examples: hydrogen has 1 proton and 1 electron. A hydrogen isotope has 1 proton, 1 neutron and 1 electron. Helium has 2 protons, 2 neutrons, and 2 electrons. Lithium normally has 3 electrons, but if you take one away you get a cation with one excess positive charge.
Newton's Law of Gravitation: the force of gravity F=(GMm)/r2.
From this we can see that gravity depends heavily on the the product of the two masses. The sun is over 99% of the mass in our solar system, so it's the largest factor in the mass part of the equation. Also, gravity drops off as 1/r2,which means if you double your distance from the sun, you cut the force of gravity between you and the sun by 1/4.
Q: What is the effect of going farther away, but also getting more massive? Would those two effects cancel themselves out, leaving F the same everywhere?
As you can see, the actual force between the sun and a planet is not entirely predictable from just looking at the masses and the distances. In these cases, the gravitational force does remain roughly constant within a range of about 100 Newtons (1 Newton=1 kg m/s2) except for Pluto. So it looks like the effect of increasing mass is in a way cancelled out by increasing the distance.
This figure shows gravity, not gravitational force, of the sun at each planet. Gravity is a type of acceleration directed in toward the planet. Going back to your high school physics, you know that F=ma, where is acceleration. Well, for the gravitational force, F=mg (just plugging in g for a), so g=F/m. In other words, g is the force per unit mass that the sun exerts on a planet. But wait a second - we know what F is: it's (GMm)/r2. Therefore, g=(GM)/r2. Now that you know exactly what is being plotted here, it should not be a surprise that this curve drops off as 1/r2as mentioned in class.
light year: the distance light travels in one year, which turns out to be about 9.454 trillion km. (You can figure this out for yourself by multiplying the speed of light (about 299,792 km/s) by the number of seconds in a year.)
Our place in the Universe
The universe is about 12 billion light years in "all" directions from us (we say "all" because we aren't really in the center).
Planetary atmospheres today are affected by the position and size of the planet, which was determined during the formation of the solar system.
Average composition: 70% H 28% He 2% heavier elements
The mass of the sun is about 99.9% the mass of the original nebula that became the solar system. It takes enormous mass to create the temperatures and pressures necessary for significant energy production. How is this energy created? Through fusion, or forcibly joining H atoms to produce He atoms and liberating some energy in the process. This is an example of a thermonuclear reaction. In one second, 650 x 106 tons of H is transformed into 625 x 106 tons of He. Energy is clearly lost in the transformation process. It changes into light or heat or energy lost in the reaction.
Table 4.1 lists planetary information.
The four planets closest to the sun - Mercury, Venus, Earth, and Mars - are called the terrestrial, rocky, inner, Earthlike, or minor planets. The next four planets - Jupiter, Saturn, Uranus, and Neptune - are called the gaseous, outer, Jovian, or major planets, or sometimes the gas giants. The inner planets are much smaller in mass and size and are denser than the outer planets, indicating that the compositions of these planets are different. The atmosphere of the inner planets is composed of heavier elements than is the outer planets' atmosphere. That is because larger planets with more mass also have higher gravity. That means it is easier to "hold on to" lighter elements, like H and He.
Pluto is not well known. It is not gaseous but rather more like a rocky planet.
Our path around the sun
Our path of revolution about the sun is not perfectly circular, but slightly elliptical. (Remember that revolution refers to one body orbiting another body whereas rotation refers to a body spinning on its axis.)
The point where Earth is closest to the sun is called perihelion and usually occurs on January 3. Our distance then is about 147 x 106 km. The point where Earth is farthest from the sun is called aphelion and usually occurs on July 4, when we are about 152 x 106 km from the sun.
Notice that we are closest to the sun during our winter season. This indicates that distance from the sun is not the most important cause of our seasons. As we shall see later, the controlling factor in the seasons is the tilt of the earth's axis (though distance does play a minor role).
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