Download Newton`s Laws of Motion

Newton’s Law
Lecture 8
By reading this chapter, you will learn
4-5 How Galileo’s pioneering observations with a telescope
supported a Sun-centered model
4-6 The ideas behind Newton’s laws, which govern the motion of
all physical objects, including the planets
4-7 Why planets stay in their orbits and don’t fall into the Sun
4-8 What causes ocean tides on Earth
Gelileo Galilei
 Galileo was one of the first people to use a
telescope to observe the heavens.
 He discovered craters on the Moon, sunspots
on the Sun, the phases of Venus, and four
moons orbiting Jupiter.
 His observations strongly suggested that the
Earth orbits the Sun, not vice versa.
Phase of the Venus
Phase of Venus  Clear Proof of incorrect Geocentric
model
Geocentric model
Heliocentric model
Gelileo Galilei
Observation Log by Galilei
 Jupiter and its 4 largest moons
Isaac Newton
Isaac Newton (1642–1727)
 Using mathematical techniques that he
devised, Isaac Newton formulated the law of
universal gravitation and demonstrated that
the planets orbit the Sun according to simple
mechanical rules.
 1st Law : An object remains at rest, or moves
in a straight line at a constant speed, unless
acted upon by a net outside foce
 2nd Law : A net outside force will result in an
accelration. F=ma
 3rd Law : Whenever one object exerts a force
on a second object, the second object exerts
an equal and opposite force on the first
object.
Newton’s Law of Gravitation
Two objects attach each other
with a force that is directly
proportional to the mass of
each object and inversely
proportional to the square of
the distance between them.
Orbit Analogy
Gravity and Orbits
If a ball is dropped from a great height
above the Earth’s surface, it
falls straight down (A). If the ball is
thrown with some horizontal speed, it
follows a curved path before hitting
the ground (B, C). If thrown with just
the right speed (E), the ball goes into
circular orbit; the ball’s path curves
but it never gets any closer to the
Earth’s surface. If the ball is thrown
with a speed that is slightly less (D) or
slightly more (F) than the speed for
a circular orbit, the ball’s orbit is an
ellipse.
Different kinds of Orbits
A conic section is any one of a family of curves obtained by slicing a cone with a plane. The orbit of one object
about another can be any one of these curves: a circle, an ellipse, a parabola, or a hyperbola.
Orbit cannot be changed in the 2-body system.
Newton’s Form of Kepler’s 3rd Law
For planets in the solar system,
m1 + m2  mass of the Sun.
So, the relation is simplified as
2
4

P2 
a3
GmSun
When we use years for P, and AU for a,
the constant become unity.
However, if we apply this formula to an
orbit around other object, we need to
pay attention to the constant.
2


4

2
3
P  
a
G(m1  m2 ) 
Origin of Tides
(a) Imagine three identical billiard balls
placed some distance from a planet
and released.
(b) The closer a ball is to the planet,
the more gravitational force the
planet exerts on it. Thus, a short
time after the balls are released,
the blue 2-ball has moved farther
toward the planet than the yellow
1-ball, and the red 3-ball has
moved farther still.
(c) From the perspective of the 2-ball
in the center, it appears that forces
have pushed the 1-ball away from
the planet and pulled the 3-ball
toward the planet.
These forces are called tidal forces.
(a) The Moon exerts different
gravitational pulls at different
locations on the Earth.
(b) At any location, the tidal force
equals the Moon’s gravitational pull
at that location minus the
gravitational pull of the Moon at
the center of the Earth. These tidal
forces tend to deform the Earth
into a non-spherical shape.
Spring tide
So, strong tide happens at New and Full Moons.
Neap Tides
 Weak tides at quarter moons
Merge of Galaxy
Key Ideas
Evidence for the Heliocentric Model: The invention of the
telescope led Galileo to new discoveries that supported a
heliocentric model. These included his observations of the
phases of Venus and of the motions of four moons around
Jupiter.
Newton’s Laws of Motion: Isaac Newton developed three
principles, called the laws of motion, that apply to the motions
of objects on Earth as well as in space.
Key Ideas
(1) the tendency of an object to maintain a constant velocity,
(2) the relationship between the net outside force on an object
and the object’s acceleration,
and (3) the principle of action and reaction.
These laws and Newton’s law of universal gravitation can be
used to deduce Kepler’s laws. They lead to extremely accurate
descriptions of planetary motions.
The mass of an object is a measure of the amount of matter in
the object. Its weight is a measure of the force with which the
gravity of some other object pulls on it.
Key Ideas
In general, the path of one object about another, such as that
of a planet or comet about the Sun, is one of the curves called
conic sections: circle, ellipse, parabola, or hyperbola.
Tidal Forces: Tidal forces are caused by differences in the
gravitational pull that one object exerts on different parts of a
second object.
The tidal forces of the Moon and Sun produce tides in the
Earth’s oceans.
The tidal forces of the Earth have locked the Moon into
synchronous rotation.
In summary…
Important Concepts
Important Terms
 Phase of Venus
 Newton’s Law of Motion
 Newton’s Law of Gravitation
 Tides
 Tidal force
Chapter/sections covered in this lecture : sections 4-5 through 4-8