Applying Bernoulli's principle on upper & lower surfaces, we get
P1+21ρ(v1)2+ρgh1=P2+21ρ(v2)2+ρgh2
Since, height difference between upper and lower surface is negligible.
⇒P2−P1=21ρ(v1)2−21ρ(v2)2
Therefore, upthrust acting due to pressure difference:
⇒AF=21ρ(v12−v22)⇒F=21×1.2×(702−652)×2
=810N