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# Cm 5 gravitation introduction (shared)

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### Cm 5 gravitation introduction (shared)

1. 1. A-level Physics A-level Physics Unit G484: The Newtonian World Gravitation -- introduction Gravitation introductionCircular motion
2. 2. Gravitation LOs Write down anything and everything you know about ‘gravity’.Circular motion
3. 3. Lesson focus • Gravitation - introduction Learning objectives At the end of the lesson you will be able to: • describe how a mass creates a gravitational field in the space around it; • describe gravitational field strength as force per unit mass.Circular motion
4. 4. 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.Circular motion
5. 5. Fields of force LOs 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?Circular motion
6. 6. Fields of force LOsCircular motion
7. 7. Fields of force LOs1/4 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).Circular motion
8. 8. Gravitational fields LOs 1. What is the gravitational field like in this lab? 2. How can we test the field?Circular motion
9. 9. Gravitational fields LOs 2/4 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.Circular motion
10. 10. Gravitational fields LOsCircular motion
11. 11. Gravitational fields LOs3/4 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.Circular motion
12. 12. The Earth’s gravitational field LOsCircular motion
13. 13. The Earth’s gravitational field LOs 1. Earth is a slightly flattened sphere - it is ellipsoidal in shape 2. GOCE senses tiny variations in the pull of gravity over Earth 3. The data is used to construct an idealised surface, or geoid 4. It traces gravity of equal potential; balls wont roll on its slopes‘ 5. It is the shape the oceans would take without winds and currents 6. So, comparing sea level and geoid data reveals ocean behaviour GOCE geoid 7. Gravity changes can betray magma movements under volcanoes 8. A precise geoid underpins a universal height system for the world BBC site 9. Gravity data can also reveal how much mass is lost by ice sheets GOCE videoCircular motion
14. 14. Gravitational fields LOs From this it can be seen that the force of gravity follows an inverse-square law. How do we write this? F  1/d2Circular motion
15. 15. What is the force of gravity? LOs4/4 • 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’ F units: N g = m kg The Earth has a gravitational field strength of approximately 9.81 N kg-1 .Circular motion
16. 16. Introductory Qs LOs1. 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.Circular motion
17. 17. Introductory Qs LOsCircular motion
18. 18. Introductory Qs LOs 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.Circular motion