• The picture showsa) 2 galaxies that are orbiting each other and which are

distorted by tidal forces.b) One peculiar galaxy with 2 bright knots in its centre and

2 tails extending in opposite directions.c) 2 clouds in our Milky Way which have radiation jets and

are forming clusters of stars.

Lecture 9 Phys 1810 • Last Class– Kepler’s Laws of Planetary

motion 2.5, 2.8• This class

– Weighing the sun, Box 2-3– Box 2-2, including escape velocity – introduction to General Relativity

p. 595-598– Radiation 3.1 to 3.4

• Coming Up– Radiation 3.4, 4.1,4.2, 4.3, 18.4 &

3.5, etc.

Test on Friday in class.

• Handout about the test on the website.• Tests are based on material in class; use textbook index to select the relevant topics to read in the book. NOT just the pages or sections that I write down. NOT the whole chapter.• Try multiple choice questions on website.

• Office hour at 1pm Monday. Come and ask me questions about the test or practice math.

velocity = distance / time. For each area in the above case:• time is the same for each area.• distance == distance travelled on the elliptical path == length of the red arc.So the planet’s velocity is proportional to the distance travelled.

The planet’s largest velocity is associated with which area (A, B, or C)? At the largest velocity is the planet at its closest or furthest approach from the sun?

Keplerian Orbit

When most of the mass is in the centre of a system (e.g. our solar system) then the velocity of the orbiting objects decrease with radius from the centre.

What does the graph look like?

For systems where most of the mass is in the centre. Does it work for galaxies? (Learn later.)

• For an orbit: Force of Inertia = Force of Gravity

Weighing the Sun

• General Relativity: Gravitational Lensing

Material on test is up to here.

At the scale of atoms, we use the description of light as particles (photons).At the macroscopic, everyday scale, we use a wave description of light.

Will start here next class.