It is difficult to sink an empty plastic bottle in water. Why ?

An empty plastic bottle has air inside it. So, the relative density of bottle is less than water. Hence, pressure given by empty bottle is very less than the upthrust given by the water and difficult to sink. 

When we immerse an empty plastic bottle in a bucket of water, the upward force (upthrust or buoyant force) exerted by water on the bottle is greater than its own weight, therefore it comes above the surface of water. ... This is because the upthrust on the bottle due to water must be balanced.The empty plastic bottle remains still because of the upward puwh applied on it by the table, which we refer as normal force (Fn). The forces acting on empty plastic bottle are also balanced.Although the bottle and the tube seem empty, both are filled with air. ... The air inside the bottle is moving slowly, so it is at a higher pressure compared with the fast-moving air in front of the bottle (the air you just blew).

Many hollow things like empty bottles, balls, and balloons will also float. That's because air is less dense than water. ... Water pushes back! So the more surface area an object has, the more water pushes back against it, helping it float.Introducing more sand into the bottle will increase the weight of the bottle and so increase its weight force resulting in it floating lower in the water.When the inverted can is placed in cold water, the steam condenses, leaving the can mostly empty, and thus with a very low pressure inside of it. The difference between the low pressure inside and the atmospheric pressure outside exerts an inward force on the walls of the can, causing it to implode.A stone of the same size as rubber ball has more mass. As mass is a measure of inertia, therefore, stone has more inertia than a rubber ball.