ARCHIMEDES' PRINCIPLE, BUOYANCY, DENSITY, AND RELATIVE DENSITY

1. This is a place where teachers and students can get physics lesson notes,example questions and exercises to enrich
both teachers and students towards better performance in teaching and learning of physics subject. this blog will deal
with O-level physics.
PHYSICSPHYSICS
ARCHIMEDES' PRINCIPLE
A plastic cup containing air (literally empty) when immersed in water upside down it
will tend to come up, likewise the piece of wood or bee wax will come up if totally or
partially immersed in water
In both cases objects rise up due to upthrust force which is greater than the weight
(mg) of objects. If the weight would be greater than the upthrust then the object
would sink but would be much lighter if measured when immersed in water.
Upthrust force is also known as buoyant force
Buoyant force is an upward force exerted by a fluid to a body which is partially or
totally immersed in it and it is equal in magnitude to the weight of fluid displaced by
a body.
ARCHIMEDES’ PRINCIPLE (LAW OF BUOYANCY)
States that, when a body is partially or totally immersed in fluid, it experiences an
upthrust force which is equal to the weight of fluid displaced.
Experiment to show upthrust is equal to the weight of the fluid displaced
A is a real weight (weight of an object in air) 4.5 N
B is apparent weight (weight of an object in fluid) 3.0 N
C is apparent loss of weight (weight of fluid displaced) 1.5 N
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INTRODUCTION TO PHYSICS
MEASUREMENT
FORCE
DENSITY AND RELATIVE DENSITY
ARCHIMEDES' PRINCIPLE
THE LAW OF FLOATATION
FORM I TOPICS
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2. Measuring the weight of displaced fluid is equal to apparent loss of weight; this
apparent loss is the buoyancy force on an object which is upthrust.
Conclusion
Apparent loss of weight is equal to the weight of displaced fluid.
Real weight = apparent weight + apparent loss
Since apparent loss is equal to upthrust
Real weight = apparent weight + upthrust
Upthrust =real weight – apparent weight.
Example 1.
A body weighs 5.5 N when measured in air and 3 N when completely immersed in
kerosene, what is upthrust experienced by a body.
Solution
Weight of body in air (Real weight) =5.5 N
Apparent weight of a body when immersed in kerosene = 3 N.
But upthrust = real weight – apparent weight loss
Upthrust = 5.5 N – 3 N
= 2.5 N.
Example 2.
A body weighs 3 N when totally immersed in water contained in an overflow can, if
weight of displaced water was 2.6 N, what is the real weight of an object.
Solution
Apparent weight = 3 N
Upthrust = 2.6 N (according to Archimedes’ principle)
But
Upthrust = real weight – apparent weight
2.6 N = Real weight – 3 N.
Real weight = 2.6 N + 3 N
= 5.6 N.
DETERMINATION OF RELATIVE DENSITY OF SOLID BY ARCHIMEDES’
PRINCIPLE
If mass is multiplied by gravitational acceleration ‘g’ becomes weight (times 10
above and below which does not change magnitude but changes the physical quantity
from mass to weight)

3. Weight of a given volume of substance: - is the weight of substance in air.
Weight of an equal volume of water: - is the weight of water displaced (liquid
has to be water because the density of solid object is compared to the density of
water, meaning of relative density)
Other formulae will be
Example 1.
A metal block weighs 108 g in air and 68 g in water, find the relative density of
metal. What possibly this metal can be? (Use the table of densities).

4. Example 2
A body weighs 135 g in air displaced 54 g of water when completely immersed, find
the relative density and volume of a body.
Volume of a body is equal to the volume of displaced water.
Density= mass/volume
Therefore 1 g/cm 3= 54 g/volume
Volume = 54 cm 3.
DETERMINATION OF RELATIVE DENSITY OF OBJECT THAT FLOATS BY
ARCHIMEDES’ PRINCIPLE
CLASS WORK
AIM: To determine the relative density of an object that floats by Archimedes’
principle.
REQUIREMENTS: floating object, sinker (piece of metal or stone), spring balance,
string, and water in a beaker
PROCEDURES:
1. Tie a sinker with a string.
2. Immerse a sinker totally in water and measure its weight (apparent weight) W 1
3. Tie a floating object just before the sinker and immerse totally the sinker leaving
the floating object in air then measure weight W 2 (weight of floating object in air
+ apparent weight of a sinker)
4. Immerse totally both the sinker and the object in water and measure the weight W 3

5. Example 1
In an Experiment to find the relative density of floating object the followings were
observed,
I. Weight of a sinker in air = 64 g
II. Weight of a sinker when totally immersed in water = 49 g
III. Weight of a sinker totally immersed and floating object in air =56 g
IV. Weight when both sinker and floating object totally immersed in water = 46 g
DETERMINATION OF RELATIVE DENSITY OF LIQUIDS BY ARCHIMEDES’
PRINCIPLE
If an object of given volume is totally immersed in different liquids it will always
displace volume of given liquid equals to its own volume but the masses of these
equal volumes of liquids displaced will differ owing to their densities(concentration
of particles)
We can simply know mass of liquid displaced by noticing the apparent loss of mass
of an object when totally immersed in liquid; this is according to Archimedes’
principle.

6. Remember, when mass is multiplied by ‘g’ (gravitational acceleration) it becomes
weight also:-
CLASS WORK
AIM: TO FIND RELATIVE DENSITY OF ‘X’
REQUIREMENTS: Liquid x in a beaker, water in a beaker, object that can sink, and
spring balance.
PROCEDURES:
1. Measure the sinking object in air and record its weight W 1
2. Measure the sinking object when totally immersed in X and record its apparent
weight W 2
3. Measure the sinking object when totally immersed in water and record its apparent
weight W 3
11 comments:
Unknown 19 April 2020 at 06:13
Very fantastic э nice notes
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Unknown 4 May 2020 at 06:41
their very helpfull
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Gaurav 19 May 2020 at 03:23
Nice work man keep doing
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Unknown 27 May 2020 at 08:13
I like these notes man
Just keep it going
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Unknown 15 June 2020 at 02:41
Very nice Notes
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Unknown 15 June 2020 at 02:42
Very nice notes...keep it
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Unknown 21 June 2020 at 05:16
I love this , keep it up
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Unknown 3 August 2020 at 14:49
I love it
Excellent!!!!!!
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Unknown 3 August 2020 at 14:50
I love it
Excellent!!!!!!
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STUDENTS GROOM. &EDUTAINMENT 1 November 2020 at 04:52
Good job I love it keep it up sir
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Kanishk Sharma 13 November 2020 at 01:34
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