }$