91Ó°ÊÓ

$m=2.75kg,∆V=10\%$

Archimedes’ principle: A body submerged in a fluid, whether totally submerged or partly, experiences an upward buoyant force that acts at the centre of mass of the displaced fluid and is equal to the weight of the fluid the body displaces.

(a)

When the fish is fully submerged the buoyant force on it must be equal to its weight. That’s why the average density of the fish is very close to the density of water.

(b)

Initially

$$\begin{gathered} F_B=w=\left(2.75kg\right)\left(9.80,/s^2\right) \\ =27N \end{gathered}$$

When the volume increases by the buoyant force also increases by a factor 1.10 and it becomes$\left(27N\right)\left(1.10\right)=29.70N$ .

(c)

From part (b), the water exerts an upward force 29.7 N.

Downward force due to gravity is$F_B=w=27N$ .

So there is a net upward force of$F_{Net}=2.7N$ ; the fish moves upward.