Physical Science by Ann J. Jacobs Genre Comprehension Skill Text Features Science Content Nonfiction Summarize Captions Chart Glossary Forces in Motion Scott Foresman Science 3.12 ì<(sk$m)=bdiebi< +^-Ä-U-Ä-U ISBN 0-328-13841-X
Vocabulary force friction Forces and Motion by Ann J. Jacobs gravity magnetism motion position relative position speed work Photographs: Every effort has been made to secure permission and provide appropriate credit for photographic material. The publisher deeply regrets any omission and pledges to correct errors called to its attention in subsequent editions. Unless otherwise acknowledged, all photographs are the property of Scott Foresman, a division of Pearson Education. Photo locators denoted as follows: Top (T), Center (C), Bottom (B), Left (L), Right (R) Background (Bkgd) Opener: Allsport Photography/Getty Images; 2 David Young-Wolff/PhotoEdit; 4 Roy Ooms/ Masterfile Corporation; 5 John Elk/Bruce Coleman Inc.; 6 (TL) Richard Megna/Fundamental Photographs, (CR) Stone/Getty Images, (BL) Tom Szuba/Masterfile Corporation; 7 Kunst & Scheidulin/Age Fotostock; 8 Jim Cummins/Corbis; 10 Rolf Bruderer/Corbis; 11 Stone/Getty Images; 12 Hennie Wiggers/Zefa/Masterfile Corporation; 14 Taxi/Getty Images; 15 AlaskaStock; 16 Bob Daemmrich/Corbis; 17 DK Images; 18 (CL) Getty Images, (R) Tony Freeman/PhotoEdit; 20 Corbis; 22 Keith Pritchard/Alamy Images; 23 Neil Rabinowitz/Corbis ISBN: 0-328-13841-X Copyright Pearson Education, Inc. All Rights Reserved. Printed in the United States of America. This publication is protected by Copyright and permission should be obtained from the publisher prior to any prohibited reproduction, storage in a retrieval system, or transmission in any form by any means, electronic, mechanical, photocopying, recording, or likewise. For information regarding permissions, write to: Permissions Department, Scott Foresman, 1900 East Lake Avenue, Glenview, Illinois 60025. 3 4 5 6 7 8 9 10 V010 13 12 11 10 09 08 07 06 05
What happens when things change position? When Things Move Suppose you spin a top. You can tell it moves because its location changes. It starts in your hand. Then it spins across the ground. If an object is in a new location, its position has changed. The top is in motion as its position changes. A top also has circular motion. This means it moves in circles around a central point. Ways of Looking at an Object s Position The position, direction, and movement of an object depend on how you look at it. For example, as you walk down a hall, a water fountain may be in front of you. After you walk by it, it is behind you. As you keep walking, the fountain seems to be moving away. A map can help you locate things. A map shows the position of objects in relation to each other. Look at the path on the map above. Tell how you would get from the classroom to the lunch room. Use position words like forward, left, right, and behind to explain your trip. 2 3
Positions of Moving Objects When one thing depends on another thing for its meaning, we say it is relative. Relative position is the position of one object compared with the position of other objects. Look at car 64 below. Its relative position is in front of the other cars. Another car might be leading the race. Then you would say car 64 is behind the lead car. The relative position of car 64 changes depending on the position of other cars. Look at the train below. The locomotive is in front. Two cars are being pulled behind it. Could the cars move in front of the locomotive? They could if the train started moving backward. Relative position can change if objects start moving in a new direction. 4 5
How Fast Things Move Speed is how fast an object changes its position. Speed can be fast. A jet plane moves fast. Speed can be slow. Honey moves slowly. Some things have such a slow speed that you can t see them move! The arms of a tuning fork move very fast. Constant Speed Objects can move at a constant speed. This means that they do not change how fast or slow they move. They are always changing position. The rate at which they change position stays the same. Variable Speed Bumper cars move at a variable speed. An object moving at a variable speed changes speed as it moves. Bumper cars can speed up, slow down, or stop. Honey moves at a slow speed. The motion of these flowers is too slow to see. 6 7
How does force affect motion? The Causes of Motion A force is any push or pull. A force can change an object s position. It can change the direction of an object s motion too. Most forces you use are contact forces. To push or pull an object, you must touch it. A bat makes contact with a ball to move it. The more force you use, the more an object s motion changes. The harder you push on a cart, the faster it goes! The mass of an object affects how it moves. You don t need much force to begin pushing an empty cart. When the cart is full of food, it has more mass. Now more force is needed to begin moving the cart. As the cart moves, its wheels rub against the floor. This causes friction. Friction is a contact force that slows an object s motion. Smooth surfaces produce less friction than rough ones. Friction is helpful when you want to slow something down. More force is needed to push three carts than to push just one. 8 9
Motion and Combined Forces You know that pushes, pulls, and friction can change an object s motion. Now think about the forces in tug-of-war. Your team s pull is a force in one direction. The other team s pull is a force in the other direction. If the forces are the same, the rope doesn t move. Suppose everyone on your team pulls together to move the rope. The rope will move in your direction if you have the stronger force. So get more friends to help! Lots of forces cause a bike to change its motion. You push on the pedals. You push on the handlebars and shift your weight to turn. Friction between the tires and the ground slows your forward motion. Going up hills takes more force. You may gain too much speed going down hills. You pull on the brakes to slow down. All these forces work together to keep your bike moving. 10 11
Gravity and Magnetism A non-contact force is a push or pull that affects an object without touching it. Gravity is a non-contact force that pulls objects toward each other. Gravity pulls everything on Earth toward Earth s center. Weight is the amount that gravity pulls on an object. Objects weigh less on the Moon because the Moon has less gravity than Earth. The pull of gravity is less the farther you are from the center of Earth. You weigh less on top of a mountain than you do at it s base. The pull of gravity also depends on matter. Objects with more matter have more mass. The more mass, the stronger the pull of gravity. Magnetism is another non-contact force. Magnets pull on objects with certain kinds of metal such as iron in them. You might use a magnet to pull a steel paper clip across your desk. Steel is a metal with iron in it. Magnets do not affect objects that lack these metals. This magnet does not attract crayons since they lack metal the magnet can attract. This magnet attracts paper clips because they have iron. Gravity pulls skydivers toward the ground. 12 13
How do simple machines affect work? Work In science, work means using force that moves an object. You do work when you rake leaves or kick a ball. How much work you do depends on how much force you use. It also depends on how far you move an object. Work is NOT done if an object does not move. Try pushing a big snowball. If the snowball does not move, no work is done. You may be using force, but you are not doing work. 14 When Work Is Done Activity Thinking about a math problem Turning a jump rope Holding a puppy Lifting a puppy Pulling on a locked door Opening an unlocked door Trying to scoop rock-hard ice cream Work No Yes No Yes No Yes No How much work do you do in a day? To find out, you would have to keep track of all the pushing and pulling you do. Then you would have to see how far those pushes and pulls moved things. Putting a book on a shelf is work. What if you put the book on a shelf twice as high? You would do twice as much work! You would also do twice as much work if you put a book that weighed twice as much on the lower shelf. 15
Some Simple Machines Machines can t reduce the amount of work that needs to be done. But they can help make your work easier. Here are six kinds of simple machines. Inclined Plane Look at the man pushing the cart below. Without the ramp, the man would have to lift the cart into the truck. That would take a lot of effort. He is using a machine called an inclined plane. An inclined plane, or ramp, is a slanting surface. It connects a low spot to a higher one. The mover pushes with less force, but he must push for a longer distance. The same amount of work gets done, but with less effort. Wedge A wedge is a simple machine. It is used to split, cut, or fasten things. A wedge has one or two slanted sides that form a sharp edge. A wedge can move through material and separate it. A knife is a wedge that can cut a pie. An axe head is a wedge that can split wood. 16 17
Screw A screw is an inclined plane wrapped around a center post. The spiral slide in the picture looks like a big screw. Screws are used to hold things together. They are used to raise and lower things too. The lid on a jar is a screw. The lid raises as you open the jar. The lid lowers as you close the jar. Support Lever A lever is another simple machine. A lever is a bar that rests on a support. Levers lift and move things. A seesaw is a lever. Push down on one side of a seesaw. Then watch the person on the other side go up! 18 19
More Simple Machines Wheel and Axle Whenever you open a door, you use a simple machine. It is called a wheel and axle. The doorknob is a wheel. The post that attaches to its center is an axle. Turning the knob takes less force than turning the small axle with your fingers. Since the knob is larger, however, you must turn it a longer distance. The rides you see below both use a big wheel and axle. Instead of turning the wheel, a motor in these rides turns the axle. The motor turns the axle a small distance. But the distance the axle turns the wheel is great. This makes the ride fun. The motor must put a great force on the axle to do this. Do you think the people on this ride are having fun? 20 21
Pulley Pulleys are simple machines that can help move things in the right direction. The sails of a boat are attached to pulleys. A pulley has a wheel that turns on an axle. The sailors pull on a rope that wraps over the wheel of each pulley. This causes the sail to move in the proper direction. You live in a world of motion. Look around. What is moving quickly? What is moving slowly? What simple machines will you use today to make moving things easier? 22 23
Glossary force friction gravity magnetism motion position relative position speed work any push or pull a force that goes against the motion of an object a non-contact force that pulls objects toward each other a non-contact force that pulls on certain kinds of metal such as iron a change in the position of an object the location of an object the position of one object compared with the position of other objects how fast an object changes its position when an object is moved by a force What did you learn? 1. What is the difference between constant speed and variable speed? 2. Would it take more force to begin moving an empty wagon or a wagon loaded with pumpkins? Why? 3. What is the difference between a contact force and a non-contact force? Give examples of each. 4. In this book you have read about work and simple machines. Write to explain how simple machines make work easier without lessening the amount of work that gets done. Use examples from the book. 5. Summarize Turn to p. 11. Summarize the forces used when riding a bike. 24