# Lesson Notes By Weeks and Term - Senior Secondary 1

Gravitational field

Term: 2nd Term

Week: 3

Class: Senior Secondary School 1

Age: 15 years

Duration: 40 minutes of 5 periods each

Date:

Subject:      Physics

Topic:-       Gravitational Field

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

1. Explain the meaning of gravitational field
2. State Newton’s law of universal gravitation

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 field Students pay attention STEP 2 EXPLANATION He explains the meaning of gravitational field Students pay attention and participates STEP 3 DEMONSTRATION He states and explains Newton’s law of universal gravitation 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

GRAVITATIONAL FIELD

ACCELERATION DUE TO GRAVITY (g)

The force of gravity is the pull of attraction between the earth and objects on the surface of the earth. The value of acceleration due to gravity is constant in the region of the laboratory.

g = GM/r2

Where

G= Gravitational constant

M= Mass of the earth

g = Approximately 9.8ms-2

NEWTON’S LAW OF UNIVERSAL GRAVITATION

Newton’s law of universal gravitation states that every particle of matter in the universe attracts every other particle with a force which is directly proportional to the product of their masses and inversely proportional to the square of radius between their centers.

Mathematically,

F = Gm1m2/r2

Where,

• F is the gravitational force between two objects measured in Newton (N).
• G is the universal gravitational constant with a value of 6.674 × 10-11Nm2kg-2.
• mis the mass of one massive body measured in kg.
• m2is the mass of another massive body measured in kg.
• r is the separation between them measured in kilometre (Km)

Solved Examples

1. What is the force of gravity acting on an object of mass 2000 kg at the Earth’s surface?

Given:

Mass of Earth (m1) = 5.98 × 1024kg

Mass of object (m2) = 2000kg

The radius of the Earth (r)= 6.38 × 106m

Acceleration due to gravity (g) = 9.8 m/s2

Universal constant (G) = 6.67 x 10-11 N m2 / kg2

Solution:

F = Gm1m2/r2

F = ( 6.67 x 10-11) (5.98 × 1024)(2 x 103)/(6.38 × 106)2

F = (7.978 x 1017)/ (4.07044  × 1013)

F = 1.959 x 10or F = 19.59 N

1. What is the force of gravity acting on an object of mass 1000 kg at 20,000 meters above the Earth’s surface?

Solution

Given:

Mass of Earth (m1) = 5.98 × 1024kg

Mass of object (m2) = 1000kg

The radius of the Earth (r)= 6.38 × 106m

Acceleration due to gravity (g) = 9.8 m/s2

Universal constant (G) = 6.67 x 10-11 N m2 / kg2

h = 2 x 104 m

F = Gm1m2/(r +h)2

F = ( 6.67 x 10-11) (5.98 × 1024)(1 x 103)/(6.38 × 106 + 2 x 104 )2

F = (3.988 x 1017)/(4.058 x 1013)

F = 9,827.50

F = 0.9827 x 104

EVALUATION:   1. Define acceleration due to gravity and state its formula

1. State and explain Newton’s law of universal gravitation
2. What is the force exerted by Big Ben on the Empire State building? Assume that Big Ben has a mass of 108 kilograms and the Empire State                                                         building 109 kilograms. The distance between them is about 5000 kilometers and Big Ben is due east of the Empire State building.
3. What is the gravitational force that the sun exerts on the earth? The earth on the sun? In what direction do these act? (Me = 5.98×1024 and Ms = 1.99×1030 and the earth-sun distance is 150×109 meters).

CLASSWORK: As in evaluation

CONCLUSION: The teacher commends the students positively