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Chapter 4

chapter 4 astronomy


Department
Astronomy & Astrophysics
Course Code
AST201H1
Professor
Stefan Mochnacki
Chapter
4

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4.1 describing motion
speed: how far it will go in a certain amount of time
velocity: describes both its speed and direction
acceleration: changes in velocity whether speed or direction
acceleration works both speeding up or slowing down
one of most important acceleration: gravity
galileo: throw weights from leaning tower of pisa learned that gravity accelerates
all objects in same amount regardless of mass
the difference in acceleration is caused by air resistance
acceleration of falling object acceleration of gravity
on earth 9.8 meters every second (around 10m)
momentum = mass X velocity
only way to change an objects momentum is to apply force to it
when net force = 0, you can be moving in a circle or curved road
angular momentum is turning momentum
when earth rotations, rotational angular momentum
when earth orbits sun, orbital angular momentum
torque -type of force that can change an objects angular momentum
torque depends on how much force applied but also where
mass amount of matter in your body (same everywhere)
weight force that a scale measures when you stand on it (depends on mass and
forces (gravity) acting on it)
elevator: when stationary or moving at constant velocity, your weight is the same
when accelerating upwards, floor exerts more force so your weight measured is
heavier
when accelerating downwards, floor exerts weaker force so your weight is much
lighter
if elevator breaks (free fall) with no resistance, you would have zero weight (floor
exerts no force on you) weightlessness
weightlessness in space not because theres no gravity but they are at free fall all
the time
orbital altitude of 28,000 km/hr if you jump that fast you would be orbiting around
earth instead of falling
4.2 newtons law of motion
newton destroyed Aristotles physics and brought earth and the heavens into one
universe
his insights became the birth of astrophysics
newton published laws of motion and gravity which govern everything from daily
movements to movements of stars throughout the universe
an object moves at constant velocity if there is no net force acting upon it
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essentially restates Galileos discovery that objects remain in motion until a force
acts to stop it
objects at rest stay at rest and objects in motion remain in motion with no change in
either their speed or direction
force = mass X acceleration
force = rate of change in momentum
standard unit of force is kg-m per second squared, aka newton
weight = mass X acceleration of gravity
for any force, there is always an equal and opposite reaction force
standing on the ground weight exerts downward force and floor is exerting
upward force (hence why you are not falling)
objects always attract each other through gravity
you always exerts gravitational force on earth & earth reacts
4.3 Conservation laws in astronomy
conservation of momentum: the total momentum of interacting objects cannot
change as long as no external force is acting on them
total momentum is conserved
object can only lose or gain momentum if some other objects momentum changes by
a precisely opposite amount
eg. Pool, the momentum of one ball is transferred to the other ball but the net
momentum is the same
conservation of angular momentum: as long as there is no external torque, the total
angular momentum of a set of interacting objects cannot change
an individual object can change its angular momentum only by transferring some
angular momentum to or from another object
angular momentum = m X v X r
orbital angular momentum: m = mass of earth, v = speed (velocity) around the orbit,
r = radius of orbit
because nothing is taking or giving to the earths angular momentum, rotation stays
the same
tell us that: earth needs no fuel to keep orbiting
also: earths angular momentum at any point in its orbit depends on product of
speed and orbital radius (distance from sun), earth is faster near the sun and slower
away from sun
rotational angular momentum: same thing, it keeps rotating at the same rate until
some outside force changes it
in fact, earth is gradually transferring some of its rotational angular momentum to
the moon, and result: its own rotation is slowing down
skaters: arms out slower (bigger radius), arms in faster (smaller radius)
conservation of energy: energy can neither be destroyed or created
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