Teaching Notes for Unit 9.2 Space Topic of NSW HSC Physics Course

1. Topic 9.2
1.1.1 Gravitational Fields

2. Mass, Weight and Gravity
 Remember that mass is a measure
of the amount of substance in an
object.
 Weight is the force of gravity pulling
down on the object's mass.
 This is expressed as the formula:
 W=mg
 What are the units of each term in this
equation?

3. The Simple Pendulum
 A simple pendulum consists of a
mass (called a bob) on a light (no
mass) string.
 The weight of the object causes it
to try to return to the lowest
position when it is disturbed.
 The time taken to make one
complete swing (Left to right to
Left) is called the TIME PERIOD, T.

4. Investigating the Simple Pendulum
 Set up a simple pendulum as
shown.
 Investigate the effect of the mass
of the bob, m, on the time period,
T, of the pendulum.
 Identify what variables need to be
controlled.
 How can you make your time
measurement as precise as possible?
 Remember keep the angle of swing
small for the best results!
 What does a graph of T against m
show?
mg
l

5. Investigating the Simple Pendulum
 Now investigate the effect of the
pendulum length, l, on the period,
T.
 Identify what variables need to be
controlled.
 How should you measure the length
of the pendulum?
 What does a graph of T against l
show?
 What does a graph of T2 against l
show?

6. Measuring g
•The time period of a simple pendulum is
given by the equation:
T = 2p l
g
•This can be written as:
2
2 = 4p
l
g
T
•Calculate g from the gradient of your T2
versus l graph.

7. Gravitational Fields
 Gravity can act over large distances,
even through space.
 To explain this we use the idea of a field.
 All objects with mass create their own
gravitational field.
 This field extends to infinity and causes
other objects to be attracted towards the
mass.
 This is similar to the electromagnetic fields.
 The gravitational field is much weaker than
the other fields.

8. Gravitational Fields

9. Uniform Gravitational Fields
 Very close to an object the
gravitational field can be
considered to be uniform.
 g has a constant value
 Further away the gravitational field
becomes weaker.
 g decreases with distance.

10. Gravitational Potential Energy
 Close to Earth, where g is constant, we know
that the energy gained by an object when it is
lifted equals the work done on it.
 EG = Work Done = Force x Distance
EG = WΔh = mgΔh
 How much GPE is gained when a 65kg object
is raised through 7.0m close to the Earth's
surface?

11. Gravitational Potential Energy
 Normally, the Earth's surface is defined
to have a GPE of 0J.
 In space, a new zero must be defined
which is the same for everyone.
 A point an infinite distance from Earth is
defined as having zero GPE.
 “Gravitational Potential Energy is a
measure of the work done to move an
object from infinity to a point within the
gravitational field”

12. Gravitational Potential Energy
 If a point an infinite distance from
Earth has zero GPE AND lifting an
object away from the Earth
increases its GPE then GPE MUST
ALWAYS BE NEGATIVE!!
 i.e. lifting an object makes its GPE
less negative!

13. Gravitational Potential Energy
 GPE is given by the formula:
EG=−G Mm
R
 Where G is the Universal Gravitational Constant
=6.67x10-11 m3kg-1s-2
 What do the other symbols mean?

14. Gravitational Potential Energy
 Calculate the GPE of a 200kg
satellite when in low Earth orbit
125km above the Earth's surface.
 What is the GPE of a 400kg
satellite in a geostationary orbit
36,000km above the Earth's
surface?
 mEarth=5.98x1024 kg
 rEarth=6.38x106 m

15. Gravitational Field Strength
 Equating our universal equation for GPE
and our close to Earth equation gives on
the Earth's surface (radius R):
mg h= G Mm
r
D -
mg R = G Mm
( )
- -
g = G M
2
0
R
R
-
 The acceleration due to gravity is not
constant! It falls off with r2.
 Notice that g is a vector and that the –
sign indicates attraction.

16. Gravitational Field Strength

17. Gravitational Field Strength
 Visit www.nineplanets.org
 Record values for the mass and
diameter of each of the 8 major
planets.
 Create a table or use a
spreadsheet to calculate values of
g for each planet.
 g=6.67E-11*mass/(radius^2)

18. Homework
 Read pages 3-4 of Keep It Simple
Science
 Complete worksheet on page 5.