Question

11 multiple choice 5 points
gravity is responsible for the
orbits of the planets around the sun.
rotation of a planet on its axis.
tilt of earths axis.
phases of the moon.
12 multiple choice 5 points
why does the moon only have 1/6 of the earths gravity?
it has no atmosphere.
it is very far away from earth.
it is made of basalt.
it has less mass than earth.
13 multiple choice 5 points
which of the following statements best describes a difference between nuclear fission and nuclear fusion reactions?
nuclei split during fission and combine during fusion.
fission forms heavier elements, and fusion forms lighter elements.
fission generates potential energy, and fusion generates kinetic energy.
nuclei gain electrons during fission and release electrons during fusion.

Explanation:

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• The gravitational pull of the Sun keeps the planets in their orbits.
• Rotation of a planet on its axis is due to its initial angular momentum from formation, not gravity.
• The tilt of Earth's axis is a result of past collisions during its formation, not gravity in the sense of this question.
• The phases of the Moon are due to the relative positions of the Earth, Moon, and Sun, not gravity.

• Gravity is related to mass. The formula for gravitational force \(F = G\frac{m_1m_2}{r^2}\), and on a planet's surface (for an object of mass \(m\)), \(g=\frac{GM}{R^{2}}\) (where \(G\) is the gravitational constant, \(M\) is the mass of the planet, and \(R\) is the radius of the planet). Since the Moon has less mass than Earth, its surface gravity is less.
• No atmosphere does not determine gravity.
• Distance from Earth (when considering surface gravity of the Moon, its own radius and mass are relevant, not its distance from Earth).
• Composition (being made of basalt) is not the factor for gravity.

• In nuclear fission, a heavy nucleus splits into lighter nuclei. In nuclear fusion, light nuclei combine to form a heavier nucleus.
• Fission forms lighter elements (by splitting a heavy nucleus), and fusion forms heavier elements (by combining light nuclei).
• Both fission and fusion release energy (usually in the form of kinetic energy of the reaction products, and then converted to other forms like heat).
• Nuclear reactions (fission and fusion) involve nuclei, not electrons (electron - related processes are in chemical reactions).

Answer:

Orbits of the planets around the Sun.

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