While taking out water from a bucket filled with water using a mug, do you notice that the mug feels lighter when it is inside water?
Answer
Yes — and it feels heavier the moment you lift it clear of the water surface.
The Earth's pull on the mug is the same in both positions; nothing has been added to or taken out of the mug. What changes is that while the mug is inside the water, the water pushes it upwards. That upward push takes over part of the load, so your hand has to supply less, and the mug feels lighter. The force applied by a liquid on an object in the upward direction is called upthrust or buoyant force.
Q2.
If we place some objects on water, some of them float, while others fall to the bottom. The gravitational force of the Earth is acting on all objects, then why don’t all objects fall to the bottom?
Answer
Because gravity is not the only force acting on an object in water. The water pushes back up with the buoyant force, and what happens depends on which of the two is larger.
Gravitational force more than buoyant force → the object sinks
Gravitational force equal to buoyant force → the object floats
So the objects that float are not escaping gravity — the Earth pulls them just as hard as it pulls the ones that sink. They float because the water beneath them happens to push up on them by exactly as much as the Earth pulls them down, and the two forces balance.
Did you know? One of the factors on which the buoyant force depends is the density of the liquid — which is why it is easier to float in very salty water than in a freshwater pond. You will learn about density in a later chapter of this book.