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Da Name __________________________________________________________ Day 1 Idea 4 Why are planets round? There are eight planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Some planets are much larger than others, and some are surrounded by rings or moons. But all planets have three things in common. They orbit the sun, they are massive enough that their gravity controls all the objects in the area around them, and they are spherical. Although you might not think of “roundness” as a particularly special or unusual quality, most objects in the solar system are not round. Millions of asteroids between the orbits of Mars and Jupiter are irregularly shaped chunks of rock. Planets are round because of their large mass and gravitational force. Mercury Mars Jupiter e Big Weekly Question Venus i l y S c i e nc WEEK 3 Vocabulary spherical SFEER-ih-kul round, ball-shaped; having a surface equally distant from the center at all points Uranus Saturn Neptune Earth A. Check the box next to the object that is spherical. !paper tube !egg !football !basketball B. What are three things that all planets have in common? 1. 3. 2. Talk Pluto would be classified as the ninth planet if it were not for the fact that there are millions of other space rocks within its orbit that its gravity doesn’t control. What are two examples of objects that are controlled by a planet’s gravity? PB-:";;>;><I" I-58E'/51:/1 111 Da Name __________________________________________________________ Day 2 i l y S c i e nc e Big Weekly Question Why are planets round? Most scientists today believe that the solar system formed when a giant cloud of gas and dust contracted into a rotating disk of matter. Most of this material coalesced in the center of the disk and became our sun. The rest formed small, rocky bodies or balls of gas. Over time, gravitational force allowed these objects to attract and collect other chunks of rock or molecules of gas in their regions of space. As these objects grew in size, their gravity increased. This allowed them to attract and gather even more material. This process, called accretion, is believed to have formed the planets, asteroids, comets, and other bodies in our solar system. Idea 4 WEEK 3 Vocabulary accretion uh-CREE-shun the process of becoming larger by adding more material coalesce koh-uh-LESS to unite; to grow together Formation of the Solar System A. Write true or false. 1. Our solar system began as a rotating disk of gas and dust. 2. As the gravity of objects in the solar system increased, the objects became bigger. 3. Most of the gas and dust in the early solar system became the planets. B. Use the vocabulary words to complete the sentences. 1. Dust and other particles of matter can to form the beginning of a planet. 2. Through the process of , an object grows in size and increases its gravitational force. 112 -58E'/51:/1I" IPB-:";;>;>< Da Name __________________________________________________________ Day 3 i l y S c i e nc e Big Weekly Question Idea 4 Why are planets round? Gravity acts to pull everything toward an object’s center of mass. The center of mass is a single imaginary point inside an object from which gravity seems to act. As an object acquires more mass, its gravity increases and more matter is pulled toward the object’s center. When an object is massive enough, gravity forces it into a round shape. This is because the only way to get all of the object’s matter as close to its center of mass as possible is to form a sphere. Every point on the surface of a sphere is equally distant from its center. Objects such as asteroids or small moons are irregularly shaped because they have much less mass than planets. Therefore their gravitational force is not strong enough to form a sphere. Unlike planets, whose center of mass is also the center of the sphere, asteroids have a center of mass that may actually be located off-center, depending on where the most mass has accumulated. WEEK 3 center of mass center of mass planet asteroid A. Check the box next to the analogy that is not correct. !Center is to sphere as center of mass is to planet. !Asteroid is to irregular-shaped as planet is to spherical. !Center of mass is to planet as axis is to Earth. !Planet is to asteroid as round is to irregular. B. Explain in your own words why an object with a lot of mass takes the shape of a sphere. PB-:";;>;><I" I-58E'/51:/1 113 Da Name __________________________________________________________ Day 4 i l y S c i e nc e Big Weekly Question Why are planets round? Even though Earth’s gravity is strong enough to pull all of the planet’s matter evenly toward its center, Earth is not a perfect sphere. This is because Earth’s rotation creates centrifugal force. Centrifugal force is the same force that causes you to slide to one side when you go around a curve on a roller coaster. On Earth, centrifugal force pushes out the planet’s surface at the equator, causing it to bulge slightly around the middle. In fact, Earth’s diameter is 26 miles wider when measured at the equator than when measured from the North Pole to the South Pole. Believe it or not, this means that you weigh slightly less at the equator than you do at the poles! This is because at the equator, you are a little farther away from Earth’s center of mass than you are at the poles. Idea 4 WEEK 3 Vocabulary centrifugal force sen-TRIF-uh-gul forss the force that tends to push an object outward when it rotates around a center diameter: 7,900 miles diameter: 7,926 miles Answer the questions. 1. Why does Earth’s surface bulge at the equator? Use the vocabulary word in your answer. 2. What would have to happen in order for Earth to become a perfect sphere? 3. Why does being farther from Earth’s center of mass cause you to weigh less? 114 -58E'/51:/1I" IPB-:";;>;>< Da Name __________________________________________________________ e Big Weekly Question Day 5 i l y S c i e nc Idea 4 Why are planets round? A. Use the words in the box to complete the sentences. WEEK 3 center of mass accretion centrifugal force coalesced spherical 1. Earth is not a perfect sphere because causes the equator to bulge out slightly. 2. Planets are so that all of their matter can as possible. be as close to the 3. Our solar system formed when gas and dust into bodies that grew larger and larger over time through the process of . B. Explain how mass and gravity cause planets to be round. C. What two qualities, besides roundness, must an object have in order to be considered a planet? 1. 2. PB-:";;>;><I" I-58E'/51:/1 115