EQUILIBRIUM OF BODIES IN LIQUIDS
Subject:
Physics
Term:
FIRST TERM
Week:
WEEK 8
Class:
SS 2
Topic:
Previous lesson:
The pupils have previous knowledge of
EQUILIBRIUM OF FORCES
that was taught as a topic in the previous lesson
Behavioural objectives:
At the end of the lesson, the learners will be able to
- Concept of Upthrust
- Archimedes Principle
- Density & Relative density
- Principle of flotation
Instructional Materials:
- Wall charts
- Pictures
- Related Online Video
- Flash Cards
Methods of Teaching:
- Class Discussion
- Group Discussion
- Asking Questions
- Explanation
- Role Modelling
- Role Delegation
REFERENCES:
- new school Physics by MW Anyakoha
- New system PHYYSICS for senior secondary schools. Dr. Charles Chew.
- Comprehensive Certificate Physics by Olumuyiwa Awe
- Senior School Physics BY PN Okeke, SF Akande
- STAN Physics.
Content:
WEEK 8:
TOPIC:
EQUILIBRIUM OF BODIES IN LIQUIDS
CONTENT:
- Concept of Upthrust
- Archimedes Principle
- Density & Relative density
- Principle of flotation
EQUILIBRIUM OF BODIES IN LIQUIDS
Boat, ship or a swimmer can float on water. This is as a result of certain forces acting on these bodies.
Consider a cube floating in water as shoe below. For the cube to be in equilibrium U = W
U
W
The force U is called the upthrust.
Upthrust can be defined as an upward force experience by object in a fluid.
Upthrust can also be defined as the loss weight experienced by an object partial or completely immersed in a fluid. for object floating in a fliud,
W = U
For object partly or wholly immersed in a fluid, (e.g bucket of water inside the water in a well weight lighter than )
U = weight loss
Consider a bucket of water of weight W in a well which is held by a string whose tension is T. When the bucket is above the water in the well, the tension in the string equals the weight of the bucket. (W = T)
When the bucket is inside the well, it experiences a weight loss which equal to the difference (W – T)
Where W is the weight of the bucket in air and T is the bucket in the well/fluid.
EXPERIMENT 6
– To measure the upthrust experienced by s body immersed in water using the spring balance, eureka can and a beaker.
Archimedes’ principle
This states that when a body is partly or completely immersed in a fluid, it experiences an upthrust which is equal to the weight of the fluid displaced.
Weight = mass x acceleration due to gravity
Weight of fluid displaced W = mg
But density of the fluid,
Where v – volume is fluid displaced. Weight of fluid displaced W = g – acceleration due to gravity
Density of a body
This can be defined as the ratio of the mass of body to its volume or mass per unit volume. In the laboratory, the density of a substance can simply be determine by measuring the mass of the substance using a triple balance and measuring the volume. With the mass and volume of the substance known, the density can be determined using:
Density is a scalar quantity and it S.I unit is kgm-3. Another unit for density is gcm-3.
Relative density
The relative density of a substance is the ratio of the density of the substance to the density of water. This has no unit. It can also be easily determine by estimating the density of the substance in kgm-3 and dividing it by 1000 kgm-3(the density of water or in g/cm3 and dividing by 1gcm-3)
Relative density of a substance can also be defined as the ratio of the mass of the substance to the mass of equal volume of water.
The relative density of a liquid can be define as the ratio of the upthrust experience by an object in the liquid to the upthrust experienced by the object in water.
EXPERIMENTS 7-11
- Experiment to determine the relative density of a liquid using the relative density bottle
- Experiment to measure the density of regular solid
- Experiment to measure the density of irregular solid using the eureka can
- Experiment to measure the density of liquids using the measuring cylinder and triple balance
- Experiment to demonstrate weight loss by an object immersed in a fluid using the spring balance.
PRINCIPLE OF FLOATATION
The law of floatation states that for a body to float in a fluid, it must displace an amount of fluid equal to it own weight.
Weight of object = weight of fluid displaced.
Application of the principle of floatation
- hydrometer
- Submarine
- Ship/boat
- Hot air balloon
- Floating iceberg
CLASSWORK
- A body of mass 20g appears to have a mass of 13g in oil and 12g in water. What is the relative density of oil? SOLUTION g R.D of oil = 7/8
- A metal block of density 900kgm-3 weighs 60N in air. find its weight when it is completely immersed in paraffin wax of density 800kgm-3 (g=10ms-2) Solution:
Density of the object
Cross multiplying,
Mass of object … …. …. …. I
Mass of object … …. …. …. II
Equating I and II
Recall, eqn ii
U= ?
But upthrust = weight loss
U = Wo –T
56 = 60 – T
T = 60 – 56
T = 4N
Weight of the block in the paraffin wax = 4N
further examples should be solved as classwork)
EVALUATION:
A piece of wood of mass 60 kg and density 600 kgm-3 float in water of density 100 kgm-3.
Calculate;
- Volume of water displaced by the wood
- Fraction of the volume of the wood immersed in water
WEEKEND ASSIGNMENT
- Differentiate between a resultant force and a equilibrant.
- Mention two differences between centre of gravity and centre of mass.
- A pencil of mass 5 g can be balanced horizontally on a knife edge at a distant of 3 cm from the plane end when a mass of 2.5 g is hung from this end. Calculate the distance of the centre of gravity of the pencil from this plane end.
- Two boys of weigh 400 N and 700N sit at the end of a seesaw 4 m long pivoted at the centre. What will be the position of a third boy whose weight is 600 N in order to balance the seesaw?
- When a mass of 50 g is hung from at the 5 cm marked of a uniform metre rule, the rule balances on a knife edge place at the 35cm mark. What is the weight of the metre rule?
- Differentiate between density and relative density.
READING ASSIGNMENT
Read up Simple Harmonic Motion in New School Physics by M A Anyakoha and answer the following questions:
- what is Simple Harmonic Motion
- Mention four examples of bodies in SHM