The Veil Nebula: a large, expanding cloud resulting from the explosion of a massive star (a supernova remnant). Life owes its existence to elements created in these massive stars and returned to space in these explosions to form new stars and planets.

Light from the original supernova explosion likely reached Earth over 5,000 years ago. Also known as the Cygnus Loop, the Veil Nebula now spans nearly 3 degrees or about 6 times the diameter of the full Moon.

Homework #2 has been posted

Questions 1-16 are multiple choice. Answers to these questions are due on Oncourse by 5:00 pm, Wednesday, September 22.

Questions 17 – 20 require short written answers. These are due by the beginning of class on Thursday, September 23.

The Activities page of the class website now has a number of out-of-class activities posted.

These activities count towards the activity points earned towards the final grade.

Geocentric Heliocentric

Two models of the Universe

A hallmark of science is that

theories are testable

Which model more accurately depicts nature?

Both make predictions for the apparent motions of the Sun, planets and stars.

The Heliocentric model, with modifications incorporating Kepler’s Laws, gives more accurate predictions

But, the Geocentric model might be made more accurate through appropriate modifications.

Need additional predictions that clearly differentiate between the two models. (need tests)

Contemporary with Kepler was

Galileo Galilei (1564-1642),the “founder of experimental science”

● First person known to point a telescope at the sky

● He wanted to connect the physics understood on earth with objects in the heaven

His work got him in trouble with the Church and led to his house

arrest for many years.

Galileo saw craters and shadows cast by the

mountains on the Moon (Moon had a

landscape; it was a “place”, not a perfect

heavenly body)

(Some of) Galileo’s Observations

Galileo saw craters and shadows cast by the

mountains on the Moon (Moon had a

landscape; it was a “place”, not a perfect

heavenly body)

Sunspots (sun not “perfect”)

(Some of) Galileo’s Observations

Galileo saw craters and shadows cast by the

mountains on the Moon (Moon had a

landscape; it was a “place”, not a perfect

heavenly body)

Sunspots (sun not “perfect”)

Rotation of sun

(Some of) Galileo’s Observations

Galileo saw craters and shadows cast by the

mountains on the Moon (Moon had a

landscape; it was a “place”, not a perfect

heavenly body)

Sunspots (sun not “perfect”)

Rotation of sun

Moons of Jupiter (Heavenly bodies existed

which did not orbit the earth)

(Some of) Galileo’s Observations

Galileo saw craters and shadows cast by the

mountains on the Moon (Moon had a

landscape; it was a “place”, not a perfect

heavenly body)

Sunspots (sun not “perfect”)

Rotation of sun

Moons of Jupiter (Heavenly bodies existed

which did not orbit the earth)

Phases of Venus: the two models of the

Universe made two very different predictions.

(Some of) Galileo’s Observations

Phases of Venus

Galileo’s observation of the phases of Venus was the final

evidence that buried the geocentric model. Geocentric Heliocentric

No gibbous or full phases! All phases are seen!

Galileo observed all phases!

With Galileo’s observations, the revolution begun by Copernicus was nearly complete… The structure of the universe had been totally changed.

The motions of the planets were understood, at least from a geometrical perspective. Earth was no longer a “special” place in the universe.

The crowning achievement was yet to come - discovering the laws of nature and that naturally led to the newly determined structure.

Sir Isaac Newton (1642-1727)

Invented calculus

Invented the reflecting telescope

Connected gravity and planetary forces

Universal Law of Gravitation Between every two objects there is an attractive force,

the magnitude of which is directly proportional to the mass of each object and inversely proportional to the square of the distance between the centers of the objects.

This force is always attractive and it applies to ALL objects possessing mass!

● Extending Kepler’s Law #1, Newton found that ellipses were not the only orbital paths.

● All orbits are “conic sections”– ellipse (bound)– parabola (unbound)– hyperbola (unbound)

Orbital Paths from Law of Gravitation

● Extending Kepler’s Law #1, Newton found that ellipses were not the only orbital paths.

● All orbits are “conic sections”– ellipse (bound)– parabola (unbound)– hyperbola (unbound)

● Orbital motion takes place around the center of mass

Orbital Paths from Law of Gravitation

The Center of Mass

In Kepler's Laws, the Sun is fixed at a point in space (a focus of an ellipse) and the planet revolves around it. Why is the Sun privileged?

Kepler had mystical ideas about the Sun, endowing it with almost god-like qualities that justified its special place.

Newton demonstrated that the the Sun does not occupy a privileged postion and in the process he modified Kepler's 3rd Law.

The center of mass is familiar to anyone who has ever played on a see-saw. The fulcrum point at which the see-saw will exactly balance two people sitting on either end is the center of mass for the two persons.

m1d1 = m2d2

Newton realized that in the planet-Sun system the planet does not orbit around a stationary Sun (a planet exerts as much gravitational force on the Sun as the Sun does on a planet).

Instead, Newton proposed that both the planet and the Sun orbited around the common center of mass for the planet-Sun system.

This led Newton to modify Kepler's 3rd Law.

Recall Kepler’s 3rd law: P2 / a3 = constant

Fg = Gm1m2/d2

P2 = 42 a3 / G (m1 + m2)

G is known as the universal gravitational constant.

Newton’s Version of Kepler’s Third Law

If you can measure the orbital period of two objects (P) and the distance between them (a), then you can calculate the sum of the masses of both objects (m1 + m2).

A Universe of Matter and Energy

What is matter?

What is energy?

Matter – material such as rocks, water, air; “stuff” composed of atoms

Energy – makes or has the potential to make matter move!

The history of the universe, including biological organisms, is based upon the interplay between

matter and energy.

Three Basic Types of Energy

• kinetic– energy of motion

• potential– stored energy; e.g., chemical,

gravitational, electrical, etc.• radiative

– energy transported by light (electromagetic radiation)

Conservation of Energy

Fundamental law of nature

Energy can be neither created nor destroyed

It can change form or be exchanged between objects.

The total energy content of the Universe was determined in the Big Bang and remains the same today.

K.E.

P.E. R.E.

Kinetic Energy (K.E.): energy of motion

K.E. = 1/2 mv2

(m is mass, v is velocity)

On the microscopic level:

temperature

is a measure of the

average kinetic energy

of particles within a substance

Temperature Scales

Temperature vs. Heat● Temperature is the average

kinetic energy.● Heat (thermal energy) is the

total kinetic energy.

lower T higher T

same T

less heat more heat

Sound waves are a form of kinetic energy on a microscopic level

(organized vibration of molecules)

Applying what we’ve learned -

pizza vs. soup

caution in the kitchen

Potential Energy:

Energy that is “stored” within an object and that has the potential of being released in a different form

Gravitational Potential Energy

● gravitational potential energy is the energy which an object stores due to its ability to fall

● It depends on:– the object’s mass (m)– the strength of gravity (g)– the distance which it can fall (d)

m

d

g

● P.E. = mgd

● gravitational potential energyg● P.E. = mgd

Mass-Energy Potential Energy

● mass-energy: energy is stored in matter itself

● this mass-energy is what would be released if an amount of mass, m, were converted into energy

E = mcE = mc22

[ c = 3 x 108 m/s is the speed of light]

Chemical Potential Energy● Chemical potential energy:

energy stored chemical bounds

There are many additional examples of potential energy. e.g.,

stretched springs, …

Energy, while conserved, can be

transformed from one type of energy to

another

Potential Kinetic

Potential Kinetic

Kinetic Potential

Orbits & Energy

MaximumKinetic Energy

MaximumPotentialEnergy

Downhill

Uphill