Amber Reed Notes

Chapter 4:

Making sense of the universe : understanding motion, energy and gravity:
Describing motion:examples form everyday life

How do we describe motion?

precise definitions to describe motion:
speed:rate at which object moves speed=distance/time (units of m/s)
example:speed of 10 m/s
velocity:speed and direction
example:10 m/s,due east
acceleration :any change in velocity; units of speed/time (m/s to the 2nd power)
All falling objects accelerate at the same rate(not friction of air resistance) 
Galileo showed that g is the same for all falling objects,regardless of their mass
Momentum =mass x velocity
a net force changes momentum,which generally means an acceleration(change in velocity)
the rotational momentum of spinning or orbiting object is known as angular momentum.

Thought question:

Is net force acting on each f the following? (answer yes or no)
a. a car coming to a stop---yes
b. a bus speeding up----yes
c. an elevator moving up at constant speed---no
d. a bicycle going around a curve--yes
e. a moon orbiting Jupiter ---yes

How is mass different from weight?

Mass--the amount of matter in an object
weight--the force that acts on an object
your are weightless in free 
Thought question:
on the moon:
a. your weight is the same;your mass is less
b. your weight is less;your mass is the same
c. your weight is more; your mass is the same
d.your weight is more;mass is less
Answer is B

Why are astronauts weightless in space? 

there is gravity in space
weightlessness is due to a constant state of free fall

Newton's Law of Motion

HOw did Newton change our view of the universe?
he realized the same physical laws that operate on Earth also operate in the heavens: ==> one universe
he discovered  laws of motion and gravity
much more:experiments with light;first reflecting telescope,calculus..


What are Newton's three laws of motion?
Newton's first law of motion :
an object moves at constant velocity unless a new force acts to change its speed or direction
Newton's second law of motion:
force=mass x acceleration
Newton's third law:
for every force, there is always an equal and opposite reaction force.

Conservation Laws in Astronomy

What keeps a planet rotating and orbiting the sun?

Conversation of momentum:
the total momentum of interacting objects cannot change unless an external force s acting on them
interacting objects exchange momentum through equal ad opposite forces
angular momentum =mass x velocity x radius
the angular momentum of an object cannot change unless an external twisting force (torque) is acting on it



where do objects get their energy?
energy makes matter move
energy is conserved but it can..
transfer form one object to another
change in form
Basic types of energy:
kinetic(motion)
radiative (light)
stored or potential
energy can change type but cannot be destroyed

Thermal energy: the collective kinetic energy of many particles(in rocks,in air,in water)
thermal energy is related to temperature but it not the same. temperature is the average kinetic energy of the many particles in substance.
thermal energy is a measure of the total kinetic energy of all the particles in a substance. it therefore depends on both temperature and density

Gravitational potential energy:
on earth it depends on
an object mass
the strength os gravity
the distance an object could potentially fall 
In space an object or gas cloud has more gravitational energy when its spread out than when it contacts
==>a contracting cloud converts gravitational potential energy to thermal energy
mass-energy mass itself is a form of potential energy 
a small amount of mass can release a great deal of energy
concentrated energy can spontaneously turn into particles
conversation of energy
energy can be neither created nor destroyed
they cannot change form or be exchanged through objects
the total energy content of the universe was determined in the Big Bang and remains the same today


The force of gravity:

what determines the strength of gravity?

the universal law of gravitation:
every mass attracts every other mass
attraction is directly proportional to the product of their masses
attraction is inversely proportional to the square of the distance between their centers

how does Newton's law of gravity extend Kepler's laws?

Kepler's first two laws apply to all orbiting object not just planets
ellipses are not the only orbital paths. orbits can be 
bound(ellipses)
unbound
parabola
hyperbola

Newton generalized Kepler's third law:
if a small object orbits a larger one and you measure the orbiting objects
orbital period and average orbital distance..then you can calculate the mass of the larger object

How do gravity and energy together allow us to understand orbits?

total orbital energy (gravitational+ kinetic) stays constant if there is no external force
orbits cannot change spontaneously
==>so what an make an object gain or lose orbital energy?
friction or atmospheric drag
a gravitational encounter

Escape velocity:
if an object gains enough orbital energy, it may escape (change form a bound to unbound orbit)
Escape velocity from Earth is approx 11km/s from sea level(about 40,000 km/hr)
how does gravity cause tides?
the moon's gravity pulls harder o near side of Earth than on far side
the difference i the moon's gravitational pull stretched Earth
size of tide depends on the phase of the moon

Tidal friction:
tidal friction gradually slows Earth's rotation(and makes the moon get farther form Earth)
moon once orbited faster (or slower);tidal friction caused it to "lock" in synchronous rotation

Amber Reed