From work energy theorem we can say, Work done by tension + work done by force (applied) + Work done by gravitational force = change in kinetic energy Here Work done by tension is zero ‌⇒0+F×AB−Mg×AC=0 ⇒F=Mg(‌
AC
AB
)=Mg[‌
1−
1
√2
1
2
] as AB=ℓsin‌45∘=‌
â„“
√2
and AC=OC−OA=ℓ−ℓ‌cos‌45∘=ℓ(1−‌
1
√2
) where ℓ= length of the string. ‌⇒F=Mg(√2−1)