Transcript Friday, Sept. 12

Lecture 4 Phys 1810
Syllabus at
http://www.physics.umanit
oba.ca/~english/2014fallph
ys1810/
(Google “Jayanne English teach”)
along supplemental
material.
REVISE DATE IS
Wed. Sept 15
(Do Honours rather than
General Science 3yr degree)
Read BEFORE coming to class:
– Kepler’s Laws of Planetary motion
2.5, 2.8 (focus on Newton’s versions)
– Tides 7.6, Chapt 8 (P. 195-196)
– Weighing the sun, Box 2-2
– Orbits, including escape velocity 2.8
– introduction to General Relativity p.
558-560
Laws of Motion and Gravity
summary
Recall column
• The moon produces tides on Earth
using the force of gravity.
Tidal Forces:
 Distortion of an object by the gravitational pull of another object.
-- nearby (e.g. Earth and Moon)
-- or very massive (e.g. Earth and Sun)
Dark Matter
Laws of Motion and Gravity
Need these to explain:
Effect of Moon on Earth (Tides)
Orbits of the Planets
Comet Crashes
Measure stellar mass
Black Holes
Peculiar Galaxies
Dark Matter
Start with Newton’s Laws and introduce General
Relativity.
2.7 Newton’s Laws
Newton’s laws of motion explain
how objects interact with each
other.
Newton’s 1st Law
Every body continues in a state of rest
or in a state of uniform motion in a
straight line, unless it is compelled to
change that state by a force acting
on it.
2.7 Newton’s Laws
Newton’s First Law:
Object at rest will remain at rest.
Object moving in a straight line at constant speed will not change its motion,
unless an external force (e.g. push or pull) acts on it.
The tendency of an object to keep moving
with the same speed in the same direction
is inertia. Demonstrate with a cart. Note change in speed.
A measure of an object’s inertia is its mass.
Mass is the total amount of matter contained
in an object. Demonstrate with a cart and mass.
(Ignoring friction.)
Harder to push the cart with weight.
Velocity:
An object’s velocity includes both
speed (e.g. m/s) AND direction.
Acceleration:
Rate of change of an object’s
speed OR direction.
Units: m/s2
1 N (==Newton) is F to
accelerate 1 kg to 1 m/s2
2.7 Newton’s Laws
Newton’s second law:
When a force is exerted on an object, its acceleration is inversely proportional to its
mass:
a = F/m
.
Or
F = ma
F == force
m == mass

a == acceleration = a
1) What happens if the F = 0 ?
1) What happens is the F = 0 ?
a= 0  v = constant
travel in straight line
If sun & its mass were suddenly to disappear, Earth
would fly off into space!
• What happens when a constant force is
applied?
1. Cart with a constant force applied.
2. Cart with same force & more mass.
• What happens when a constant force is
applied?

1) The speed changes  a.
2) If m increases, a decreases.
2.7 Newton’s Laws
Newton’s third law:
To every action there is an equal and opposite reaction.
When object A exerts a force on object B, object B exerts an equal and opposite
force on object A.
(2 carts pushing equally on each other)
(Glass of water)
2.7 Newton’s Laws
Gravity
Look up:
– how astronauts
experience gravity.
– feather & hammer
landing in vacuum
On supplemental page
2.7 Newton’s Laws
Gravity
For two massive objects, gravitational
force is proportional to the product of
their masses divided by the square of
the distance between them:
Newton’s Law of Gravity
G
• == Gravitational Constant
• measured experimentally
G=
(will give G on a test but you may have to convert units)
Since units of Newton:
Homework: Convert to units used by textbook Appendix 3
2.7 Newton’s Laws
Gravity
The constant G is called the gravitational constant; it is
measured experimentally and found to be:
G = 6.67 x 10-11 m3/s2/kg
• If
we keep the distance the same (r =
constant)
m
F
2 * m1
2*F
3 * m1
3*F
• The force felt on m1 due to m2 is
equal to the force felt by m2 due to m1.
Gravity is inversely proportional to
DISTANCE! The closer the object the
stronger the Fg.
Keep m constant:
r
1
F

1
2

¼
3

1/9
etc.
Question:
Recall column
summary
• The mass of the Earth is a few millionths
that of the Sun. Therefore
a) The gravitational force of the earth on
the sun will be roughly 1/1,000,000
that of the gravitational force of the
sun on the earth.
b) The gravitational force of the earth on
the sun will be equal to the
gravitational force of the sun on the
earth.
Example:
Recall column
summary
Example:
Recall column
summary
summary
Example:
Recall column
7) calculate
Practise Exercise:
What is gravitational force between the Earth & Moon?
Is it smaller or larger than between the Earth & Sun? By how much?
Examples of the effects of Gravity:
NASA’s Solar System Simulator
Orbits
Define Orbit:
Recall column
summary
The balance between tendency of an
object to move in a straight line
(inertia) and gravitational pull from a
massive body, can cause the object
to move in a continuous path around
that massive body. This path is called
an orbit.
Examples of the effects of Gravity:
How fast is the earth going around the sun?
Orbits
summary
Example:
Recall column
collect powers of ten
calculate
2.7 Newton’s Laws
Escape speed: the speed necessary
for a projectile to completely escape a
planet’s gravitational field
Escape speed:
needed for
projectile to escape
gravitational field.
We will do an example in
one of the following classes.
ESA’s Rosetta Mission to Comet:
Recall column
• Uses orbital assists.
summary
Rosetta’s Comet
summary
Recall column
• 67P/Churyumov-Gerasimenko from a
distance of 285 km. The image
resolution is 5.3 metres/pixel.
Examples of the Effects of Gravity: Tides
Tides at the Bay of Fundy
Effect of the Moon on Earth
Gravity (
)  Tidal Forces:
== Distortion of an object by the gravitational pull of another
object.
-- nearby (e.g. Earth & Moon)
OR
-- very massive (e.g. Earth & Sun)
Tidal Force Examples:
Asteroid Belt –tidal force of Jupiter prevented formation of planet
between Mars & Jupiter.
Tidal Force Examples: The tidal force of Jupiter (and Europa)
cause
- deformation of Io’s interior
-> heat
- volcanism
Split apart
Comet
Shoemaker-Levy
q
Saturn raises tides on its moons.
Gravity  orbits of material in rings.
Peculiar Galaxies
Tidal Tail