why do objects float or sink in water

Hey samadhan, An object may float or sink when placed on the surface of water because of the principle of buoyancy. Which is quite simply the upward force that is exerted on an object by the fluid on whose surface the object is placed. Let's take a very simple example of a paper boat placed on a water. The boat at all times experiences a force that is upward in direction from the water. This force is exerted because the water is resisting the weight of the boat. This force increases as we go deep in to the water. That is why it is highly unlikely that a paper boat will not sink near the bottom of the water body, because its weight is not able to resist the higher force at the bottom, but it can resist the weaker force at the top. Any object whose density is greater than the fluid in which it is placed, will sink. That is why a heavy boat made of metal will sink when placed in a small tumbler of water, because the weight of the boat is more than the buoyant force that the water is exerted in this case.


However, a big ship made of metal is afloat in a sea because the buoyant force in this case is balanced by the weight of the ship. The shape of the object also places an important role and a boat also because of its aerodynamics manages to stay afloat on water. The archimedes principle explains this very succinctly. "Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object. "
Why do some things float and others sink? The first thing that comes to mind for many people is that it depends on how heavy an object is. While an object's weight, or more properly its mass does play a role, it is not the only factor. If it were, we could not explain how a giant ocean liner floats while a small pebble sinks. Mass matters, but there is more to it. The ability of an object to float is described as its buoyancy. The buoyancy of an object is its tendency to float on or rise in a liquid.


An object that floats in water is said to be positively buoyant. An object that sinks is negatively buoyant. To determine an object's buoyancy, both its mass and volume must be taken into consideration. The relationship between object's volume and mass is called its density. Density is defined as the mass of an object per unit volume. Mathematically, this relationship is described using the following equation ). In order to explain how an object's density influences its buoyancy, the behavior of an object placed in water must be understood. When an object is placed in water, even a floating object displaces some of that water. The amount of water displaced is a function of the object's mass. The object sinks into the water until it displaces an amount of water equal to its own mass. A 1 g object will sink until it displaces 1 g of water. This is independent of its size or shape. Since water has a density of 1 g/cm, a 1 g object will displace 1 cm3 of water.


An object with a mass of 25. 2 g can displace up to 25. 2 cm of water. If the object has a volume greater than 25. 2 cm, it will stop sinking before it is fully immersed in the water. In other words, it will float. If its volume is less than 25. 2 cm, it will not stop before it is fully immersed. It will sink. This means whether or not an object will float or sink depends on its own density and the density of the liquid it is placed in. In the case of water, an object with a density less than 1 g/cm will float. The closer its density is to 1 g/cm, the more of it will sit below the water level. An object with a density of 0. 5 g/cm will sit half in and half out of the water. Three quarters of an object with an density of 0. 75 g/cm will be submerged. Another way to look at the buoyancy of an object is as an interaction of two forces. The force of gravity (Fg) pulling an object down. This is the weight of the object; its mass time the acceleration due to gravity (9. 8 ms on Earth).


It is a force and is expressed in Newtons (N). The buoyant force (Fb) holding the object up. This can be measured as the force of gravity acting on a mass of water equal to the amount of water the object displaces when fully immersed. This is also expressed in Newtons. Example 1: An object with an mass of 10 g (0. 01 kg) Fg = 0. 01kg x 9. 8 ms = 0. 098 kg m s = 0. 098 N Fb = 5 cm3 water = 5 g water = 0. 005 kg x 9. 8 ms = 0. 049 kg m s = 0. 049 N Fg Fb - the object will sink. Example 2: An object with an mass of 10 g (0. 01 kg) and a volume of 20 cm Fg = 0. 01kg x 9. 8 ms = 0. 098 kg m s = 0. 098 N Fb = 20 cm3 water = 20 g water = 0. 02 kg x 9. 8 ms = 1. 96 kg m s = 0. 196 N Fg Fb - the object will float. The illustration below shows a block placed in water. it explores the relationship between the blockвs volume, mass and density, and how this relationship determines the blockвs buoyancy. Move the sliders to adjust the mass and volume of the red block.

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