cm 7 gravitational field strength (shared)
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A-level Physics
Unit G484: The Newtonian World
Gravitation - introduction
A-level Physics
Unit G484: The Newtonian World
Gravitation – Newton’s law
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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.
Temperature LOs
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Learning objectivesAt the end of the lesson you will be able to:
Lesson focus• Gravitational field strength
• 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.
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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.
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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
What is the force of gravity? LOs
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At the Earth’s surface, the gravitational force on an object is
F = mg (the weight of the object)
G Me m
r2= - where, Me – the mass of the Earth
F =
G Me m
r2= - mg
G Me
r2= - g
The link between g and G LOs
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G Me
r2= - g
To do
1. Refer to the equation and sketch a graph to show how g changes with distance r above the surface of 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
C
3. On your graph show how g varies with r inside of the Earth.
4. What is the value of g at the centre of the Earth?
g
r 2r 3r0 distance from the centre of the Earth
the mass of this ‘shell’ can be ignored
The variation of g with r LOs
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A2 Physics Challenge, 2011
The variation of g with r LOs