Term: 1^st Term

Week: 8

Class: Senior Secondary School 1

Age: 15 years

Duration: 40 minutes of 5 periods each

Date:

Subject: Physics

Topic:- Pressure, Archimedes’ principles, upthrust & laws of floatation

SPECIFIC OBJECTIVES: At the end of the lesson, pupils should be able to

Explain the meaning of pressure
Discuss the Archimedes’ Principles & Upthrust
State the laws of Floatation

INSTRUCTIONAL TECHNIQUES: Identification, explanation, questions and answers, demonstration, videos from source

INSTRUCTIONAL MATERIALS: Videos, loud speaker, textbook, pictures

INSTRUCTIONAL PROCEDURES

PERIOD 1-2

PRESENTATION	TEACHER’S ACTIVITY	STUDENT’S ACTIVITY
STEP 1 INTRODUCTION	The teacher reviews the previous lesson on Density and relative density	Students pay attention
STEP 2 EXPLANATION	He explains the meaning of pressure. He discusses the Archimedes principles and upthrust.	Students pay attention and participates
STEP 3 DEMONSTRATION	He further states and explains the laws of floatation	Students pay attention and participate
STEP 4 NOTE TAKING	The teacher writes a summarized note on the board	The students copy the note in their books

NOTE

PRESSURE, ARCHIMEDES’ PRINCIPLES, UPTHRUST & LAWS OF FLOATATION

Pressure is defined as force per unit surface area. It is a scalar quantity & measured in N/m² or Pascal (pa).

P = F 1

Where P-pressure,

F- force

& A-area

NOTE: 1 bar = 10⁵ N/m² = 10⁵ pa

Example –

A force of 40N acts on an area of 5m². What is the pressure exerted on the surface?

Solution

F = 40N, A = 5m², P =?

P = F/A = 40/5 = 8pa

Pressure in Liquid

Pressure in liquid has the following properties

Pressure increases with depth
Pressure depends on density
Pressure at any point in the liquid acts equally in all direction
Pressure at all points at the same level within a liquid is the same
It is independent of cross-sectional area

P = ρgh 2

Where: p-pressure,

ρ-density,

h-height

& g-acceleration due to gravity.

ARCHIMEDES’ PRINCIPLES AND UPTHRUST

Archimedes’ principle is a law that explains buoyancy or upthrust. It states that when a body is completely or partially immersed in a fluid it experiences an upthrust, or an apparent loss in weight, which is equal to the weight of fluid displaced.

From pressure, P Is given by p = hρg, where:

h is The height of the fluid column

ρ is The density of the fluid

g is The acceleration due to gravity

Let us confirm this principle theoretically. On the figure on the left, a solid block is immersed completely in a fluid with density ρ. The difference in the force exerted, d on the top and bottom surfaces with area a is due to the difference in pressure,

given by

d = h2aρg – h1aρg

= (h2 – h1)aρg

But (h2– h1) is the height of the wooden block.

So, (h2 – h1)a is the volume of the solid block, V.

d = Vρg

Upthrust = Vρg

Weight in air – upthrust = weight in fluid

Upthrust = weight in air – weight in fluid

Upthrust = Apparent loss in weight

NB: When an object is wholly immersed, it displaces its volume of fluid. So;

Upthrust = weight of fluid displaces

Upthrust = Volume of fluid displaced x its density x g Upthrust= volume of object x density of fluid x g

Determination of Relative Density by Archimedes’ Principle

Relative density of solid

The body is weighed in air w1, and then when completely immersed in water w2

Relative density of solid = Weight of solid in air

Weight of equal volume in water

= w1

W1-W2

Relative density of liquid

A solid is weighed in air (w1), then in water (w2) and finally in the given liquid (w3)

Relative density of liquid = apparent loss of weight of solid in liquid apparent loss of weight of solid in water.

= W1–W3

W1–W2

Example –

The mass of a stone is 15g when completely immersed in water and 10g when completely immersed in liquid of relative density 2.0. What is the mass of the stone in air?

Solution:

Relative density = upthrust in liquid

upthrust in water

Let W represents the mass of the stone in air 2 = w – 10

w – 15

2(w – 15) = w –10

2w – 30 = w – 10

2 w – w = -10 + 30

w = 20g

LAW OF FLOATATION

A floating object displaces its own weight of the fluid in which it floats or an object floats when the upthrust exerted upon it by the fluid is equal to the weight of the body. When an object is floating freely (i.e. neither sinking nor moving vertically upwards), then the upthrust must be fully supporting the object’s weight.

We can say

Upthrust on body = Weight of floating body.

By Archimedes’ principle, Upthrust on body = Weight of fluid displaced.

Therefore, Weight of floating body = Weight of fluid displaced. This result sometimes called the “principle of floatation”, is a special case of Archimedes’ principle

EVALUATION: 1. Define pressure

State three characteristics of pressure in liquids
State Archimedes’ principle.
Explain the following terms:

(i) viscosity

(ii) terminal velocity

State two (i) effect of viscosity (ii) applications of viscosity
What is the pressure due to water at the bottom of a tank which is 20cm deep and is half of water? (Density of water = 10³kg/m³& g = 10m/s²)

CLASSWORK: As in evaluation

CONCLUSION: The teacher commends the students positively

Lesson Notes By Weeks and Term - Senior Secondary 1