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Force and Motion a Science A–Z Physical Series Word Count: 1,484

Force and Motion a Science A–Z Physical Series Word Count: 1,484

Force and A Science A–Z Physical Series Word Count: 1,484

Written by Ron Fridell

Visit www.sciencea-z.com www.sciencea-z.com Key elements Used in This Book Force and The Big Idea: Force and motion are fundamental to all matter in the . A force is anything that can push or pull on an object. Forces influence objects that are at rest or that are already in motion. Isaac ’s three laws of motion involve , , , and Motion . Key forces include , , and . A force is required to do , and generating a force requires . Energy can be stored as energy, or it can have —the energy of motion. Energy can also be converted and exchanged through energy transfer. Objects move in predictable ways. By learning about force and motion, we come to understand how using forces can produce that allow us to be safe and to enjoy ourselves. Key words: attract, direction, , , , energy, energy transfer, , force, friction, gravity, heat energy, inertia, kinetic energy, law, lines of force, magnetism, mass, momentum, motion, , , rest, sound energy, , velocity, , work Key comprehension skills: Cause and Effect Other suitable comprehension skills: Classify information; compare and contrast; elements of a genre; identify facts; interpret charts, graphs, and diagrams; main idea and details Key reading strategy: Visualize Other suitable reading strategies: Ask and answer questions; connect to prior knowledge; retell; summarize

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Illustration Credit: pages 12, 18, 19: © Learning A–Z; page 21 (bottom): Signe Nordin/© Learning A–Z

Written by Ron Fridell Force and Motion © Learning A–Z. Written by Ron Fridell All rights reserved. www.sciencea-z.com www.sciencea-z.com Table of Contents ro ll spinning Introduction...... 4 i spinning p n spinning

s spinning g l Motion Needs a Force...... 5 i spinningri jumping th e ng The Laws of Motion...... 7 Newton’s First Law of Motion...... 8 creeping Newton’s Law of Motion...... 10 Introduction Newton’s Third Law of Motion...... 12 The words above tell about motion. They Types of Forces...... 13 describe different ways things move. Motion Gravity...... 13 is all around you. There is even motion inside Friction...... 16 you. Your blood is moving right now. Magnetism...... 18 For every motion, there is a force. In this Force, Motion, and Work...... 20 book, you will learn about motion and the Energy...... 21 forces that make things move. Potential and Kinetic Energy...... 21 Energy Transfer...... 23 Conclusion...... 24 Glossary...... 25 Index...... 26

3 4 Motion Needs a Force can be a force. A huge engine You need a force to move something. can push a into Lifting, pushing, and pulling are all forces. . A small electric Where do forces come from? motor can the Sometimes the force comes from a blades of a fan or person. When you your books, a toy car. you are the force. If you push a car, Nature is also a you are the force. If you pull a force. Breezes make rope to play tug-of-war, you leaves shake in the are the force. treetops. Ocean push make boats bob in the water. Earthquakes lift shake the ground.

pull

The outer layer of is made of pieces called plates. Earth’s plates move in several ways. The force of these moving plates causes earthquakes. 5 6 The Laws of Motion

Scientists have done tests A law is a rule that people must follow. But on motion for hundreds of in science, law has another meaning. A law explains how things in the world always work. years. British scientist discovered the force we call gravity. You cannot Newton’s First Law of Motion see gravity. But it causes Isaac Newton is known for his three laws things to fall toward the ground. of motion. These laws explain how things move. The first law has two parts. Some people think that Newton discovered gravity when a falling apple hit him on the Part one says that an object at rest will not head. That’s not true. But Newton did notice move unless a force moves it. For example, that things always fell down toward the your bike will stay parked until something ground. He would watch closely and make moves it. notes. That is how he discovered gravity. Part two says that an object will keep SIr Isaac Newton (1642–1727) Isaac Newton lived on a farm in England. Instead of farming, moving in the same he chose to study math and science. He made many way unless a new important discoveries. At age 27, Newton did tests on light. He was the first force changes how person to state that light was it moves. It will keep made up of all moving at the same Why is it important to wear the colors of the a seat belt? Think about rainbow. He is now speed and in the same Newton’s first law of motion. one of the most famous scientists direction. If you pedal of all ! your bike faster or turn, your bike will change speed or direction. If you brake, it will stop.

7 8 Inertia (in-ER-shuh) makes an object keep Newton’s Second Law of Motion doing what it is doing. An object at rest will Newton’s second law of motion has to not move unless a force moves it. An object’s do with mass and speed. Mass is the amount motion will not change unless a force stops of matter, or physical stuff, in an object. For it or changes its speed or direction. Newton’s example, a huge boulder has a lot more mass first law of motion is also called the law than a tiny pebble. of inertia. This law says that an object’s motion depends on how much mass it has and how much force is needed to move it. A boulder has more mass than a small rock, so more force would be needed to move it or to stop it from rolling.

An object has inertia when it is at rest or moving. A force is needed to make the object move, stop, or change direction. 9 10 Newton’s Third Law of Motion Newton’s third law of motion says that every force or has an equal and opposite reaction. So, if you lift a 9 kg (20 lb.) box, the box This train has a large mass and a high speed. This means it has pulls down with an equal a lot of momentum. It would take a very strong force to slow or stop this train! force of 9 kg (20 lbs.).

The second law is also about speed. The This law helps you faster an object moves, the more speed it understand why a balloon full has, and the more force is needed to stop it. of air goes flying if you let it go before you tie it. Air rushes out Think about a train speeding down a track. the open end. An opposite force pushes It has a lot of mass and speed. So a strong on the far end. This opposite force makes force is needed to overcome its inertia and the balloon fly. make it stop.

The force in a moving object is called B momentum. The more speed and mass an object has, the more momentum it has. A 1. Air rushes out the open end (arrow A).

2. An equal, opposite Velocity is an object’s speed in a certain direction. force pushes on the Scientists measure momentum by multiplying an far end of the balloon object’s mass by its velocity. (arrow B). mass x velocity = momentum 3. The balloon flies around the room! 11 12 Types of Forces Imagine standing next to a tall building. You and the building both have mass. So you You have read about how people, machines, both pull on each other because of gravity. and nature can be forces. Now you will read The building has more mass, so it pulls on about three other important forces. you more than you pull on it. Gravity Isaac Newton learned how objects pull on each other because of gravity. The power of gravity’s pull depends on two things. One is the mass of each object, and the other is the distance between them.

Greater have a stronger pull than small masses do. If they are close, the pull is stronger. If they are far apart, the pull is weaker.

Mass, Distance, and Gravity In which situation is the gravitational pull strongest? In which situation is the gravitational pull weakest? However, both you and the building are A B standing on Earth. Earth has much more mass than either you or the building. So Earth pulls • two small masses • two small masses on you and the building much more than you • short distance • greater distance and the building pull on each other. C D On Earth, an object’s mass is the same as its

• two large masses • two large masses weight. The greater its mass, the more the force

• short distance • greater distance

eakest: B eakest: W ; C Strongest: Strongest: of gravity pulls on it, and the more it weighs. 13 14 Friction Friction is another force you cannot see. Friction slows down moving things. It also heats them up. When you rub your hands together, they create friction. This friction produces heat energy, which warms your skin.

A moving bowling ball has another kind If you were up in space, the pull of gravity would be weaker. You of friction. Rolling friction also slows things would still have the same mass, but you would weigh much less. down, but not as much as sliding friction. Earth is huge, but the Sun has way more Placing rollers under a box makes it easier mass than anything else in our solar system. to push. Rolling reduces friction. That means it has a much stronger pull of gravity than anything else nearby. This pull of gravity keeps the from flying off Rough surfaces make more into space. Instead, they circle the Sun. friction against each other than smooth surfaces do. Distance is important to gravity, too. You Putting a lubricant such as are much closer to Earth than to the Sun. So oil or grease between two surfaces will reduce friction. Earth’s gravity keeps you on the ground. 15 16 friction happens when an object Magnetism moves through water or air. Your finger Magnetism is an invisible force. It pulls and moves through water faster than honey pushes objects made of certain metals, such as because honey creates more fluid friction. iron. also push and pull each other. Fluid friction can be The force of Magnetic Around a weaker than sliding friction. A puck magnetism can lines of force floating on air move an object in an air hockey without game moves touching it. faster than How? Invisible a puck sliding lines of force from the magnet pull along a solid magnet surface. on the object. Each magnet has a north and south pole. Two opposite poles attract each other. But two of the same poles push each other away. S N

Imagine that N S you wanted S S to race your friends Opposite poles attract. down a snowy hill. How could thinking N N about friction help you win the race? Like poles repel. 17 18 Force, Motion, and Work

In science, work happens when you move something. The amount of work you do depends on the distance the object moves and the force it takes to move it.

Scientists measure work by multiplying the distance an object moves by the force used to move it. Generators like these use magnets to make electricity flow through wires. force x distance = work

Magnets can send electricity through wires. The magnets past wires to make electricity flow. most work

Magnets can also produce electricity. But electricity can also turn some metals into magnets. Electricity and magnetism are part 4 blocks, no baby 4 blocks, with baby of a single force called electromagnetism.

least work

2 blocks, no baby 2 blocks, with baby What would happen It takes less work to push an empty stroller than to push a to the paper clips stroller with a baby in it. The empty stroller has less mass. if the wire stopped Also, if you move the stroller four city blocks instead of two, touching the battery? you will do twice as much work. 19 20 Energy Can you guess which type of energy the gas in a car has? Fuels have potential energy. Energy is the ability Burning gas to move a car changes that to do work. If you have potential energy into kinetic energy. more energy, you can do more work.

Potential and Kinetic Energy Why can’t a There are two main kinds of energy: motorcycle that is out potential and kinetic. Suppose you pull back of gas be ridden? Think about potential and kinetic energy. on a . It stretches. You have given it potential energy. This energy is stored and The food you eat also has potential energy. ready to use. Let it go, and what happens? When you eat food, you store up energy. Your The rubber band flies away. You turned that body needs that energy. You use energy every potential energy into kinetic energy. Kinetic time you run, jump, or smile. energy is energy in motion.

21 22 Energy Transfer Conclusion Energy can be changed from one kind You have learned that it takes force to make to another. Energy can also be moved from things move or stop. Pushing, pulling, and one object or place to another. Changing lifting are all forces. Other forces include gravity, and moving energy is called energy transfer. friction, and magnetism. You have also learned The energy stored in about Newton’s three laws of motion. foods can move to your It takes energy to move anything, which is muscles. Then it gets called doing work. This energy can be potential changed into motion (stored) or kinetic (motion). We can transfer in your arms. You can energy by moving it from one object to another. move this motion to Or we can change it from one kind of energy sticks that make a to another. Forces and motion are everywhere, drum vibrate. Now all around you. the motion energy becomes sound energy.

Energy in batteries can make a toy move or a flashlight shine. What other examples of energy transfer can you think of? 23 24 Glossary mass the amount of matter, measured on Earth by its direction the way or course toward which weight (p. 10) something moves or faces (p. 8) momentum the strength or force that keeps distance the amount of space between something moving (p. 11) things (p. 13) motion the act of going from one place electromagnetism the combined force of electricity to another; movement (p. 4) and magnetism (p. 19) potential energy the energy a body has because energy transfer the movement of energy from of its , electrical one object to another or the charge, or structure; stored change of energy from one energy (p. 21) form to another (p. 23) speed the rate of movement (p. 8) force the strength or energy that moves an object (p. 4) weight how heavy something is, determined by the pull of friction a force that slows down gravity on the object’s mass moving things (p. 16) (p. 14) gravity the force that pulls things work the act of using force to move toward the center of Earth something over a certain or any other object that has distance (p. 20) mass (p. 7) Index inertia the tendency of an object to resist change in the direction lines of force, 18 third law of or speed of its motion (p. 9) Newton, Isaac, 7–10, motion, 12 12, 13, 24 speed vs. velocity, 11 kinetic energy the energy that a moving body first law of motion, 8, 9 types of friction has because of its motion (p. 21) second law of fluid friction, 17 magnetism a force that pushes and pulls motion, 10, 11 rolling friction, 16 certain metals (p. 18) sliding friction, 16

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