Cm 5 gravitation introduction (shared)

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A-level Physics

Unit G484: The Newtonian World

Gravitation - introduction

A-level Physics

Unit G484: The Newtonian World

Gravitation - introduction

Circular motion

Write down anything and everything you know

about ‘gravity’.

Gravitation LOs

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Learning objectivesAt the end of the lesson you will be able to:

Lesson focus• Gravitation - introduction

• describe how a mass creates a gravitational field in the space around it;

• describe gravitational field strength as force per unit mass.

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Learning outcomes

All of you should be able to• explain the meaning of gravitational field;

• use field lines to show the gravitational field in a small region of the Earth’s surface, and around the whole Earth;

• explain the meaning of gravitational field strength.

Most of you will be able to• solve problems involving the shape and strength of a gravitational

field.

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Draw a diagram to show the magnetic field around a bar magnet.

Questions

1. Where is the field strongest?

3. How does your diagram show this?

4. How could you show that there is a ‘field’ around the magnet?

Fields of force LOs

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Fields of force LOs

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A field is a region where a specific test object experiences a force. A test object interacts with the field but is small enough not to change the field.

Fields are represented with field lines. These show the direction of the force on the object. The strength of the field is shown by the spacing of the field lines (closer = stronger).

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Fields of force LOs

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1. What is the gravitational field like in this lab?

2. How can we test the field?

Gravitational fields LOs

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A gravitational field is a region in which a mass experiences a force

of gravity. Gravitational field lines show the direction of force on a

test mass. Over small regions of the Earth’s surface they are

vertical, parallel and evenly spaced.

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Gravitational fields LOs

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Gravitational fields LOs

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The gravitational field of the Earth as a whole is (broadly*) radial – field lines

come from infinity and converge at the centre of the Earth.

* Density variations within the Earth mean that it is not perfectly radial.

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Gravitational fields LOs

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The Earth’s gravitational field LOs

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GOCE geoid

BBC site

1. Earth is a slightly flattened sphere - it is ellipsoidal in shape2. GOCE senses tiny variations in the pull of gravity over Earth3. The data is used to construct an idealised surface, or geoid4. It traces gravity of equal 'potential'; balls won't roll on its 'slopes‘5. It is the shape the oceans would take without winds and currents6. So, comparing sea level and geoid data reveals ocean behaviour7. Gravity changes can betray magma movements under volcanoes8. A precise geoid underpins a universal height system for the world9. Gravity data can also reveal how much mass is lost by ice sheets

GOCE video

The Earth’s gravitational field LOs

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From this it can be seen that the force of gravity follows an

inverse-square law.

How do we write this? F 1/d2

Gravitational fields LOs

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• The force of gravity is an attractive force between masses.

• It is the weakest of the four fundamental forces.

Gravitational field strength

The strength of a gravitational field is found by placing a test mass in the

field and is expressed as the force per unit mass, ‘g’

The Earth has a gravitational field strength of approximately 9.81 N kg-1 .

F units: Ng = m kg

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What is the force of gravity? LOs

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1. In a solar eclipse the Sun, Moon and Earth lie in a

straight line. Show on a diagram the gravitation

forces which are exerted on the Moon at this point

and the direction in which they act. A scale diagram

is not required.

2. In travelling from the Earth to the Moon, American

astronauts experienced gravitation forces. On a

suitable graph, sketch the variation in gravitational

field strength that they experienced during the trip.

1. In a solar eclipse the Sun, Moon and Earth lie in a

straight line. Show on a diagram the gravitation

forces which are exerted on the Moon at this point

and the direction in which they act. A scale diagram

is not required.

2. In travelling from the Earth to the Moon, American

astronauts experienced gravitation forces. On a

suitable graph, sketch the variation in gravitational

field strength that they experienced during the trip.

Introductory Qs LOs

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Introductory Qs LOs

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3. If it were possible to position a huge mass of small size (e.g. a

neutron ball) just above the Earth’s surface, what would be the

shape of the gravitational field around the mass?

4. A mass of 7 kg is placed on a spring balance 1 m above the

surface of the Earth and experiences a force of 68.67 N.

Calculate the gravitation field strength at that point.

5. The gravitational force on a mass m on the Moon is 1.7m. Use

this information to calculate the ratio of g on the Earth to that on

the Moon.

Introductory Qs LOs