A100 Solar System
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Transcript of A100 Solar System
Today’s APOD
Review Chapter 1, Kepler’s LawsRead Chapter 2: Gravity & Motion2nd Homework due FridayKirkwood Obs. open tonight, 8:30-
10:30IN-CLASS QUIZ TODAY!!
The Sun Today
A100 Solar System
Extra Announcements
Caty can’t do office hours this week… (send email with questions)
Nathalie will lecture on Friday
Postpone due date for HW2 until Monday, Sept. 29 (web problems, use alternate URL)
astro.unl.edu/naap/pos/animations/kepler.html
Kepler’s 3 Laws of Planetary
Motion Planets move in elliptical orbits
with the Sun at one focus of the ellipse
A planet’s orbital speed varies in such a way that a line joining the Sun and the planet will sweep out an equal area each month
P2 = a3 (the square of the period of a planet orbiting the sun is equal to the cube of the semi-major axis of the planet’s orbit)
But WHY????????
The Problem of Astronomical
Motion
Galileo investigated this connection with experiments using projectiles and balls rolling down planks
He put science on a course to determine laws of motion and to develop the scientific method
Astronomers of antiquity did not connect gravity
and astronomical motion
inertia!
Demonstrated the ideas of inertia and forcesWithout friction…
a body at rest tends to remain at resta body in motion tends to remain in motion
Galileo experimented
with inclined planes
Isaac Newton, the Laws of Motion, and the Universal Law of
GravitationNewton
Born same year Galileo diedAttempts to understand motion of the
MoonLeads him to deduce the law of gravity (as we
still use it today!)Requires him to invent new mathematicsLeads him to deduce the general laws of
motion
A body continues in a state of rest
or uniform motion in a straight line unless made to
change that state by forces acting
on it
Galileo’s ideas of inertia became Newton’s First Law of Motion:
Newton’s First Law
Important ideasWhat is a force?
A push or a pull
The sum of all the forces on an object is the net force
If the forces all balance, the net force is zero, and the object’s motion will not change
If the speed or
direction of motion of an object changes, then a nonzero net force must be present
Astronomical Motion
Planets move along curved (elliptical) paths, or orbits
Speed and direction are changing
Must there be a net force on the planets?
Yes!
Gravity is that force
Gravity gives the Universe its structurea universal force that causes all objects to
pull on all other objects everywhereholds the Earth in orbit around the Sun,
the Sun in orbit around the Milky Way, and the Milky Way in its path within the Local Group
Everything attracts everything else!!
Newton’s Law of Gravity
Orbital Motion
and GravityNewton
Explained the Moon’s motion with force that pulls the Moon from a straight, inertial trajectory
Showed that the force must decrease with distance
defined the properties of gravity
wrote the equations of motion with gravity
The Moon moves “parallel” to the Earth’s surface at such a speed that its gravitational deflection toward the surface is offset by the surface’s curvature away from the projectile
Orbital Motion Using Newton’s First Law
At a sufficiently high speed, the cannonball travels so far that the ground curves out from under it.
The cannonball literally misses the ground!
Newton’s 2nd Law: Acceleration
Acceleration An object increasing or decreasing in speed along a
straight line is accelerating An object changing direction, even with constant speed,
is accelerating Acceleration is produced by a force Acceleration and force are proportional (double the force,
double the acceleration
Newton’s Second Law: Mass
Mass is the amount of matter an object contains
Technically, mass is a measure of an object’s inertia
Mass is generally measured in kilograms
Mass should not be confused with weight, which is a force related to gravity – weight may change from place to place, but mass does not
Newton’s Second Law of Motion
The amount of acceleration (a) that an object undergoes is proportional to the force applied (F) and inversely proportional to the mass (m) of the objectThis equation applies for any force,
gravitational or otherwise
F = maF = ma
F = ma
Newton’s Third Law of MotionWhen two objects
interact, they create equal and opposite forces on each other
This is true for any two objects, including the Sun and the Earth!
To explain the motion of the planets, Newton developed three ideas:
1. The laws of motion2. The theory of universal gravitation3. Calculus, a new branch of mathematics
Newton solved the premier scientific problem of his time --- to explain the motion of the planets.
“If I have been able to see farther than others it is because I stood on the shoulders of giants.”
--- Newton’s letter to Robert Hooke,probably referring to Galileo and Kepler
221
r
mGmF
F = ma
ASSIGNMENTSthis week
Review Chapter 1, Kepler’s LawsRead Chapter 2: Gravity & Motion2nd Homework due FridayKirkwood Obs. open tonight, 8:30-
10:30