Chapter 13 Review

Gravitational Fields:
Gravity is a field force (a force that will act between two ojects not in contact).
The strength of the gravitaitonal field (at any point) is the acceleration caused by the object at that point.
The earths gravitational field can be represented by imaginary field lines.
The strength of the Earth's gravitational field is shown through the equation g=F/M(the force per unit mass exerted by the earth).
At the surface of the Earth, the gravitational field strength is the familiar gravitational acceleration constant. G=9.8 (m\s^2)
To find the gravitational field strength between two objects, you would use the equation: g=G(m/r^2) where G=6.673*10^ -11((n*m^2)/r^2))
external image sink.png
http://www.lightandmatter.com/html_books/4em/ch05/figs/sink.png

Gravitational Field Inside a Planet:
The gravitational field of the earth exists inside the earth as well as outside.
The equation above for the gravitional field strength assumes that all of the mass of the planet is on one side of you, if you burrow down into the planet then some of the planet's mass will be on the other side of you partially canceling the force from the rest of the mass and reduces the effect.
At the center of a planet, the gravitational attraction from one side would be exactly balance by the attraction from the other side and you would feel no net gravitation.

Weight and Weightlessness
The force of gravity like, any force,causes acceleration. Objects under the influence of gravity are pulled toward each other and accelerate.
Because of this, we are almost always in contact with the earth and gravity is keeping us against the earth. This pressing against the earth is your weight
If you are in an enclosure that is being accelerated, you would feel a change in your apparent weight.
For example when your in elevator moving upward or downward, the speed is changing. if the elevator moves upward you will feel heavier, if it moves downward you feel lighter.
Suppose the cables break in an elevator, you would feel weightless relative to the elevator because you and the elevator are both falling at the same rate.
Weightlessness=is not the absence of gravity but rather the absence of a support force.
Weight=the force you exert against a supporting floor.
external image elevator.gif
http://www.thebigview.com/spacetime/elevator.gif

Ocean Tides:
The moon has different gravitational pulls on both sides of the planet Earth.
On any solid object, this would not be noticeable,however,on liquids such as the water of the ocean it is pulled closer to the side closet to the moon causing tidal bulges
Newton showed that ocean tides are caused by differences in the gravitational pull of the moon on opposite sides of the earth.
The moons attraction is stronger on the earth's oceans closer to the moon and weaker on the oceans futher from the moon.
This is becaused the gravitational force is weaker with increased distance.

external image wave1.jpg
http://www.oceantidesgroup.com/graphics/wave1.jpg

Tides and the Earth and Atmosphere:

Since the earth is not completely rigid, the earth's surface is affected by the tidal forces.
The affect of the tidal forces is not noticable because everything on the earths surface moves with it.
Twice each day the solid surface of the earth rises and falls.Its been found to move less than 25cm.
Atmospheric Tides also occur on earth but they are relatively small as well.

Black Hole:

There are two main processes occuring in stars like the sun. One process is gravitation which tends to crunch all solar material toward the center of the earth.
The other process is thermonuclear fusion tends to blow solar material outward. When the two processes balance each other out, the result is the sun at a given size.
The gravitational force pulling the gas toward the center and the nuclear fusion in the core pushes the gas back out affects the stars gasy surface. A black hole is formed when the nuclear inside of the star weakens and the gravitational force pulling the gas inward collapses the star. The amount of gas that makes a black hole occur is proportional to the mass that was in the star previous to collapsing. Anything that was previously orbiting the star will now orbit into the black hole. Since the black hole is much smaller than the star was the orbiting mass can get much closer to the center of the black hole. The pull from the center of the black hole is so great that even light can't escape the pull of the hole.external image blackhole_300_300.jpg
http://www.sr.bham.ac.uk/xmm/images/xray/blackhole_300_300.jpg

Important Vocabulary
Field Force- a force that will act between two objects.
gravitational field- A kind of field force that surrounds massive objects.
Weight -the force you exert against a supporting floor.
Weightlessness- is not the absence of gravity but rather the absense of a supporting force.
black hole-a mass that has collapsed to so great a density that its enormous local gravitational field prevents light from escaping.