Question

When a boat in a river enters the sea water, then it:
A. Sinks a little
B. Rises a little
C. Remains same
D. Will drown

Answer 1

Hint: The density of seawater is greater than that of river water and it has more density. When density is more, buoyant force on the boat is more.

Formula used: ${{\text{F}}_{\text{b}}}\text{= dVg}$
Where ${{\text{F}}_{\text{b}}}$ is the buoyant force, d is the density of water, V is the volume of displaced water, g is the acceleration due to gravity.

Complete step by step answer:
Buoyancy is the upthrust on a body which is equal to the volume of the liquid displaced by that body. We know that the density of seawater is more than river water because of the presence of a number of salts dissolved in it. So, when the boat enters the sea, the water is now the saline sea water, not the fresh river water anymore. From the equation for buoyant force, ${{\text{F}}_{\text{b}}}\text{= dVg}$, we can understand that as the density increases, the upward thrust or buoyant force also increases as a result of which the boat rises a little.

So, the answer is Option B.

Note:
The average salinity of the oceans is 35 grams per 1000ml of water. As the density of seawater is more the mass and hence weight of water is more for the same volume of water displaced. Saltwater weighs 2.5% more than the same volume of freshwater. Therefore, the upthrust is more in the seas.