1 Just as we know that the sun will rise every morning, we expect the planets and the moon to stay in their orbits. And rightly so. For 400 years, people have understood that the movements of Earth, the moon, the planets, and all the other bodies in the universe are determined by precise and predictable laws of force and motion. These laws tell us that the path of an object is controlled by two forces: gravity and inertia. Inertia keeps an object moving in a straight line and at a constant speed until it is pushed or pulled on by a force. Everything with mass has inertia, and the more massive something is, the more inertia it has. In the absence of all other forces, such as air resistance or gravity, inertia would allow a baseball that was hit out of the park to travel in a straight line all the way to the edge of the universe! inertia ih-nur-shuh a property of matter that keeps an object moving in a straight line and at a constant speed A. What two forces govern the movement of the planets and all other bodies in the universe? 1. 2. B. Cross out the incorrect word in each sentence below and write the correct word above it to make the statement true. 1. Gravity keeps an object moving in a straight line at a constant speed. 2. The path of a baseball is unaffected by the laws of motion. 3. The more mass an object has, the less inertia it has. 117
2 Planets move in well-defined and separate paths, obeying the laws of force and motion as they orbit the sun. What keeps planets in their orbit is the balance between the opposing forces of gravity and inertia. The sun s gravity pulls planets toward it, and inertia keeps planets moving in a straight line. The combined result is an elliptical orbit, meaning that the planets revolve around the sun in oval-shaped paths. The sun is almost, but not quite, at the center of this oval. Similarly, the orbit of the moon around Earth is elliptical. Unlike a baseball hit through the air, objects in space don t encounter air resistance or other types of friction that could slow their motion. So the planets just keep circling the sun. And in the absence of all other forces, they will continue to do this forever. elliptical ee-lip-tih-kul having the shape of an oval or flattened circle gravitational pull of the sun Earth s orbit around sun Earth s inertia A. Complete the sentences, using words from the passage. 1. The balance between gravity and keeps planets in their orbits. 2. Air resistance is a kind of that slows down an object s motion. 3. Gravity and inertia maintain the moon s around Earth. B. Complete the analogy. Circle is to round as oval is to. 118
3 Astronauts orbiting Earth in a spacecraft are subject to the same laws of gravity and inertia that control the motions of the moon and planets. When you see astronauts floating inside their spacecraft, they may appear to be weightless, but they are actually still in the grip of Earth s gravity. Astronauts float because they are in free fall. During free fall, the gravitational force that causes a spacecraft and its contents to fall toward Earth is exactly matched by the forces of inertia that keep the spacecraft traveling in a straight line. The result is a curved trajectory an elliptical orbit around Earth. Because downward fall is balanced by sideways motion, the spacecraft doesn t ever actually hit Earth. You, too, may have experienced free fall momentarily on an amusement park ride. During a sudden drop, there is a brief sensation of weightlessness as your seat takes the plunge and inertia attempts to keep you in place. But hold on to your belongings, because coins and sunglasses will seem to float out of your pockets! free fall FREE fall the unchecked fall of an object trajectory trah-jek-tuh-ree the path of an object moving through space Use the vocabulary words and information from the passage to complete the paragraph below. Every object that moves through space follows a. Even astronauts who appear to be floating are actually in. This means that the downward force of is exactly balanced by. 119
4 Although the balance between inertia and gravity keeps the planets in stable, predictable trajectories around the sun, this was not the case when the solar system first formed. At that time, small, solid bodies that coalesced early in the solar system s history did not have stable orbits. These bodies crashed into each other, and through the process of accretion, became the large planets orbiting the sun today. Objects such as comets and asteroids may be the leftovers. One of the most spectacular collisions between large bodies in our solar system may have happened right in our own neighborhood. Scientists hypothesize that very early in Earth s history, an object the size of Mars collided with Earth and punched out a huge mass of material that became our moon. Luckily for us, a similar collision is probably impossible today. comet KAH-mit an icy chunk of rock from a region of space beyond the orbit of Neptune Write true or false. 1. When the solar system first formed, objects in space moved in predictable trajectories. 2. Comets may be remains from the formation of the solar system. 3. The moon was formed when Earth and Mars crashed into each other. Talk In the past, Earth was a planet where collisions with asteroids, meteorites, and comets happened regularly. How do you think these collisions helped shape life on Earth today? 120
5 A. Write the word from the box that completes each sentence. elliptical comets trajectory free fall inertia 1. When astronauts float in space, they are really in. 2. The orbit of Earth around the sun is, instead of completely circular. 3. An object s is the path it takes through space. 4. is the property of matter that keeps it moving in a straight line. 5. Scientists believe that may be leftovers from the solar system s formation. B. Label gravity, inertia, and orbit in the diagram below. 121