Question

Statement 1: Consider an object that floats in water but sinks in oil. When the object floats in water with half submerged, if oil is slowly poured into the water till it completely covers the object, the object moves up.
Statement 2: As the oil is poured in the situation of Statement 1, pressure inside the water will increase everywhere resulting in an increase in upward force on the object.

• A. Statement 1 is true, Statement 2 is true; Statement 2 is correct explanation for Statement 1
• B. Statement 1 is true, Statement 2 is true; Statement 2 is not correct explanation for Statement 1
• C. Statement 1 is true, Statement 2 is false
• D. Statement 1 is false, Statement 2 is true

Answer 1

The correct option is A Statement 1 is true, Statement 2 is true; Statement 2 is correct explanation for Statement 1

As the oil is poured till it covers the object completely, pressure in water at all points keep on increasing. As a result, upward force on object exerted by water increases and the object moves up for the given duration.
So, Statement 1 is true, Statement 2 is true; Statement 2 is correct explanation for Statement 1.

OR

Let volume of block be $V$
Initially when block floats in water, equating force of buoyancy with blocks weight, we get
$F_B=mg$
${\rho }_w\frac{V}{2}g=mg$

Now, when oil is poured till the block is covered, we get
Force of buoyancy as ${\rho }_o\frac{V}{2}g+{\rho }_w\frac{V}{2}g$
We can observe that,
Net upward buoyancy ${\rho }_o\frac{V}{2}g+{\rho }_w\frac{V}{2}g>mg$ downward force.
Hence, block moves up.
As oil is poured on to the water, net pressure inside the water increases due to weight of oil above it.