CBSE Class 9 - Science - Chapter 10: FLOATATION

CBSE Class 9 - Science - Ch10: FLOATATION: Buoyancy

BUOYANCY AND FORCE OF BUOYANCY

When a body is immersed fully or partially in a fluid, it displaced the fluid whose volume is equal to the volume of the body immersed in the fluid. This displaced fluid exerts an upward force on the body called the force of buoyancy.

This tendency of the displaced fluid to exert an upward force) is called buoyancy.     OR,

The tendency of an object to float in a liquid or the power of liquid to make an object float is called buoyancy

Up thrust or buoyant force depends on:

(i) The size or volume of the body immersed in a liquid.

(ii) The density of the liquid in which the body is immersed.

Quest Time  (Answer these questions)

Q. What are the factors the buoyant force depends on?

Answer: 1. Volume of the object immersed in the liquid. 2. Density of the liquid.

Q. Name the factors the buoyant force does not depend on?

Ans:  Buoyant force does not change with depth.

Q .In what direction does the buoyant force on an object immersed in a liquid act?

Ans: in the upward direction

Q. Why does a block of plastic released under water come up to the surface of water?

Ans: It is because  the upward buoyant force acting on plastic block  is greater than the downward gravitational force

Q. The volume of 50 g of a substance is 20 cm³. If the density of water is 1 g cm⁻³, will the substance float or sink?

Ans: Density of substance: masss/volume = 50g/20cm³= 2.5g/cm³

The density of the substance is more than the density of water (1 g cm−3). Hence, the substance will sink in water.

Q. The volume of a 500 g sealed packet is 350 cm3. Will the packet float or sink in water if the density of water is 1 g cm−3? What will be the mass of the water displaced by this packet?

Ans: Density of substance: masss/volume =500/350 =1.428

The density of the substance is more than the density of water (1 g/cm³). Hence, it will sink in water.

The volume of water displaced by the packet =  the volume of the packet= 350 cm³

The mass of water displaced by the packet  = dxv = 1 g/cm³ x 350 cm³  =350g

Floating and Sinking: if density of object is more than the density of liquid object float unless sink

Statement of Archimedes principle:

When a body is immersed partially or completely in a fluid (liquid or gas), it experiences an up thrust or buoyant force which is equal to the weight of the fluid displaced by the body.

Applications based on Archimedes’ principle are:

a. Designing of ships and submarines

b. Lactometers, which are used to determine the purity of a sample of milk

c. Hydrometers used for determining density of liquids

Proof of Archimedes Principe :

Consider a cylindrical body of cross-sectional area ‘a’ submerged in a liquid of density r . Let the upper face of the body is at a depth h1 below the surface of the liquid and the lower face is at a depth h2 below the surface of the liquid. The pressure exerted by the liquid on the upper surface of the body is given by p1 = h1dg .

Downward thrust on the upper surface of the body is ,

F1 = p1 × a = h1 d g × a

Pressure exerted by the liquid at the lower surface of the body,

P2 = h2 d g

Upward thrust on the lower surface of the body is,

F2 = P2 × a = h2 d g × a

The horizontal thrusts acting on the vertical sides of the body being equal and opposite from all the sides cancels out.

Resultant upthrust or buoyant force acting on the body

F = F2 - F1 = h2 d g a - h1 d g a = (h2 - h1) d g a  Since volume of the body, V = (h2 - h1)a

F = V dg

Thus, when a body is submerged in a liquid, it experiences an upward thrust equal to the weight of the liquid displaced by the body.