1. A-level Physics
A-level Physics
Unit G484:
The Newtonian
World
Gravitation – introduction
Gravitation - Newton’s law
Circular motion

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. 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. 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. 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. 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 = -
r2
Circular motion

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 ignored
Circular motion

8. The variation of g with r LOs
A2 Physics Challenge, 2011
Circular motion