AST 201 Midterm
* The definition of "Universe"
The universe is the totality of all space, time, energy, and matter.
* The age and size of the Universe
13.7 billion year old; the universe has no outside
* The observable Universe
* Light travel time and the light year
1 light year almost 10 trillion km
A light year is the distance light travels in one year.
* The contents of the Universe
Planet, star, solar system, galaxy, super cluster,
* Our location in the Universe
Earth, Solar System, Milky Way Galaxy, Local Group, Virgo Super cluster
Summary from slides
•The Universe is the totality of everything
•The Universe is 13.7 billion years old, but we haven’t talked about why or how we know
•We measure astronomical distances in light years
•A light year is almost ten trillion km
•Looking far away means looking back in time
Science is falsifiable. Part2
Chapter 4 Understanding Motion, Energy, and Gravity
* Gravity and motion
Motion: 1 speed: rate at which object moves speed = distance/time
2 velocity: speed and direction
3 acceleration: any change in velocity units of speed/time
Gravity: all falling objects accelerate at same rate of g = 10m/s(square)
speed increases 10m/s with each second of falling
regardless of their mass
* Forces and Newton's Laws
1 law: an object moves at constant velocity unless a net force acts to change its speed or
2 law: force = mass x acceleration
3 law: for every force, there is always an equal and opposite reaction force
Newton changed our view of the universe by discovered laws of motion and gravitation;
realized these same laws of physics were identical in the universe and on Earth.
* Mass vs. weight
Mass: the amount of matter in an object never changes!!
Weight: the force that acts upon an object
objects are weightless in free-fall(nothing to stop acceleration due to gravity)
Falling without any resistance to slow the object down
free-fall makes one weightless
Question: why are astronauts weightless in space?
there is gravity in space weightlessness is due to a constant state of free-fall
Momentum = velocity * mass
Only way to change an object’s momentum is to apply a force on it.
Generally acceleration could change in velocity and change momentum.
Angular momentum = velocity * mass * radius
for a particle moving in a circle
Question ask for no force find the option with no acceleration no change in
direction or velocity
Conservation of momentum 动能守恒
The total momentum of interacting objects cannot change unless an external force is
acting on them.
Interacting objects exchange momentum through equal and opposite forces
Conservation of angular momentum
Angular momentum = mass x velocity x radius
The angular momentum of an object cannot change unless an external twisting force is
acting on it: therefore angular momentum stays constant, i.e. increase of radius
decrease of velocity
Earth experiences no twisting force as it orbits the Sun, so its rotation and orbit will
continue indefinitely (not exactly true: tidal forces of Sun and Moon exert small changes
* Conservation of energy and types of energy
Energy can be neither created nor destroyed, but 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. Types: kinetic (motion) radiative (light) potential (stored)
Thermal energy: the collective kinetic energy of many particles; is related to temperature
temperature is the average kinetic energy of the many particles in a substance
Gravitational potential energy:
on earth, depends on m/g/distance object could potentially fall
in space, an object or gas could has more gravitational energy when it is spread out than
when it contracts a contracting cloud converts gravitational potential energy to thermal
* Mass-energy equivalence
E = mc 2
C-speed of light
Summery from slides:
• Why do objects move at constant velocity if no force acts on them?
– Conservation of momentum
• What keeps a planet rotating and orbiting the Sun?
– Conservation of angular momentum
• Where do objects get their energy?
– Conservation of energy: energy cannot be created or destroyed but only transformed
from one type to another.
– Energy comes in three basic types: kinetic, potential, radiative.
* Kepler's Laws, especially the third law
1 orbits are ellipses
2 the area swept out in 30-day periods are all equal near perihelion bigger v; near
aphelion smaller v
(Result of/ a statement of conservation of angular momentum) 3 P 2 =A3
P- period (year); A- average distance from Sun(AU)
Kepler's Laws For planets in the Solar System:
1. Planets move in elliptical orbits, the Sun at one focus.
2. As a planet moves around its orbit, it sweeps out equal areas in equal times.
3. More distant planets orbit the Sun at slower average speeds, precisely obeying:
(Period in years)2 = (average distance from Sun in AU)3
Universal Law of Gravitation:
1 every mass attract every other mass
2 attraction is directly proportional to the product of their masses
3 attraction is inversely proportional to the square of the distance between their centers.
* Newton's law of gravity
his laws of gravity and motion showed that the relationship between the orbital period
and average orbital distance of a system tells us the total mass of the system.
Newton’s version of Kepler’s Third Law for 2 bodies of comparable mas
A = G (M +1 )P2/(42 2)
Summery from slides:
• What determines the strength of gravity?
– Directly proportional to the product of the masses (M x m)
– Inversely proportional to the square of the separation
• How does Newton’s law of gravity allow us to extend Kepler’s laws?
– Applies to other objects, not just planets.
– Includes unbound orbit shapes: parabola, hyperbola (e.g. Orbits of comets)
– Can be used to measure mass of orbiting systems.
Chapter 5 Light and Matters * The nature of light
Particles of light are called photons.
The energy of a photon depends on its frequency.
* Colours, wavelengths, and frequenc