Transcript unit1-17 - Caldervale High School

Notes p.34
Gravitation
Additional reading: Higher Physics for CfE, p.55 – 63.
Newton’s “Thought Experiment”
Sir Isaac Newton, as well as giving
us the three laws, came up with an
ingenious thought experiment for
satellite motion that predated the
first artificial satellite by over 300
years!
Flat surface.
Object takes a curved
path and lands.
Surface curves away.
Object takes longer to
land.
Newton proposed that, if
the object has a fast
enough horizontal
velocity and is launched
from a suitable height
above the atmosphere,
so that there is no air
resistance, then the
object will continue to
fall but never land.
The Earth’s surface
curves “away” from the
object with the same
flight path as the object.
The object has become a
satellite in orbit around
Earth.
Of course there is no
mountain this high …
satellites are taken to
their launch position in
rockets!
Points to Note
lower
Higher altitude satellites require ___________
launch velocities.
Objects in continual freefall (like satellites or
space stations or astronauts in space stations)
weightless .
appear to be ___________
At an altitude of almost 36 000 km a satellite
needs to travel at 11 300 kmh-1. This orbit has a
24 h making the satellite
period of _____
_______________.
geostationary
Which Statement is Correct?
“A geostationary satellite is one which
remains above the same point on the Earth’s
surface.”
OR
“A geostationary satellite is one which
remains at the same point above the Earth’s
surface.”
Find Out About …
1.
“Escape velocity” of a planet.
What’s Earth’s?
2.
“Slingshot effect” …
checkout this cool link and try the
animation.
http://galileoandeinstein.physics.virginia.edu/
more_stuff/flashlets/Slingshot.htm
Gravity and Mass
Notes p.35
If there is one thing that causes confusion in
physics [aye, it would be good if it was only
one thing!] then the distinction between
mass and weight could be it.
matter
Mass is a measure of how much __________
an object contains. This will only change if
matter is added or taken away.
_________
We already know that large masses, for
gravitational
example planets and stars, have __________
fields around them.
In fact ALL ________
masses produce gravitational
fields it’s just that for small masses the field
is negligible.
I am attracted
to you
I am even attracted
to you
Gravitational field strength is defined as
the
force, caused by gravity, acting on each
__________________________________
1
kg of mass. (Or the “weight per kg”)
__________________________________
Body
Gravitational Field Strength
Sun
274.13
Moon
1.62
Mercury
3.59
Venus
8.87
Earth
9.81
Mars
3.77
Jupiter
25.95
Saturn
11.08
Uranus
10.67
Neptune
14.07
(Nkg-1)
Notes p.36
Newton’s Universal Law of Gravitation
Newton devised the following formula to
calculate the force of attraction caused by
gravity between two objects:
F =
G m1 m2
r2
where:
F = force between the masses (in N)
G = the Universal Constant of Gravitation
(= 6.67 x 10-11 m3kg-1s-2)
m1 = mass of first object (in kg)
m2 = mass of second object (in kg)
r = distance between the mass centres (in m)
Worked Example 1
Mass of Earth = 6 x 1024 kg
Mass of Moon = 7.3 x 1022 kg
Mean distance between the centres of the Earth and
the Moon = 3.84 x 108 m
Determine the gravitational force between the Moon
and the Earth.
m1 = 6 x 1024 kg
m2 = 7.3 x 1022 kg
r = 3.84 x 108 m
G = 6.67 x 10-11 m3kg-1s-2
F =
=
G m1 m2
r2
6.67 x 10-11 x 6 x 1024 x 7.3 x 1022
(3.84 x 108)
= 1.98 x 1020 N
2
Worked Example 2
Determine the gravitational force between 2 pupils
of masses 50 kg and 65 kg who are sitting 1 m apart.
m1 = 50 kg
m2 = 65 kg
r=1m
G = 6.67 x 10-11 m3kg-1s-2
F =
=
G m1 m2
r2
6.67 x 10-11 x 50 x 65
(1)
2
= 2.2 x 10-7 N
1m
Consequences and Applications of Gravity
Read Notes page 37!
Really, do, it’s interesting stuff! All about …
How stars and planets are formed … GRAVITY!
The slingshot effect for space travel … GRAVITY!
For most situations gravity is well
approximated by Newton’s Universal Law of
Gravitation.
However …
In modern Physics, gravitation is most
accurately described by the General
Theory of Relativity (proposed by Einstein).
This describes gravitation as a consequence
of the curvature of space - time.
Don’t worry … this is outwith the scope of the
Higher course 
Complete Problems from Tutorial IV
Gravitation Q. 1 - 6
Answers
1. F = m1 m2 / r2
2. 417 N
3. 2.7 x 10-4 N
4. 3.6 x 10-47 N
5. 3.54 x 1022 N
6. 4.00 x 10-15 m