• Earth has two tidal bulges at all times. Approximately where are these bulges located?
o One faces the Moon, and one faces opposite the Moon.
o The tidal bulges face toward and away from the Moon, because they are caused
primarily by the gravitational attraction between Earth and the Moon. (Friction
explains why the bulges are slightly ahead of the Earth-Moon line, rather than
directly on the Earth-Moon line; but we'll ignore that detail for now.)
• Most people are familiar with the rise and fall of ocean tides, but do tides also affect land?
o Yes, though land rises and falls by a much smaller amount than the oceans.
o Tides affect the entire Earth, but they are much more noticeable for the oceans
because water flows so much more easily than land. Still, the land rises and falls
measurably (by about 1 centimeter) with the tides.
• Any particular location on Earth experiences any particular location on Earth experiences
o any location on Earth passes through both tidal bulges and both tidal minima (the
places where the tides are smallest) each day, which means two high tides and two
low tides. Again, recall that this is true both of land and oceans, though tides are
more noticeable in the oceans because water flows so much more readily than land.
• One tidal bulge faces toward the Moon because that is where the gravitational attraction
between Earth and the Moon is strongest. Which of the following best explains why there
is also a second tidal bulge?
o The second tidal bulge arises because gravity weakens with distance, essentially
stretching Earth along the Earth-Moon line.
o Tides are created by gravity, and the tidal force is caused by the fact that gravity
weakens with distance. Therefore, the parts of Earth that are closer to the Moon feel
a stronger gravitational attraction to the Moon, and the parts of Earth that are
farther away feel a weaker gravitational attraction to the Moon. This varying
gravitational attraction essentially stretches Earth along the Earth-Moon line,
creating tidal bulges on both sides.
• the tidal bulges are largest and the tidal minima are smallest at full moon and new moon, because those are the
times when the tidal forces due to the Sun and Moon are aligned (and therefore add to one another). Therefore,
high tides are higher and low tides are lower at these times, which are called spring tides. (In contrast, we have
neap tides at first- and third-quarter moons, when high tides are not as high and low tides are not as low.)
• High tides occur at the locations of the tidal bulges.
• You have found that tides on Earth are determined primarily by the position of the Moon,
with the Sun playing only a secondary role. Why does the Moon play a greater role in
causing tides than the Sun?
o The Sun exerts a stronger gravitational force on Earth, which is why Earth orbits the
Sun. However, tides are caused by the variation in the gravitational attraction
across Earth. Even though the gravitational attraction between Earth and the Moon
is smaller than the attraction between Earth and the Sun, the Moon's much closer
distance makes this attraction vary more across Earth. That is why tides are due
primarily to the Moon, with only a secondary effect from the Sun.
• Tidal friction caused by Earth's stretching from the Moon's gravity is gradually slowing
down the rotation of Earth.
• The Moon is slowly moving away from Earth. • At which lunar phase(s) are tides least pronounced (e.g., the lowest high tides)?
o both first and third quarters
• The tides on Earth are an example of
o the universal law of gravitation.
• Which of the following best describes the origin of ocean tides on Earth?
o Tides are caused by the difference in the force of gravity exerted by the Moon
across the sphere of the earth.
• At which lunar phase(s) are tides most pronounced (e.g., the highest high tides)?
o both new and full Moons
• Fusion requires bringing two positively charged nuclei so close together that the strong nuclear force can hold
them against the electromagnetic repulsion of their positive charges. What conditions make fusion possible?
o extremely high temperature and high density
o It takes high temperature for nuclei to get into close enough contact for fusion, and
high density for this to happen with enough likelihood that fusion will continue. An
increase in either the temperature or density therefore will increase the fusion rate.
• When the Sun’s core contracts, the core temperature
o Just as when you compress air in a bicycle tire pump, compressing the Sun’s core
increases its temperature. Now, remember that the fusion rate is very sensitive to
• Which of the following changes would cause the fusion rate in the Sun’s core to increase?
o An increase in the core temperature
o A decrease in the core radius
o An increase in the core temperature increases the fusion rate because the fusion
rate is very sensitive to temperature. A decrease in the core radius causes the core
to heat up and increase in density, which therefore leads to an increased fusion
• The core of a star is in equilibrium when its temperature, size, and the rate of nuclear
fusion all hold steady.
• Suppose you could somehow start nucl