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Gravitation

Buoyancy

Buoyancy is the upward force exerted by fluids over the surface are of contact of an object which is immersed in fluids. Buoyancy is also known as upward thrust.

Why does an object sink or float over water

When an object is immersed in water, it exerts pressure over water due to its weight. At the same time water also exerts upward thrust over the object. If the force exerted by the object is greater than the upward thrust or buoyancy by water, the object sinks in water otherwise it floats over water.

Buoyancy in everyday life –

(a) Swimming in water :– Anyone can be able to swim in water because of upward thrust exerted by water.

(b) Flying of bird or aero plane :– Since air is a fluid, thus it also exerts upward thrust over the object. Therefore, because of upward thrust of air a bird or aero plane can fly in air.

Factors which affect buoyancy

  •  Volume of the object
  • Density of the fluid

Volume of the object – Buoyancy or upward thrust exerted by a fluid increases with the volume of the object immersed in it.

Density of the fluid – The buoyant force or upward thrust increases with increase in density of the fluid. Denser liquid exert more upward thrust.

This is the cause that it is easier to swim in sea water rather than fresh water. Sea water is saline. Salts dissolved in sea water increase the density and hence it exerts more upward thrust than fresh water.

Density

Mass per unit volume of an object is called density or mass density. Density is denoted by Greek letter rho (ρ).

Buoyancy & Pressure Notes | Physics for Grade 10

Where, m is mass of object and V is the volume of that object.

SI Unit of density

The SI unit of mass is kg and SI unit of volume is cubic meter.

Therefore, by substituting the unit of mass and volume in the expression of density we get

Buoyancy & Pressure Notes | Physics for Grade 10

Relative density

For convenience the density of water is expressed in comparison of water. Thus, when density of a substance is expressed in comparison with water, it is called relative density.

Buoyancy & Pressure Notes | Physics for Grade 10

Relative density has no unit, because it is the ratio of similar quantity.

Practical application of density

Water is taken as reference material because it is easily available at most of the places and it is easy to carry out the measurement related to density in field visit.

When the relative density of a substance is less than 1, it will float in water otherwise it will sink in water.

The relative density of ice is 0.91, thus, it floats in water.

If the relative density of an object is less than 1, compare to the liquid in which it is immersed, the object will sink otherwise object will float.

Archimedes’ Principle

Archimedes’ Principle states that when an object is immersed fully or partially in a liquid, it experiences an upward force which is equal to the weight of liquid displaced by the object.

Application of Archimedes’ Principle

When the weight of displaced liquid by an object is greater than the weight of object, the object will float in liquid and when the weight of liquid will be smaller than that of the weight of the object, the object will sink in liquid.

(a) In submarines – In submarines, there is a tank which can be filled or emptied as per requirement. It is called buoyancy tank.

When submarines have to go inside the water, the buoyancy tank is filled with water, so that, the weight of the submarines would become more than the weight of water displaced by it. And when submarines have to come over the surface of water, the buoyancy tank is emptied and water is replaced by air. In this condition, the weight of water displaced by submarines will become more than the weight of submarines and submarine comes over the surface of water.

(b) Air balloons – To raise the air balloon in air; the air inside the balloon is heated. Air expands because of heat and becomes lighter. Thus, the balloon gets and upward thrust from the surrounding air and rises up.

(c) Ships – A ship is much heavier than water, yet it floats on water. This happens because of unique shape of the ship. Because of its shape, the volume of the ship is larger compared to its weight. Due to this, water displaced by the ship provides a proper upward thrust to the ship and the ship floats on water.

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FAQs on Buoyancy & Pressure Notes - Physics for Grade 10

1. What is buoyancy and how does it work?
Ans. Buoyancy is the upward force exerted by a fluid on an object immersed in it. It is caused by the difference in pressure between the top and bottom of the object. According to Archimedes' principle, the buoyant force is equal to the weight of the fluid displaced by the object. This force helps objects float or rise in a fluid.
2. How does the density of an object affect its buoyancy?
Ans. The density of an object determines whether it will float or sink in a fluid. If the density of the object is less than the density of the fluid, it will float because the buoyant force is greater than the object's weight. On the other hand, if the density of the object is greater than the density of the fluid, it will sink because the buoyant force is less than the object's weight.
3. What factors affect the buoyant force acting on an object?
Ans. The buoyant force acting on an object is influenced by two main factors: the volume of the object submerged in the fluid and the density of the fluid. The greater the volume of the object submerged, the larger the buoyant force. Similarly, the denser the fluid, the greater the buoyant force.
4. How does pressure change with depth in a fluid?
Ans. Pressure increases with depth in a fluid. This is because the weight of the fluid above exerts a force on the fluid below, which in turn increases the pressure. The relationship between pressure and depth is linear, with an increase of 1 unit of depth resulting in an increase of 1 unit of pressure.
5. Can you explain the concept of pressure in terms of buoyancy?
Ans. In the context of buoyancy, pressure plays a crucial role. The pressure difference between the top and bottom of an object submerged in a fluid creates the buoyant force. The greater the pressure difference, the stronger the buoyant force. This pressure difference is due to the weight of the fluid above the object, which compresses the fluid below and creates higher pressure.
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