From Boyle's law :- P α 
Initial Pressure P₁  = atmospheric pressure + pressure of liquid cloumn
= Hρg + Hρg
Where h 2 depth of the pond
H 2 Assume 10m of water
V₁ = \(\frac { 4 }{ 3 } \pi \)r³ ; P₂ = Hρg
V₂ = \(\frac { 4 }{ 3 } \pi \)(3r)³ = \(\frac { 4 }{ 3 } \pi \)(27r³)
P₁V₁ = P₂V₂ 

​​​​​​​h+10 =10x27
h =270-10
depth of the pond, h = 260m

i) The rolling friction is smaller than sliding friction
ii) By using ball bearings sliding friction is replaced by rolling friction
iii) A ball bearing is used in a cycle hub to reduce friction.

i)Pour soap powder or oil into the sea near their ship.
ii) It decreases the surface tension of sea water.
iii) It reduces the impact of the violent water current.

Frictional force arises when two or more bodies in contact move or tend to move, relative to each other. It acts always in the opposite direction of the moving body.
Examples:
(i) To light a matchstick.
(ii) To stitch a cloth in a sewing machine.

i) No, there is no possibility of making a special device to measure the magnitude of friction directly. The magnitude of friction is directly proportional to irregular surface of contact, and the magnitude of the force exerted on it.
ii) The magnitude of friction of a body remains changing and self adjusting. So it cannot be measured directly
iii) With Tribometer we can calculate co-efficient of friction (u) and then we can calculate the magnitude of friction from it.