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# Cm 7 gravitational field strength (shared)

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### Cm 7 gravitational field strength (shared)

1. 1. A-level Physics A-level Physics Unit G484: The Newtonian World Gravitation – introduction Gravitation - Newton’s lawCircular motion
2. 2. Temperature LOs 1. What is a gravitational field? 2. Explain the meaning of gravitational field strength. 3. Gravitational field lines: what information is given by i) their directions, ii) their spacing? 5. Describe the gravitational field over a small part of the Earth’s surface. 6. Describe the gravitational field of the Earth as a whole.Circular motion
3. 3. Lesson focus • Gravitational field strength Learning objectives At the end of the lesson you will be able to: • describe gravitational field strength as force per unit mass; • select and apply the equation g = -GM/r2 for the gravitational field strength of a point mass.Circular motion
4. 4. Learning outcomes All of you should be able to • explain the meaning of gravitational field strength; • derive the equation for gravitational field strength; • use this equation to solve basic problems. Most of you will be able to • use the equation for gravitational field strength to solve more complex problems.Circular motion
5. 5. What is the force of gravity? LOs 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
6. 6. The link between g and G LOs At the Earth’s surface, the gravitational force on an object is F = mg (the weight of the object) G Me m where, Me – the mass of the Earth = - r2 G Me m ∴ mg = - r2 G Me ∴ g = - r2Circular motion
7. 7. The variation of g with r LOs g G Me ∴ g = - r2 To do 0 r 2r 3r distance from the 1. Refer to the equation and sketch a graph to show how g changes centre of with distance r above the surface of the Earth. the Earth 2. Inside the Earth, the gravitational field strength is due only to the mass that is closer to the centre of the Earth than you are. i.e. at A, g is due to the mass inside the sphere with radius CA •A 3. On your graph show how g varies with r inside of the Earth. •C 4. What is the value of g at the centre of the Earth? the mass of this ‘shell’ can be ignoredCircular motion
8. 8. The variation of g with r LOs A2 Physics Challenge, 2011Circular motion