Astronomy Picture of the Day

Review: Kepler's Laws

1. Planets travel aound the sun in elliptical orbits with the sun at one focus of the ellipse.

2. A line connecting the Sun and a planet sweeps out equal areas in equal times.

3. The square of a planet's orbital period is proportional to the cube of its semi-major axis. (P2 α a3)

Review: Newton's Laws of Motion

1. Every object continues in a state of rest or a state of uniform motion in a straight line unless acted on by a force. (Inertia)

2. F = m*a

3. For every action there is an equal and opposite reaction.

Newton's Law of Gravity: F =G m

1

m2

R2

Orbit of Earth around Sun

Gravity and Orbits

• Throwing an object fast enough will put the object into orbit! (Neglecting air resistance)

• Moon is continually “falling” towards the Earth in its orbit (Gravity vs. inertia)

Correction to Kepler’s Third Law

Earth and sun actually rotate about their common center of mass

Corresponds to a point inside sun

Used to detect extrasolar planets

Question

The force of gravity between two objects decreases rapidly with the:

A) product of the two masses.

B) elipticity of the orbit.

C) separation distance.

D) orbital velocity.

Question

The acceleration experienced by a falling object due to Earth's gravity _____ the mass of the object.

A) increases with

B) decreases with

C) is independent of

That one was complicated, because we need to remember:

Fg = G m m

E

R2

AND

a =

Fg

m

So, a =G m m

E

m R2 =

G mE

R2

Chapter 2: EM Waves

What is a wave?

What are the main properties of waves?

What two things do all waves transport?

Waves are a type of disturbance that can propagate or travel.Waves carry information and energy.

Properties of a wave

wavelength ()

crest

amplitude (A)

velocity (v)trough

is a distance, so its units are m, cm, or mm, etc.

Period (T): time between crest (or trough) passages

Frequency (f): rate of passage of crests (or troughs), f

v = f

1T(units: Hertz or cycles/sec)

Equilibrium position

Waves bend when they pass through material of different densities.

swimming pool

air

water

prism

airairglass

Refraction

All radiation (including visible light) travels as Electromagnetic waves.

That is, waves of electric and magnetic fields travelling together.

What are some examples of objects with magnetic fields:

What are some examples of objects with electric fields:

All radiation (including visible light) travels as Electromagnetic waves.

That is, waves of electric and magnetic fields travelling together.

What are some examples of objects with magnetic fields:

a bar magnetthe Earththe Sun

What are some examples of objects with electric fields:

} "charged" particles that make up atoms.

Anything with too many or to few electrons!

Electrical appliancesLightningProtonselectrons

Electric Force - opposites attract, likes repelOscillating charges radiateAll objects have temperatures greater than absolute zero - random thermal motionAll objects radiate!Why don’t we see the radiation coming from many ordinary objects?

Dog whistle analogy

Electromagnetic Radiation

(How we get information about the cosmos)

What are some examples of electromagnetic radiation and what are the different ways in which we might experience them or make use of them?

Electromagnetic Radiation

(How we get information about the cosmos)

What are some examples of electromagnetic radiation and what are the different ways in which we might experience them or make use of them?

Light (see)Infrared (heat)Ultraviolet (sunburn)Microwaves (cooking, communication)AM radio (communication)FM radio (communication)TV signals (communication)Cell phone signals(communication)X-rays (medical applications)