Question

A submarine is accelerating through the water at a constant depth. Four forces act on it as shown: Identify the pair of forces that balance one another. Find the net force acting on the submarine.

Answer 1

As the submarine is accelerating through water at a constant depth, it means net force acting on the submarine is only in horizontal direction and no force is acting in vertical direction.

Thus upthrust force acting on the submarine balances its weight.

Net force in the horizontal direction $F_{net}=F_p-F_w$

where $F_p$ and $F_w$ are the propelling force and water resistance force.