A force is a push or pull on an object. For example, when you sit at a picnic table, you exert a force on the bench and the bench exerts a force back on you. Every force has a certain strength and direction.
1. Force and Motion A Science A–Z Physical Series Word Count: 1,746 Written by Ron Fridell Visit www.sciencea-z.com www.sciencea-z.com
3. Table of Contents ro nn gg g iiin n g spinning i n n n lling p n Introduction............................................................... 4 p s iiin sl ssp ppr n Motion Needs a Force............................................... 5 jumping i s e in th g The Laws of Motion.................................................. 7 Newton’s First Law of Motion................................. 8 c r e e p i n g Newton’s Second Law of Motion........................... 10 Introduction Newton’s Third Law of Motion.............................. 12 Each word above tells how objects can move. Types of Forces........................................................ 13 Some travel in a straight, flat line, such as a ball Gravity................................................................... 13 rolled along the ground. When you kick the ball Friction................................................................... 16 into the air, it flies in a curved path until it bounces Magnetism............................................................. 18 on the ground. A merry-go-round turns in circles, Force, Motion, and Work........................................ 20 while lightning strikes in a jagged path. Energy....................................................................... 21 Motion is all around you. It’s even inside you Potential and Kinetic Energy................................. 21 as your blood moves through your body. Energy Transfer...................................................... 23 For every motion, there is a force. In this book, Conclusion............................................................... 24 you will learn about the forces that make things Glossary.................................................................... 25 move, stop, and change direction or speed. Index......................................................................... 26 3 4
4. Motion Needs a Force Machines can provide force to create motion. A To make something move, a force is needed. huge engine can produce Lifting, pushing, and pulling are all forces. At enough force to propel a times, a person provides the force. massive rocket into space. When you lift a stack of books, Motors in cake mixers, push a broken-down car, or pull fans, and blenders use a rope in a tug-of-war game, you electrical force to make are the force. things move. push Nature can also be a strong force. Breezes make leaves dance in the treetops. Ocean waves make boats bob up and down. Forces deep underground lift cause earthquakes that make the ground tremble. pull Earth’s crust is made up of moving plates. These plates can shift in several ways. The force of these shifting plates causes earthquakes. 5 6
5. The Laws of Motion Newton’s First Law of Motion Isaac Newton is known for his three laws of Much of what we know about motion, which explain how things move. The motion comes from scientists first part of the first law says that an object at who lived hundreds of years ago. rest will remain at rest unless a force acts upon it. They conducted tests on moving For example, your bike will stay parked where it objects. British scientist Isaac is unless something moves it. Newton discovered gravity, the invisible force that causes things to fall toward the ground. You may have heard that he was sitting under a tree one day when a falling apple In government, a law is a rule that hit him on the head. The story goes that this people must obey. In science, a law is accident led him to discover gravity. a statement that explains how things always work in the physical world. Isaac wasn’t really hit on the head by a falling apple. That’s just a myth. But over time, he The second part of this law states that an object observed that objects always fall down toward in motion will continue moving in the same way the ground. And that led him to discover gravity. unless a new force changes the motion. It will keep moving at the same speed and in the same direction. SIr Isaac Newton (1642–1727) So your moving bike will continue moving at the Isaac Newton lived on a farm in England. Instead of farming, he chose to study math and science in London. same speed and in the same direction until you At age 27, Isaac did experiments with light. He was the pedal faster, first to suggest that light was actually coast, brake, made up of all the or turn. colors of the rainbow. He is now one of the most famous scientists of all time! Why is it important to wear a seat belt? Think about Newton’s first law of motion. 7 8
6. Every object, at rest or in motion, has inertia. Newton’s Second Law of Motion Inertia is what makes an object keep doing what Mass is the amount of matter in an object. it is already doing. An object at rest will remain Newton’s second law of motion deals with mass at rest unless a force moves it. An object in motion and motion. It explains that an object’s motion will remain in motion unless a force stops it. depends upon its mass and the amount of force Newton’s first law of motion is sometimes called needed to move that mass. the Law of Inertia. Why is it easier to move a small rock than a boulder? Since a boulder has much more mass than a small rock, much more force is needed to overcome the boulder’s inertia. Much less force is needed to make small rocks move, stop, or change direction. An object has inertia whether it is at rest or in motion. In either case, a force must overcome that inertia. The force may create motion, stop it, or change its direction. 9 10
7. Newton’s Third Law of Motion Newton’s third law of motion says This train has a large mass and a high that for every force or action, there speed, so it has a is an equal and opposite reaction. lot of momentum. It would take a very So, if you lift a 9 kg (20 lb.) box, strong force to slow the box pulls down with or stop this train! an equal force of 9 kg (20 lbs.) in the opposite This law of motion also deals with speed. direction. Speed measures how far something travels in This law explains a certain amount of time. The faster an object why a balloon full of air moves, the more force is needed to stop it. goes flying when you let Think about a locomotive speeding down a it go without tying it. Air track. It has a lot of mass and speed. So a great rushes out the open end. deal of force will be needed to overcome the An opposite force pushes inertia of the train’s motion and make it stop. on the far end of the balloon, making it fly. Momentum is the amount of force in a moving object. The more speed and mass a moving object has, the more momentum it has. A small, slow scooter can stop much faster than a massive, fast train can. As air rushes out of the B open balloon (arrow A), an equal force pushes in the opposite direction A (arrow B). In which Velocity describes an object’s speed in a certain direction will it fly? direction. Scientists calculate momentum by using this formula: mass x velocity = momentum 11 12
8. Types of Forces Imagine standing next to a tall building. Both you and the building have mass, so you both You’ve learned that people, machines, and exert a gravitational pull. The building has much nature can all provide a force. Here are three more mass, so it pulls on you much more than more forces you should be familiar with. you pull on it. Isaac Newton observed that all objects pull on each other due to gravity. His findings became known as the Universal Law of Gravity. The pull of gravity depends on the mass of the objects and the distance between them. Greater masses have a stronger pull. If they move farther apart, the pull between them gets weaker. Mass, Distance, and Gravity In which situation is the gravitational pull strongest? In which situation is the gravitational pull weakest? A B However, both you and the building are standing on Earth. Planet Earth is far more massive than either you or the building. So • two small masses • two small masses Earth exerts a much stronger gravitational • short distance • greater distance pull on you and the building than you and C D the building exert on each other. On Earth, an object’s mass is measured as weight. The greater the mass of an object, the • t wo large masses • two large masses more the force of Earth’s gravity pulls on it, •s hort distance • greater distance Strongest: C; Weakest: B and the more it weighs. 13 14
9. Friction Friction is an invisible force that both slows down moving things and heats them up. One kind of friction you know well is sliding friction. When you rub your hands together, they create sliding friction. This friction produces heat energy, causing your skin to warm up. A rolling bowling ball has another kind of friction—rolling friction. Rolling friction will slow down a moving object, but not as much as sliding friction does. Placing rollers If you were in space, twice as far from the center of Earth as you under a box are now, the pull of gravity would be only one-quarter as strong. You would still have the same mass, but you would weigh only makes it easier one-quarter of what you weigh on Earth. to move. Rolling reduces friction. Now let’s move from Earth to the Sun. The Sun has much more mass than anything else in our solar system. It exerts enough gravitational pull to keep all the planets from flying off into space. Instead, they orbit the Sun. Rough surfaces exert more Remember, distance is also important. While friction against each other the Sun exerts more gravitational pull than Earth, than smooth surfaces do. you are much closer to Earth than to the Sun. Putting a lubricant such as oil or grease between two So Earth’s gravity keeps you from getting pulled surfaces will reduce friction. up to the Sun! 15 16
10. Fluid friction happens when an object moves Magnetism through a fluid, such as water or air. You can Magnetism can push or pull objects made of move your finger through water faster than honey certain metals, such as iron. Magnets also push because honey creates more fluid friction than and pull each other. water. Engineers design airplanes to reduce the fluid friction caused The force of lines of force by the air. Fluid magnetism can move objects N+ friction can help + ++ without touching + objects move more + +– them. How? + – easily, such as – – a puck floating Invisible lines – – – on air in an air of force enter and – S hockey game. leave at opposite ends, or poles, of magnetic a magnet. The field magnet attracts certain objects that enter its magnetic field. Each magnet has a north and south pole. Two opposite poles attract each other. But two of the same poles repel, or push each other away. Imagine that S N you wanted S S to race your friends N S down a snowy hill. How could thinking N N Opposite poles attract. about friction help you win the race? Like poles repel. 17 18
11. 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. force x distance = work Generators like these use magnets to make electricity flow through wires. Magnets can make electricity flow through most work wires. Power plants use magnets to produce On the other hand, electricity can turn some 4 blocks, no baby 4 blocks, with baby metals into magnets. Electricity and magnetism are part 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 a stroller with to the paper clips a baby in it because an empty stroller has less mass. Also, if the wire weren’t pushing the stroller four city blocks requires twice as much work touching the battery? as pushing it only two blocks. 19 20
12. Energy Can you guess which type of energy a tank full of gasoline contains? Fuels such as gasoline have Energy is the ability to potential energy. Burning gas to make a car move do work. The more energy changes that potential energy into kinetic energy. you have, the more work you can do. Potential and Kinetic Energy Energy comes in two Why can’t a motorcycle that is basic types: potential and kinetic. Take a rubber out of gas be ridden? Think about band, for example. When you pull it back, it has potential and kinetic energy. potential energy. This energy is stored and ready to use. The farther back you pull the rubber band, the more potential energy it has. When you let it go, The food you eat also has potential energy. the rubber band flies away. That potential energy Plants gather energy from the Sun. Animals get changed into kinetic energy. Kinetic energy is energy from eating plants and other animals. energy in motion. When you eat food, you store up energy. Then that energy makes your heart beat and makes your lungs expand and contract. 21 22
13. Energy Transfer Conclusion Energy can be changed, or converted, from You’ve learned that it takes force to make things one kind to another. Energy can also be moved, move or stop and that forces affect the speed and or transferred, from one object or place to another. direction of a moving object. You read about Isaac Changing and moving energy is called energy Newton’s laws of motion. You learned that the mass of an object is related to the force that makes Solar energy becomes it move. Simple pushes and pulls as well as lifting stored energy in plants. are examples of force. Other types of force include When you eat, the stored gravity, friction, and magnetism. People, machines, energy in plants can be and nature can all exert a force. transferred to your muscles Work is done when a force moves an object and converted into motion in over a distance. It takes energy to do work. your arms. Then this motion This energy is either potential (stored) or kinetic can be transferred to moving (motion). Energy can transfer by moving or by drumsticks. The sticks can changing from one type of energy to another. make a drum vibrate, Forces and motion are everywhere, all around you. creating sound energy. The energy stored in batteries can make a toy move and make sounds. It can also be converted into light energy in a flashlight. What other examples of energy transfer can you think of? 23 24
14. 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. 4) 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 position, 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. 4) 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) inertia the tendency of an object to Index resist change in the direction lines of force, 18 Universal Law of or speed of its motion (p. 9) Newton, Isaac, 7–10, Gravity, 13 kinetic energy the energy that a moving body 12, 13, 24 speed vs. velocity, 11 has because of its motion (p. 21) first law of motion, 8, 9 types of friction second law of fluid friction, 17 magnetism a force that pushes and pulls motion, 10, 11 rolling friction, 16 certain metals (p. 18) third law of sliding friction, 16 motion, 12 25 26
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