Gravity. in the Solar System. Beyond the Book. FOCUS Book

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1 FOCUS Book Design a test to find out whether Earth s gravity always pulls straight down. A pendulum is a weight that hangs from a string or rod that can swing back and forth. Use string and metal washers to make pendulums of different lengths and es. Attach your pendulums in different places and release them from different positions. Vary the way you launch each pendulum. Watch as each pendulum settles and stops moving. Record your observations about how the pendulum swings, settles, and stops, including its final position. Explain how gravity affects where the pendulum stops. Gravity in the Solar System Beyond the Book Use the Internet to learn more about how weightlessness affects people who travel into space.

2 Gravity in the Solar System FOCUS Question How does gravity affect objects in the solar system? Scale, Proportion, and Quantity Photo Credits: Front cover: SPL/Science Source; page 2: Science Source; page 3 (top): mopic/123rf; page 3 (bottom): Tony Hallas/Science Faction/Corbis; page 4: Elke Van De Velde/Corbis; page 5 (top left): shtanzman/123rf; page 5 (top right): istock/kenneth Keifer; page 5 (bottom): Graça Victoria/123RF; page 9: Universal Images Group Limited/Alamy Illustration Credits: page 6 an 8: Q2A Media Inc. Gravity in the Solar System Learning A Z All rights reserved. 2 Gravity and Weight Imagine walking on the Moon. You have to wear a heavy spacesuit to protect you because there is no oxygen. Imagine how hard it would be to move around with all that heavy gear. Actually, it would be much easier than you think! On Earth, your weight is caused by gravity pulling your toward the center of the planet. The Moon has much less, or total amount of matter, than Earth, so its gravitational pull is much weaker. On the Moon, your does not change, but you would weigh one-sixth of what you weigh on Earth. The gear you are wearing would also be much lighter. Your muscles, however, are just as strong on the Moon as on Earth. It is much easier to lift your body and your spacesuit off the surface. Astronauts on the Moon had to move very carefully to avoid stepping or jumping too far. It s all about and gravity, but what is gravity?

3 Gravity and Distance Gravity and Mass Gravity is a force that attracts all objects to each other. How strongly they are attracted depends on two key factors: the es of the objects and the between them. Two large, rocky asteroids in space attract one another because of gravity. If they are far apart, the force is very weak. If they begin to move toward one another, the force increases. They may pull together and form one larger asteroid. Artist s rendering of an asteroid that orbits the Sun and passed close to Earth Are you and the classmate sitting next to you pulled together by each other s gravitational force? Yes, you are! Gravity attracts you to your pencil and to the bird outside your window and to any other objects that have, but it is hard to notice small pulls of gravity. Even large buildings are not ive enough to cause noticeable attraction. Earth s pull on these objects is much greater than the pull of these objects on each other, because Earth is so much more ive than they are. The more ive, the stronger the attraction. Objects like moons, planets, and stars have a noticeable gravitational pull. of building: 7500 kg of ice cream cone: 0.2 kg of student: 35 kg A meteor is a piece of rock that gets close to Earth. Earth s gravity pulls it down. You might see a streak of light in the sky as the meteor burns up in our atmosphere. The Leonid meteor shower From weakest to strongest, order the strength of the gravitational attraction between Earth and each object in this picture: the student, the building, and the ice cream cone. Force and Motion Gravity in the Solar System 3 4

4 Gravity and the Moon Tides Caused by Gravitational Force of the Moon low tide high tide Earth gravitational force of the Moon Moon The Sun and the Planets You probably know that Earth orbits the Sun. It is the Sun s gravity that keeps Earth and the other planets in the solar system orbiting around it. Can you guess what determines the strength of the gravitational pull between each planet and the Sun? It is the of each planet and its from the Sun. low tide high tide Earth s gravity keeps the Moon orbiting Earth once every 27.3 days. The Moon pulls on Earth, too, causing ocean tides. Water on the side of Earth facing the Moon is pulled toward the Moon. This causes a high tide. As the Moon pulls on Earth, the water on the opposite side gets left behind. This causes a high tide on that side, too. Low tides occur between the high tides. As Earth rotates, each coastal location passes through high and low tides twice a day. Uranus Earth Venus Sun Neptune Mercury Saturn High tide Low tide Mars Jupiter Not to scale Force and Motion Gravity in the Solar System 5 6

5 Orbiting the Sun Think about how the Sun s gravity would act on each planet based on its and its from the Sun. Planet Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Mass Earth s 0.81 Earth s 5.97 x kg* 0.11 Earth s 320 Earth s 95 Earth s 14 Earth s 30 Earth s Distance from Sun 0.39 Earth s 0.72 Earth s about 150 million kilometers (93 million miles) 1.5 Earth s 5.2 Earth s 9.6 Earth s 19 Earth s 30 Earth s *5,972,600,000,000,000,000,000,000 kilograms Time to orbit Sun about 88 Earth days about 225 Earth days 1 Earth year = 365 days almost 2 about 12 about 29.5 about 84 about 165 When planets move around the Sun, they do not travel in a circle. Their orbit is an ellipse. An ellipse looks like a circle that has been flattened a little on two sides. The amount of flattening is too small to be obvious in a drawing like the one below. The path of planets around the Sun is mostly smooth and follows the elliptical curve. However, planets are attracted to each other, too! The nearer planets get to each other, the more they are pulled. This causes them to travel a little out of their paths and towards each other. Before Neptune was ever observed, astronomers predicted that it existed and where it was located. They calculated how the Sun and the large planets, Jupiter and Saturn, affect the motion of Uranus. Then they found that the real orbit of Uranus was slightly different. That meant there was another Neptune large planet Saturn to be found. This turned out Sun to be Neptune. Uranus wobbles a little bit in its orbit as it is pulled by the other three large planets of the solar system. Uranus Mercury Jupiter Force and Motion Gravity in the Solar System 7 8

6 Gravity and You Astronauts floating in the International Space Station Would you like to be taller? Try going into space for a while! Astronauts on the International Space Station grow taller while in space. Gravity on Earth pulls down on and compresses the vertebrae (bones) in our spine. But in space, where the force of Earth s gravity is much weaker, astronauts vertebrae decompress (loosen) and expand. Astronauts come back to Earth taller than when they left! After being back on Earth for several months, however, their spines go back to normal. They return to their original height. Gravity causes tides and orbits, and it even affects how tall we are! That is a very attractive force! Write your answers on separate paper. Use details from the text as evidence. 1 Define weight in your own words. 2 Use the words weaker and stronger to complete these two sentences: A The more two objects have, the the gravitational attraction between them. B The farther apart two objects are, the the gravitational attraction between them. 3 Look at the two photos of tides at the bottom of page 5. Based on what you know about tides and the Moon s gravity, explain why the water level changes. 4 Jupiter is farther from Earth than Mars is. Why does Jupiter have a stronger gravitational pull on Earth than Mars does? 5 Read this sentence: Planets in the solar system are affected only by the Sun s gravity. Is this a true statement? How do you know? FOCUS Question How does gravity affect objects in the solar system? Write a paragraph to explain how the es of the Sun and the planets affect how objects in the solar system move. Force and Motion Gravity in the Solar System 9 10