Why does a plastic ball released under water come up to the surface of water?
The density of a hollow plastic ball is less than the density of water due to the air in the ball. So, the weight of the water displaced by the ball is more than the weight of the ball. Thus, by the principle of floatation the ball floats in water. Even if you push the ball into the water the buoyant force will push it upwards and keep it floating once you release your hand.
There may be two answers
1. If the plastic ball is hollow from inside that is it has air trapped inside it. And since air is lighter than water therefore it come up WITH the plastic ball.
2. If it is not hollow from inside, then because of its lesser density it comes up to the surface. Even if we think we would find that the light density of plastic ball is caused mostly due to the air trapped inside small pockets of the ball during the making process of that ball.