Gravity in the solar system and galaxies

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レッスンノート

Gravity Holds Objects in Orbit

Everything that has mass has gravity — gravity is the pulling force of attraction between any two objects with mass.
The Sun is the most massive object in the solar system, so it exerts the greatest gravitational pull; its gravity holds Earth and every other planet in orbit around it.
The greater an object's mass, the stronger its gravitational pull on other objects.
The Sun's gravity makes the planets move in curved, orbital paths around it instead of travelling off in a straight line.

The direction of Earth's gravitational pull

Gravity, Mass, and Distance

Gravity gets weaker as objects move further apart: doubling the distance between two objects cuts the gravitational force between them to a quarter; tripling the distance cuts it to a ninth.
The Sun is about 333,000 times more massive than Earth, but because we are so far from the Sun, Earth's own gravity holds us to its surface far more strongly than the Sun's gravity does.
Weight is the force of gravity acting on an object's mass, so it changes depending on where the object is; mass — the amount of matter in the object — stays the same everywhere.
The Moon's gravity is about one-sixth of Earth's, so an object weighs one-sixth as much on the Moon as it does on Earth, even though its mass has not changed.

An astronaut's mass and weight on Earth and the Moon

Inertia and Orbital Motion

Inertia is the tendency of an object to keep moving the way it is already moving; objects with more mass have more inertia.
Earth does not fall into the Sun: its forward motion (inertia) constantly carries it sideways while the Sun's gravity constantly pulls it inward, so gravity and inertia together make Earth fall around the Sun instead of into it.
This combined motion is called orbital motion. It happens whenever an object is moving forward while gravity pulls it toward another object.
The Moon orbits Earth the same way: Earth's gravity pulls the Moon toward Earth, while the Moon's inertia keeps carrying it forward, so it continually falls around Earth instead of crashing into it.

Gravity and inertia keep the Moon in orbit around Earth

Solar System Objects & What Makes a Planet

In orbit around the Sun are the eight planets, their moons, dwarf planets, and many smaller objects such as asteroids, Kuiper Belt objects, and comets.
Terrestrial planets (Mercury, Venus, Earth, Mars) have solid rocky surfaces; gas giants (Jupiter, Saturn, Uranus, Neptune) are much larger and made mainly of gas.
To count as a planet, an object must: orbit the Sun; be large enough that its own gravity pulls it into a spherical shape; and have cleared other objects out of its orbital path.
Pluto was reclassified from a planet to a dwarf planet in 2006 because, although it orbits the Sun and is nearly round, it has not cleared its orbit of similar-sized objects.

The five known dwarf planets, shown to scale with Earth

Terrestrial vs Gas Giant Planets

Terrestrial planets are close to the Sun, small, rocky, dense, and slow-spinning, with thin atmospheres, few or no moons, and no rings.
Gas giants are far from the Sun, large, made mainly of gas, less dense, fast-spinning, with thick atmospheres, many moons, and rings.
In the early solar system it was hot close to the Sun, so only metals and rock could stay solid there; further out it was cold enough for ices and gases to collect, which is why the outer planets grew so much larger.
Moon counts show the pattern clearly: Mercury 0, Venus 0, Earth 1, Mars 2, compared with Jupiter 67, Saturn 62, Uranus 27, Neptune 13. Saturn's rings alone are over 200,000 km wide but only a few tens of metres thick.

The relative sizes of the terrestrial planets and gas giants

Planetary Temperatures

Planets closer to the Sun are usually warmer, and planets farther away are usually colder.
Venus is the hottest planet (464°C) even though Mercury is closer to the Sun, because Venus's thick carbon dioxide atmosphere traps heat in a strong greenhouse effect.
Neptune is the coldest planet (-200°C), as it is the farthest planet from the Sun.
Excluding Venus, planetary temperatures decrease with distance from the Sun.

Surface temperatures of the planets

Small Bodies: Asteroids, Kuiper Belt, and Comets

Asteroids are small rocky leftovers from the solar system's formation, mostly found in the asteroid belt between Mars and Jupiter; more than 100,000 have been catalogued there.
The Kuiper Belt is a region of icy objects beyond Neptune's orbit; its two largest known members, Pluto and Eris, are both dwarf planets.
Comets are icy, dusty objects that orbit the Sun from the Kuiper Belt or the far more distant Oort Cloud; they only become visible from Earth when they travel close to the Sun.
As a comet nears the Sun, heat evaporates its ices, forming a dust tail that points back along its path, and a second ion tail, pushed by the solar wind, that always points directly away from the Sun.

The Kuiper Belt and the Oort Cloud

Scale and Distances

The average distance from Earth to the Sun is called an astronomical unit (AU), about 150 million km.
The Kuiper Belt lies between about 30 and 50 times Earth's distance from the Sun.
The Oort Cloud is a predicted cloud of icy objects at about 5,000 to 100,000 times Earth's distance from the Sun.
Gravity holds all of this together across enormous distances: the Sun's pull, though far weaker at greater distances, still keeps even the most distant objects in orbit around it.

The orbits of the planets in the solar system

The Milky Way Galaxy

A galaxy is a massive collection of stars, gas, and dust held together by gravity.
The Milky Way has about 200 billion stars, including our Sun, arranged in a disk with a central bulge and spiral arms.
Our Sun sits in a small spiral arm called the Orion (or Local) Arm, roughly halfway out from the galaxy's centre.
Gravity keeps the whole galaxy turning: the Sun and the rest of the solar system orbit the centre of the Milky Way once every 225 million years, even while travelling at about 250 km/s.

The Milky Way seen from the side

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練習問題

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1.What is the name of the force that keeps the planets in orbit around the Sun?
Easy
- AGravity
- BMagnetism
- CFriction
- DInertia
2.The Sun makes up 98% of the total mass of the solar system.
Easy
True or false?
3.Which of the following is NOT a criterion for an object to be classified as a planet?
Easy
- AIt must orbit the Sun
- BIt must be large enough to be spherical due to its own gravity
- CIt must have cleared its orbit of other objects
- DIt must have a moon
4.The asteroid belt is located between the orbits of Mars and Jupiter.
Easy
True or false?
5.Which of the following is a gas giant?
Easy
- AMars
- BJupiter
- CVenus
- DMercury
6.What is the main component of the atmosphere of Venus?
Easy
- ACarbon dioxide
- BOxygen
- CNitrogen
- DMethane
7.The Sun is located at the centre of the Milky Way Galaxy.
Easy
True or false?
8.What is a galaxy?
Easy
- AA collection of stars, gas and dust held together by gravity
- BA single star with planets
- CA cloud of gas and dust
- DA group of planets orbiting a star

もっと問題

9.All the stars you can see with your naked eye belong to the Milky Way Galaxy.
Easy
True or false?
10.Which planet is known as the Red Planet?
Easy
- AMars
- BVenus
- CJupiter
- DMercury
11.What is the main reason why Venus has the highest surface temperature of all the planets?
Easy
- AIts thick atmosphere of carbon dioxide
- BIts close distance to the Sun
- CIts rapid rotation
- DIts volcanic activity
12.The Oort Cloud is a region of icy objects surrounding the Sun at the very edge of the solar system.
Easy
True or false?
13.The Sun takes about 225 million years to complete one orbit around the centre of the Milky Way.
Easy
True or false?
14.Which of the following is the correct order of planets from the Sun?
Easy
- AMercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
- BMercury, Earth, Venus, Mars, Jupiter, Saturn, Uranus, Neptune
- CMercury, Venus, Earth, Mars, Saturn, Jupiter, Uranus, Neptune
- DVenus, Mercury, Earth, Mars, Jupiter, Saturn, Uranus, Neptune
15.All gas giants have rings.
Easy
True or false?
16.The Sun is the largest object in the solar system.
Easy
True or false?
17.Which of the following is NOT a dwarf planet?
Easy
- APluto
- BEris
- CCeres
- DTitan
18.The table shows the number of known moons of each planet. | Planet | Number of moons | |---|---| | Mercury | 0 | | Venus | 0 | | Earth | 1 | | Mars | 2 | | Jupiter | 67 | | Saturn | 62 | | Uranus | 27 | | Neptune | 13 | Which planet has the most moons?
Easy
- AJupiter
- BSaturn
- CUranus
- DNeptune
19.Which of the following is the correct definition of a light year?
Easy
- AThe distance light travels in one year
- BThe time it takes light to travel from the Sun to Earth
- CThe distance from the Sun to the nearest star
- DThe time it takes for Earth to orbit the Sun
20.The terrestrial planets are much larger than the gas giants.
Easy
True or false?
21.Which planet has the highest average surface temperature?
Easy
- AVenus
- BMercury
- CMars
- DJupiter
22.Match each type of object in the solar system with its correct description.
Easy
- Asteroid
- Kuiper Belt object
- Comet
- Small rocky object found mainly between Mars and Jupiter
- Icy object found beyond Neptune
- Icy, dusty object that forms a tail near the Sun
23.Arrange the following objects in order from closest to the Sun to farthest from the Sun.
Medium
- Asteroid belt
- Kuiper Belt
- Oort Cloud
- Orbit of Neptune
24.Which of the following are criteria for an object to be classified as a planet? (Select all that apply)
Medium
- AIt orbits the Sun
- BIt has a solid surface
- CIt has cleared its orbit of smaller objects
- DIt has a moon
- EIt is large enough for its gravity to make it spherical
25.The terrestrial planets are more dense than the gas giants.
Easy
True or false?
26.Fill in the blank: The four planets closest to the Sun are called ____ planets because they have solid rocky surfaces.
Easy
The four planets closest to the Sun are called ____ planets.
27.Fill in the blank: The Kuiper Belt is a region of space filled with trillions of small objects that lies past the orbit of ____.
Easy
The Kuiper Belt lies past the orbit of ____.
28.Fill in the blank: The Sun is located in a small spiral arm called the ____ Arm.
Easy
The Sun is located in the ____ Arm.
29.Fill in the blank: The average distance between the Sun and Earth is called an astronomical unit and is equivalent to ____ million kilometres.
Easy
1 AU = ____ million km.
30.Fill in the blank: The largest volcano in the solar system, Olympus Mons, is on ____.
Easy
Olympus Mons is on ____.
31.Fill in the blank: The thin band of light that you see in the night sky is actually the stars in the ____ arm as you look towards the centre of the galaxy.
Easy
The Milky Way band is the stars in the ____ arm.
32.Fill in the blank: The four gas giants are Jupiter, Saturn, Uranus, and ____.
Easy
The gas giants are Jupiter, Saturn, Uranus, and ____.
33.Fill in the blank: The two largest known members of the Kuiper Belt are Eris and ____.
Easy
The two largest Kuiper Belt objects are Eris and ____.
34.Fill in the blank: The Milky Way is a ____ galaxy.
Easy
The Milky Way is a ____ galaxy.
35.Fill in the blank: The four terrestrial planets are Mercury, Venus, Earth, and ____.
Easy
The terrestrial planets are Mercury, Venus, Earth, and ____.
36.Why is Venus so hot?
Medium
37.Why does the Sun's gravity hold the planets in orbit around it, rather than the planets holding the Sun in orbit around them?
Easy
- AThe Sun has by far the greatest mass in the solar system, so it exerts by far the greatest gravitational pull
- BThe Sun is hot, and heat creates a stronger gravitational pull
- CThe Sun spins faster than the planets
- DThe planets have no gravity of their own
38.If two objects are moved twice as far apart, the gravitational force between them becomes four times weaker.
Easy
True or false?
39.The table shows the strength of gravity on some planets, relative to Earth (Earth = 1). | Planet | Gravity relative to Earth | |---|---| | Mercury | 0.378 | | Venus | 0.907 | | Mars | 0.377 | | Jupiter | 2.36 | On which of these planets would a person weigh the most?
Easy
- AMercury
- BVenus
- CMars
- DJupiter
40.The International Space Station (ISS) moves forward very fast while Earth's gravity constantly pulls it toward Earth. Why doesn't the ISS either fly off into space or fall to the ground?
Easy
- AIts forward motion (inertia) and Earth's gravity combine, so it continually falls around Earth instead of into it
- BThere is no gravity in space, so nothing pulls on the ISS
- CThe ISS fires engines constantly to hold itself up
- DThe ISS is too light for gravity to act on it
41.The diagram shows the Sun, Earth, and the Hubble Space Telescope. Earth and Hubble each complete an orbit around the Sun in one year, while Hubble also completes a much smaller orbit around Earth in about 95 minutes. Explain, using both gravity and inertia, why Hubble stays in orbit around Earth, and why Earth stays in orbit around the Sun.
Medium
42.The diagram shows Jupiter's moon Io orbiting Jupiter, with Jupiter orbiting the Sun. A student claims: "The Sun exerts a gravitational force on Jupiter, but not on Io." Is this claim correct?
Medium
True or false?

復習ノート

## Gravity in the Solar System and Galaxies ### Key Concepts - **Gravity** is the force that keeps planets orbiting the Sun and moons orbiting planets. It also holds galaxies together. - **Orbits** are curved paths objects follow due to gravity. Most orbits are elliptical (oval-shaped), not perfect circles. - **Kepler's Laws**: 1. Planets orbit the Sun in ellipses, with the Sun at one focus. 2. A planet moves faster when closer to the Sun and slower when farther away. 3. The square of a planet's orbital period (T²) is proportional to the cube of its average distance from the Sun (a³). For any two planets, \(\frac{a^3}{T^2}\) is constant. - **Newton's Law of Gravitation**: Gravity is an attractive force between all objects with mass. The force depends on mass and distance. For a planet orbiting a star, \(T^2 \propto \frac{a^3}{M}\), where M is the mass of the star. - **Center of Mass**: Two objects actually orbit their common center of mass. If one is much more massive (e.g., Sun vs. Earth), the center is inside the larger object. ### Examples in the Solar System - Earth orbits the Sun once every 365.25 days at an average distance of 1 AU (astronomical unit). - Jupiter is about 5.2 AU from the Sun and takes ~11.86 years to orbit. Check Kepler's third law: \(5.2^3 / 11.86^2 \approx 1.0\). - The Moon orbits Earth due to Earth's gravity. ### Galaxies - The Milky Way is a spiral galaxy held together by gravity. Stars, gas, and dust orbit the galactic center, which contains a supermassive black hole. - Gravity causes galaxies to cluster and interact over cosmic time. ### Historical Models - Early models (Ptolemy) used complex circles (epicycles) to explain planet motions. - Copernicus placed the Sun at the center, simplifying the model. - Kepler used Tycho Brahe's data to discover elliptical orbits. - Newton explained orbits using gravity and calculus. ### Key Formula (from Newton) - \(T^2 \propto \frac{a^3}{M}\) where T = orbital period, a = semi-major axis (average distance), M = mass of central object. ### Study Tips - Remember: gravity is the "glue" of the universe. It determines orbits, from moons to galaxies. - Practice using Kepler's third law: if you know distance, find period, or vice versa. - Understand that all orbits are ellipses; circles are a special case.

Orbit: The curved path of an object around a star, planet, or moon, caused by gravity.
Gravity: A force that attracts objects with mass toward each other; it keeps planets in orbit and holds galaxies together.
Kepler's Laws: Three laws describing planetary motion: (1) planets move in ellipses with the Sun at one focus; (2) planets sweep equal areas in equal times; (3) the square of the orbital period is proportional to the cube of the average distance from the Sun.
Ellipse: An oval shape; the path of a planet around the Sun is an ellipse.
Orbital Period: The time an object takes to complete one full orbit around another object.
Astronomical Unit (AU): The average distance from Earth to the Sun, about 150 million kilometers; used to measure distances in the solar system.
Center of Mass: The point around which two orbiting objects both revolve; for a planet and a much larger star, it lies inside the star.
Milky Way: The spiral galaxy that contains our solar system; held together by gravity.
Inverse-Square Law: A law stating that the force of gravity between two objects decreases with the square of the distance between them.

フラッシュカード

What is an orbit?

An orbit is the curved path of an object around another object due to gravity, such as a planet around a star or a moon around a planet.

What shape are most orbits in the solar system?

Most orbits are elliptical (oval-shaped), not perfect circles, with the larger object at one focus of the ellipse.

What force keeps planets in orbit around the Sun?

Gravity is the force that keeps planets in orbit around the Sun, pulling them toward the Sun while their forward motion keeps them from falling in.

What is Kepler's first law of planetary motion?

Kepler's first law states that planets orbit the Sun in elliptical paths, with the Sun at one focus of the ellipse.

What is Kepler's second law of planetary motion?

Kepler's second law says that a planet moves faster when it is closer to the Sun and slower when it is farther away, sweeping equal areas in equal times.

What is Kepler's third law of planetary motion?

Kepler's third law states that the square of a planet's orbital period is proportional to the cube of its average distance from the Sun (T² ∝ a³).

How does distance from the Sun affect a planet's orbital period?

Planets farther from the Sun have longer orbital periods; for example, Jupiter (5.2 AU) takes about 11.86 years, while Venus (0.723 AU) takes about 0.615 years.

What is an astronomical unit (AU)?

An astronomical unit (AU) is the average distance from Earth to the Sun, about 150 million kilometers, used to measure distances within the solar system.

What did Isaac Newton contribute to our understanding of orbits?

Newton showed that gravity follows an inverse-square law and that Kepler's laws can be derived from his law of universal gravitation.

What is the inverse-square law of gravity?

The inverse-square law states that the gravitational force between two objects decreases with the square of the distance between them.

What is the role of gravity in the solar system?

Gravity holds the solar system together, causing planets to orbit the Sun, moons to orbit planets, and keeping all objects in their paths.

How does gravity affect galaxies?

Gravity holds galaxies together, causing stars, gas, and dust to orbit the galactic center, and it governs the motions of galaxies in clusters.

What is the Milky Way?

The Milky Way is the galaxy that contains our solar system; it is a large spiral galaxy with billions of stars, all held together by gravity.

What is the difference between revolution and rotation?

Revolution is the orbit of one object around another (e.g., Earth around Sun), while rotation is the spinning of an object on its own axis.

What did Johannes Kepler discover about planetary orbits?

Kepler discovered that planets orbit in ellipses (not circles), that their speed varies with distance from the Sun, and that orbital period relates to distance.

What is a natural satellite?

A natural satellite is a moon or other celestial body that orbits a planet or larger object, such as Earth's Moon or Jupiter's moons.

What is an artificial satellite?

An artificial satellite is a human-made object placed into orbit around Earth or another celestial body for communication, observation, or other purposes.

How does the mass of a star affect the orbits of its planets?

A more massive star exerts a stronger gravitational pull, affecting the orbital speeds and periods of its planets; the combined mass of the system influences orbit size and period.

What is the center of mass in an orbiting system?

The center of mass is the point around which two objects orbit; for a planet and a star, it is usually inside the star due to the star's much larger mass.

What are Lagrangian points?

Lagrangian points are positions in space where the gravitational forces of two large bodies (e.g., Sun and Earth) balance, allowing smaller objects to stay in place.

How did the prediction of Neptune demonstrate Newton's laws?

In 1846, Urbain Le Verrier used Newton's laws to predict Neptune's position from perturbations in Uranus's orbit, confirming the accuracy of Newtonian gravity.

What is a perturbation in orbital mechanics?

A perturbation is a small change in an orbit caused by the gravitational pull of other bodies, such as the effect of Jupiter on an asteroid's path.

What is the difference between a geocentric and heliocentric model?

A geocentric model places Earth at the center of the universe, while a heliocentric model places the Sun at the center; the heliocentric model is correct.

Why do planets not fall into the Sun?

Planets have forward momentum (inertia) that balances the Sun's gravity, causing them to fall around the Sun in a curved path rather than straight into it.

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