Class: Senior Secondary School 1 (SS1, SS 1, SSS1, SSS 1)
Term: 2nd Term
Week: 6
Age: 15 years
Duration: 45 minutes
Subject: Physics
Curriculum Theme: Physics
Previous Lesson: Description and Properties of Fields.
Topic: GRAVITATIONAL FIELD
Subject Matter: Concept of gravitational field, acceleration due to gravity, factors affecting g, shape and dimension of the earth, using ticker-timer method to determine acceleration due to gravity, interpreting simple motion tape results

Specific Objectives

By the end of the lesson, pupils should be able to:

Cognitive Domain:

Define gravitational field.
Define acceleration due to gravity.
State at least three factors affecting acceleration due to gravity (g).
Describe the shape and dimensions of the Earth in relation to gravity.
Interpret simple motion tape results to determine velocity and acceleration.

Affective Domain:

Appreciate the importance of understanding gravitational phenomena.
Show interest in the practical determination of acceleration due to gravity.

Psychomotor Domain:

Demonstrate the setup and use of a ticker-timer for motion experiments.
Calculate a simple estimate for acceleration due to gravity from ticker-timer data.

Collaborate with peers during practical demonstrations and discussions.

Reference Materials

The following resources were used in planning this lesson:

9 Years Basic Education Curriculum for Physics (Senior Secondary).
State Unified Scheme of Work for Senior Secondary Schools.
New School Physics by P.N. Okeke and Anyakoha.
https://www.thephysicsclassroom.com/class/circles/Lesson-3/The-Value-of-g
https://www.bbc.co.uk/bitesize/guides/zx8xng8/revision/1

Instructional Materials

The teacher will teach this lesson with the aid of:

Ticker-timer
Ticker tape
Power supply (AC/DC)
Trolley or heavy ball setup for free fall
Meter rule
Calculator (optional)
Earth diagram chart

Rationale for the Lesson

This lesson helps pupils understand the fundamental concept of gravity, which affects all objects on Earth. It enables them to appreciate why objects fall and how the Earth’s properties influence this force, providing a practical method to measure it.

Prerequisite/Previous Knowledge

Pupils have prior knowledge of force, motion, speed, velocity, acceleration, and basic measurements from their Junior Secondary School science classes.

Lesson Content/Board Summary

GRAVITATIONAL FIELD

1. Gravitational Field

A gravitational field is the region or space around a mass where another mass experiences a gravitational force. It is a vector quantity, meaning it has both magnitude and direction.

2. Acceleration Due to Gravity (g)

Acceleration due to gravity (g) is the acceleration experienced by an object due to the gravitational pull of a celestial body, such as the Earth. It is the rate at which the velocity of a freely falling body increases. The standard approximate value of ‘g’ on Earth’s surface is 9.8 m/s² or 10 m/s².

3. Factors Affecting Acceleration Due to Gravity (g)

The following factors affect the acceleration due to gravity (g):

Altitude (Height above sea level): As altitude increases, ‘g’ decreases because the distance from the Earth’s center increases.
Depth (Below Earth’s surface): As depth increases, ‘g’ decreases because the effective mass pulling the object reduces.
Shape of the Earth: The Earth is not a perfect sphere; it is an oblate spheroid (bulges at the equator and is flattened at the poles). This causes ‘g’ to be slightly greater at the poles than at the equator.
Rotation of the Earth: The Earth’s rotation creates a centrifugal force that slightly reduces the effective gravitational pull, especially at the equator.
Local Geology: Variations in the density of rocks beneath the surface can cause minor local variations in ‘g’.

4. Shape and Dimensions of the Earth

The Earth is an oblate spheroid, meaning it is slightly flattened at the poles and bulges at the equator. This results in the radius at the equator being slightly larger than the radius at the poles. Since ‘g’ is inversely proportional to the square of the distance from the Earth’s center (g = GM/R²), objects at the poles are closer to the Earth’s center than objects at the equator, leading to a higher ‘g’ value at the poles.

5. Determination of Acceleration Due to Gravity (g) using Ticker-Timer Method

The ticker-timer is a device used to study motion by marking dots on a ticker tape at regular, short intervals. When an object (e.g., a falling mass) pulls the tape through the ticker-timer, the spacing between the dots provides information about its velocity and acceleration.

To determine ‘g’ using a ticker-timer:

A heavy mass is attached to a ticker tape.
The tape is pulled through the ticker-timer as the mass falls freely.
The ticker-timer marks dots on the tape at a known frequency (e.g., 50 Hz, meaning 50 dots per second, so the time interval between two consecutive dots is 0.02 s).
The tape is then cut into strips of equal time intervals (e.g., 10 dots per strip, representing 0.2 s).
The length of each strip represents the distance covered in that time interval.
A graph of velocity (length of strip / time interval) against time is plotted. The gradient of this graph gives the acceleration due to gravity, ‘g’.

6. Interpreting Simple Motion Tape Results

When interpreting motion tape results:

Constant velocity: Dots are evenly spaced.
Increasing velocity (acceleration): Dots become progressively farther apart.
Decreasing velocity (deceleration): Dots become progressively closer together.

To calculate velocity from a tape:

Velocity = (Length of a section of tape) / (Time taken for that section)

To calculate acceleration from a tape:

Acceleration = (Change in velocity) / (Time taken for the change)

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction/Prior Knowledge Activation
Teacher’s Activity: The teacher greets the pupils and asks questions about their previous knowledge of force and motion, specifically asking why objects fall to the ground. The teacher then introduces the topic: Gravitational Field.
Pupils’ Activity: Pupils respond to questions and listen attentively to the introduction.
Learning Point: Pupils recall prior knowledge and are introduced to the lesson topic.

Step 2: Concept of Gravitational Field

Time: 7 minutes
Teaching Skill: Explanation/Definition
Teacher’s Activity: The teacher explains what a gravitational field is, using examples like the Earth’s pull on objects. The teacher draws a simple diagram on the board to illustrate the field lines.
Pupils’ Activity: Pupils listen, take notes, and ask questions for clarification. They attempt to define a gravitational field in their own words.
Learning Point: Pupils understand and can define a gravitational field.

Step 3: Acceleration Due to Gravity (g)

Time: 7 minutes
Teaching Skill: Explanation/Calculations
Teacher’s Activity: The teacher defines acceleration due to gravity, states its approximate value (9.8 m/s² or 10 m/s²), and explains that it is constant for all objects near the Earth’s surface, regardless of their mass (neglecting air resistance).
Pupils’ Activity: Pupils define ‘g’ and note its value. They participate in discussions about free fall.
Learning Point: Pupils understand the concept and value of acceleration due to gravity.

Step 4: Factors Affecting ‘g’ and Earth’s Shape

Time: 8 minutes
Teaching Skill: Elaboration/Visual Aid
Teacher’s Activity: The teacher discusses the factors affecting ‘g’ (altitude, depth, shape of Earth, rotation) using the Earth diagram chart to illustrate the Earth’s oblate spheroid shape and how it influences ‘g’ at different locations.
Pupils’ Activity: Pupils observe the chart, listen to explanations, and list the factors affecting ‘g’.
Learning Point: Pupils understand that ‘g’ is not uniform across the Earth and can state the factors causing variations.

Step 5: Ticker-Timer Method Demonstration

Time: 7 minutes
Teaching Skill: Demonstration/Practical Application
Teacher’s Activity: The teacher sets up the ticker-timer with a power supply, ticker tape, and a falling mass (e.g., trolley or heavy ball). The teacher demonstrates how to obtain a motion tape by allowing the mass to fall, pulling the tape through the timer.
Pupils’ Activity: Pupils observe the demonstration carefully and ask questions about the setup and process.
Learning Point: Pupils visualize how a ticker-timer works to record motion.

Step 6: Interpreting Ticker Tape and Calculating ‘g’

Time: 6 minutes
Teaching Skill: Guided Practice/Analysis
Teacher’s Activity: The teacher guides pupils on how to interpret the ticker tape (e.g., increasing spacing means acceleration). The teacher explains how to measure sections of the tape to calculate velocity and then acceleration (g) using simple calculations.
Pupils’ Activity: Pupils analyze sample ticker tapes (if available) or the one from the demonstration. They attempt simple calculations for velocity and acceleration with teacher guidance.
Learning Point: Pupils learn to interpret motion tapes and understand the principle of calculating ‘g’ from the data.

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

Define gravitational field.
What is acceleration due to gravity?
State any three factors that affect the value of ‘g’.
Briefly explain how a ticker-timer can be used to determine ‘g’.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 3 minutes
Teaching Skill: Summarization/Assignment
Teacher’s Activity: The teacher summarizes the key points of the lesson, reiterating the importance of gravity in daily life. The teacher assigns homework: “List possible sources of error when determining ‘g’ using the ticker-timer method.”
Pupils’ Activity: Pupils listen to the summary and copy the homework.
Learning Point: Pupils consolidate their learning and are given a task to reinforce understanding.

Lesson Keywords

Gravitational Field – The region around a mass where another mass experiences a gravitational force.
Acceleration Due to Gravity (g) – The acceleration of an object caused by the gravitational pull of a planet.
Ticker-Timer – A device used to record motion by marking dots on a tape at regular intervals.
Oblate Spheroid – The shape of the Earth, flattened at the poles and bulging at the equator.
Altitude – Height above sea level.

Differentiation

For pupils who grasp concepts quickly, they can be challenged to research the exact formula for ‘g’ and derive the relationship between ‘g’ and the Earth’s mass/radius. For pupils needing more support, the teacher will provide simplified explanations and extra guidance during the ticker-timer demonstration and calculations, possibly providing pre-cut tapes for analysis.

Note for teachers using this lesson plan

Ensure the ticker-timer is in good working condition and the power supply is appropriate. Prepare sample ticker tapes beforehand if a live demonstration is not feasible or time-consuming. Emphasize safety during the practical demonstration. Encourage pupils to relate the concepts to everyday experiences.