Universal Law of Gravitation

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Textbook

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Additional information

Useful websites

1. Cavendish experiment - This link gives a simple sketch of the Cavendish experiment.

This website demonstrates the set-up of the Cavendish experiment.

1. 
   This is a college lecture by Prof Walter Levin explaining gravitation. Click here to see the video.
   
   
   
   
2. Conceptual Physics
3. Physics Classroom

Reference Books

The following textbooks are good references

1. Conceptual Physics, Paul Hewitt, 10th Edition
2. Physics for Scientists and Engineers, Douglas C. Giancoli, 3rd Edition
3. Resnick & Halliday with Jearl Walker, 8th Edition
4. NCERT Class 9 textbook Motion
5. NCERT Class 9 textbook Laws of Motion
6. NCERT Class 9 textbook Gravitation

Teaching Outlines

Concept #1 - Nature of Gravitational Force

Learning objectives

1. Inertial and Gravitational Property of Mass
2. Idea of a force field and that forces can act at a distance. The distance over which the effect of a force can be felt is called its field.
3. Gravitational force acts over a distance through a field and is one of the four fundamental forces
4. Gravitational force is a very small force

Notes for teachers

These are short notes that the teacher wants to share about the concept, any locally relevant information, specific instructions on what kind of methodology used and common misconceptions/mistakes.

• Mass has two aspects – inertial and gravitational. We are familiar with mass; it is an intrinsic property of matter. An intrinsic property is something that cannot be separated from the object. The mass of an object can be understood in terms of its gravitational aspect and inertial aspect. The intrinsic property of matter which makes it resist change is what we have called as mass. But this same mass possesses another property – the ability to “attract” mass. This we call the gravitational aspect. The gravitational aspect of the mass is responsible for the attractive force between two masses. The gravitational aspect is what makes masses interact with one another.
  

• Gravitation is a fundamental force. Gravitational force, or gravity, is the force by which physical bodies attract each other with a force directly proportional to their mass and inversely proportional to the square of the distance between them. This is one of the four fundamental forces in nature. Gravity is the force which makes masses interact with one another. Gravitational force is mutually attractive. Another fundamental force is the electromagnetic force, due to another intrinsic property of matter – charge. These forces are due to charges possessed by an object and their movement.
  
• Strong and weak nuclear forces are other fundamental forces that act within the nucleus of an atom, holding it together. These forces however act only at the subatomic level. Above the atomic level, gravitational and electromagnetic forces can satisfactorily explain all events.

Activity No # 1 - Inertial property of mass

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• Estimated Time - 30 minutes
• Materials/ Resources needed
• Prerequisites/Instructions, if any
• Multimedia resources
  

This video describes the inertial property of mass

• Website interactives/ links/ simulations
• Process (How to do the activity)
  

1. Let the students watch the video
2. Ask them to write situations where they have faced inertia

• Developmental Questions (What discussion questions)

1. When do we say something has inertia? (When it does not change its position)
2. What does inertia do? (Resists change)
3. Does it depend on size of the object? (Yes/ No/ Not always)
4. If not on size, what does it depend on? (It depends on how much matter ther?e is - mass)
5. When we observe inertia, we can say object has mass

• Evaluation (Questions for assessment of the child)

1. Does inertia increase with size?
2. Will an object have inertia when it moves?

• Question Corner

1. How can I tell that an object has mass or not?

Activity No #2 Mass and Weight

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• Estimated Time - 30 minutes
• Materials/ Resources needed

1. A piece of iron
2. A piece of wood
3. A ball of yarn or cotton

• Prerequisites/Instructions, if any
• Multimedia resources
• Website interactives/ links/ simulations
• Process (How to do the activity)

1. Hold each object in your palm
2. What do you feel when you hold each one of them (ask them to answer as light--->heavy)

• Developmental Questions (What discussion questions)

1. Is there a difference when you hold each of them?
2. Is this due to inertia? Am I changing anything when I am holding?
3. What might this difference be due to?
4. This is due to a property called weight which is because the mass of this object interacts with another mass - that of the earth. We call this weight
5. We will study more about this - but mass also interacts with other masses; this is called gravitational property
6. The Earth is not touching the object, but still its mass has an effect on the mass on my hand
7. Gravity can act at a distance between two masses

• Evaluation (Questions for assessment of the child)

1. Does mass have both inertial and gravitational property?
2. Does gravitational property depend on size? Or mass?

• Question Corner

1. If an object has inertia, when I drop the object why does it fall?
2. Do you feel the gravitational effect of the Earth?

Concept #2 - Newton's law of universal gravitation

Learning objectives

1. Gravitational Force is small
2. The importance of inverse square law

Notes for teachers

These are short notes that the teacher wants to share about the concept, any locally relevant information, specific instructions on what kind of methodology used and common misconceptions/mistakes.

Newton's law of universal gravitation states that every particle in the universe attracts every other particle with a force that is proportional to the product of their masses and inversely proportional to the square of the distance between them. Click on the links below to read more.

1. Newton's Law of Gravitation
2. Inverse Square Law
3. Law of gravitation and relativity

Activity No # 1 Demonstration of the gravitational force

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• Estimated Time - 45 minutes
• Materials/ Resources needed
• Prerequisites/Instructions, if any
• Multimedia resources
• Website interactives/ links/ simulations

Click here for PhET simulation

• Process (How to do the activity)

1. Open the simulation
2. Work with the simulation, change the parameters and discuss the following questions o

• Developmental Questions (What discussion questions)

1. When $m_{1}$ is moved towards $m_{2}$ what is the the force on each sphere?
2. What is the direction and magnitude of the force?
3. When the spheres are moved towards each other, does the force increase or decrease?
4. What is this due to? (Inverse square law)

• Evaluation (Questions for assessment of the child)

1. If the gravitational force is the same for both objects, why do we see the apple falling but not the ground moving up?
2. What can you say about the magnitude of gravitational force?

• Question Corner

1. If you were to go inside a mine, deep into the Earth will the gravitational force increase or decrease?
2. If there was a hollow channel through the centre of the earth and if you drop a ball inside, how would the ball move?

Activity No #

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• Estimated Time
• Materials/ Resources needed
• Prerequisites/Instructions, if any
• Multimedia resources
• Website interactives/ links/ simulations
• Process (How to do the activity)
• Developmental Questions (What discussion questions)
• Evaluation (Questions for assessment of the child)
• Question Corner

Fun corner

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