Download CHAPTER 13: GRAVITATIONAL INTERACTIONS 13.1

CHAPTER 13: GRAVITATIONAL INTERACTIONS
13.1 GRAVITATIONAL FIELDS
A magnetic field is a force field, because magnetic materials in it experience a force
The gravitational field around Earth is similar
A mass in the field region experiences a gravitational force
The field is strongest at the _________________________________________________
_______________________________________________________________________
g is the acceleration due to gravity
G is the universal gravitational constant
g is the gravitational field vector
They are all different quantities that are related
G and g have the same magnitude and their units are equivalent: N/kg = m/s2
g = GM/R2
g = 9.8 N/kg is valid only near the planet’s surface
as R increases, g decreases
The strength of Earth’s gravitational field is the force per unit mass exerted by Earth on
any object
g=F
m
g = 9.8 N/kg = 9.8 m/s2
g weakens with increasing distance from Earth
QUESTIONS
If Earth had the same size but twice the mass, what would be the acceleration of freely
falling objects at its surface?
__________________________________________________________________
If Earth has the same mass but half the size, what would be the acceleration of freely
falling objects at its surface?
__________________________________________________________________
If Earth had twice the mass, and half the size, what would be the acceleration of freely
falling objects at its surface?
__________________________________________________________________
What evidence would you look for to tell whether or not you were in a gravitational
field?
__________________________________________________________________
CHAPTER 13: GRAVITATIONAL INTERACTIONS
WEIGHT & WEIGHTLESSNESS
Weight: _________________________________________
We are as heavy as we feel
Weightlessness: __________________________________________________________
The queasy feeling you experience when in a car that speeds over the top of a hill
FOR EXAMPLE: A SCALE
Stand on a bathroom scale on the floor
The gravitational force between you & Earth ___________________________________
_______________________________________________________________________
By Newton’s 3rd law, the floor and scale in turn push upward on you
NOW…WEIGHT IN AN ELEVATOR
UP, UP AND AWAY
If the elevator accelerates upward, the bathroom scale and floor would push harder
against your feet
The scale would show _____________________________________________________
GROUND FLOOR PLEASE
If the elevator accelerated downward, the scale would show _______________________
_________________________________
The support force of the floor would now be less
ELEVATOR CABLE BROKE
Elevator falls freely, the ____________________________________________________
According to the scale, you would be weightless and feel weightless
Your insides would no longer be supported by your legs and hips
QUESTION
Why would you feel weightless in an elevator with a broken cable?
__________________________________________________________________
13.4 OCEAN TIDES
Think of someone pulling on your coat
If some one pulled only on the sleeve, the coat could tear
But if every part of your coat were pulled equally, it and you would accelerate, but the
coat wouldn’t tear
It tears when one part is pulled harder than another because of a difference in forces
acting on the coat
CHAPTER 13: GRAVITATIONAL INTERACTIONS
In a similar way, the spherical Earth is “torn” into an elliptical shape by differences
________________________________________________________________________
Ocean tides are caused by differences in the ____________________________________
________________________________________________________________________
The moon’s attraction is stronger on Earth’s oceans closer to the moon, and weaker on
the oceans farther from the moon
__________________________________________________________________
WHY ARE THERE TWO TIDES A DAY?
There would only be one tide per day, if the Earth were “nailed down” in one place and
held stationary, except for a daily rotation
But the Earth and moon are in _______________________________________________
The ocean nearest the moon is pulled upward toward the moon, while the main body of
Earth is pulled toward the moon
away from the ocean on the far side
Earth is closer to the moon than the far-side ocean is, so Earth’s waters get slightly
_______________________________________________________________________
Earth makes one complete turn per day beneath these ocean bulges creating ___________
________________________________________________________________________
The part of Earth that passes beneath one of the bulges ___________________________
The tide cycle is every 24 hours and 50 minutes
The ____________________________________________,
but not as much as the moon.
The tilt of Earth’s axis is another factor
QUESTIONS
Which pulls harder on the oceans of Earth, the sun or the moon?
__________________________________________________________________
Which is most effective in raising tides?
__________________________________________________________________
The difference in pulls decreases as the cube of the distance between the centers of the
bodies
__________________________________________________________________
__________________________________________________________________
Only relatively close distances result in appreciable tides, and so the nearby moon “outtides” the enormously more massive, but far away sun
CHAPTER 13: GRAVITATIONAL INTERACTIONS
When the sun, Earth, and the moon are lined up, the tides due to the sun and the moon
coincide
__________________________________________________________________
These are called ____________________________________
They have nothing to do with spring
If the alignment is perfect, _________________________________________
ECLIPSE
A ____________________ is produced when Earth is directly between the sun and moon
A _________________ is produced when the moon is directly between the sun and Earth
PHASES OF THE MOON
When the Earth is between the sun and the moon –__________________________
When the moon is between the sun and Earth – ___________________________
Spring tides occur during a new moon and full moon
During half-moons, the tides due to the sun and the moon partly cancel each other
__________________________________________________________________
__________________________________________________________________
These are called __________________________
QUESTION
At the time of extra high tides, will extra low tides follow in the same day?
__________________________________________________________________
__________________________________________________________________
13.5 TIDES IN EARTH AND THE ATMOSPHERE
There are tides within Earth, which is mostly molten lava
There is a greater probability of earthquakes and volcanoes when there is ____________
________________________________________________________________________
This is when Earth experiences spring tides-greater stresses on Earth’s crust
IF THE MOON WERE CLOSER
___________________________________________________________ and so the tidal
forces on the moon’s crust would be greater
If the moon were too close, Earth’s tidal forces would tear the moon into a billion pieces,
_______________________________________________ similar to those around Saturn
CHAPTER 13: GRAVITATIONAL INTERACTIONS
13.6 BLACK HOLES
Occurs with stars that are at least two to three times more massive than our sun
Once the flame of thermonuclear fusion is extinguished, __________________________
_______________________________________________________________________
Gravitation is so enormous that nothing can get back out-_________________________
A collapsed star represents condensed mass and therefore condensed gravity.
_________________________________________________
t
hat collapsed to form it
Hence the gravitational field of the star and the black hole are the ___________________
_______________________________________________________________ of the star
QUESTION
Consider a satellite companion to a star that collapses to become a black hole. How will
the orbit of the companion satellite be affected by the star’s transformation to a black
hole?
__________________________________________________________________
SUMMARY
Earth can be thought of as being _____________________________________________
t
hat interacts with objects and causes them to experience
onal
gravitati
forces.
The gravitational field, g, is equal to the acceleration of a freely falling object.
Objects in orbit around Earth ________________________________________________
even though they may appear to be weightless.
___________________ (and even tides within the solid Earth and within the atmosphere)
are caused by differences in the gravitational pull of the moon (and sun) on opposite sides
of Earth.
_______________________________________________________________________,
it collapses under gravitational forces. Sufficiently massive stars collapse to form black
holes.
CHAPTER 13: GRAVITATIONAL INTERACTIONS