CHAPTER 8 Motion and LESSON 3 What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After 5. To apply a force, one object must be touching another object. 6. If an object is at rest, there are no forces acting on it. Key Concepts What are different types of forces? What factors affect the force of gravity? What happens when forces combine? How are balanced and unbalanced forces related to motion? What is force? Today you might have pushed open the classroom door or pulled the zipper on your backpack. In both of these cases, you are applying a force. A force is a push or a pull on an object. Force has size and direction. Just as you used arrows to show the size and direction of velocity and acceleration, arrows can show the size and direction of a force. The unit for force is the newton (N). You use about 1 N of force to lift a stick of butter or a medium-sized apple. Contact When a karate expert breaks a board, his or her hand applies a force to the wood. A contact force is a push or a pull one object applies to another object that is touching it. Contact forces can be small, such as a finger pushing a button. They can also be large, such as a wrecking ball crashing into a building. Noncontact Have you ever held a magnet near a paper clip and made the paper clip move without touching the two objects together? A force that one object applies to another object without touching it is a noncontact force. The force that pulls the paper clip is a noncontact force. Another noncontact force, an electrical force, sometimes causes socks to stick together when they are pulled out of the dryer. Identify the Main Ideas Fold a sheet of paper into three columns. Label them (K ) for what you already know about forces, (W ) for what you want to learn, and (L) for the facts that you learned. Fill in the third column after you have read this lesson. Key Concept Check 1. Apply What are some examples of contact and noncontact forces you have experienced today? Reading Essentials Motion and 137
Key Concept Check 2. State What factors affect the force of gravity? Visual Check 3. Predict Look at the objects on the left side of the figure. What would happen to the force of gravity if the mass of the objects in A became as large as the objects in B? A B 1 m Gravity A Noncontact Force When you jump off a step, the force of Earth s gravity pulls you toward Earth. Did you know that your gravitational force pulls Earth and other objects toward you? Gravity is an attractive force that exists between all objects that have mass. Mass is the amount of matter in an object. Both you and Earth have mass, so both you and Earth pull on each other. Why doesn t Earth move toward you? Earth does move toward you. However, Earth s mass is much greater than your mass, so it is easier to see the effect of Earth s force on you than that of your forces on Earth. The size of a gravitational force depends on the masses of the objects and the distance between them. Gravitational force depends on mass. See the left side of the figure below. It shows that, if the mass of an object increases, the gravitational force increases between it and another object. The gravitational force between you and Earth is large because of Earth s large mass. The force holds you on Earth s surface. The gravitational force between you and your pencil is small because you and the pencil have relatively small masses. Gravitational force depends on distance. See the right side of the figure below. It shows that the gravitational force between two objects 1 m apart is four times greater than the gravitational force between the same objects when they are 2 m apart. Effect of Mass and Distance on Force of Gravity Effect of Mass Effect of Distance on Force of Gravity on Force of Gravity 1 m 1 m 2 m The distances between the marbles in diagrams A and B are the same. The force of attraction between the marbles in B is greater than in A because the marbles in B have more mass. C D The masses of the marbles in diagrams C and D are the same. The force of attraction between the marbles in D is less than in C because the distance between the marbles is greater. 138 Motion and Reading Essentials
Mass and Weight in Different Locations Location Mass (kg) Weight (N) Earth 90 900 Interpreting Tables 4. Infer What would be the weight of a girl on the Moon if she weighs 600 N on Earth? Moon 90 150 Mass and weight are different. Weight is a measure of the gravitational force acting on an object s mass. Therefore, weight depends on the masses of the objects and the distance between them. When comparing the weight of two objects at the same location on Earth, the object with more mass has a greater weight. The weights of the same objects on the Moon are less because the mass of the Moon is less. The relationship between weights and mass on Earth and the Moon is shown in the table above. Because it has less mass, the Moon s gravity is only 1/6 that of Earth s. Therefore, an astronaut s weight on the Moon is 1/6 the astronaut s weight on Earth. Friction A Contact Force Rub your finger across your desk. Then rub it across a piece of your clothing. What did you feel? It s easy to run your finger over your desk because it is smooth. On your clothing, you felt a force called friction. Friction is a contact force that resists the sliding motion of two surfaces that are touching. Rough surfaces tend to produce more friction. Smooth surfaces produce less friction. Effects of Friction Push your book across your desk. The book stops when you stop pushing it. The force of friction acts in the opposite direction of the book s motion. A heavier book is more affected by friction than a lighter one. If you want to move a box of heavy books by sliding the box across the floor, you have to use a large pulling force. The force of friction between the box and the floor acts in the opposite direction to the force of your pull. To move the box, your pulling force must be greater than the friction force. Air Resistance When you drop a sheet of paper, it slowly drifts downward. Friction between the air and the paper s surface slows its motion. Air resistance is the frictional force between air and objects moving through it. When you crumple the paper into a ball, less surface area is in contact with the air. As air resistance decreases, the ball of paper falls more quickly. 5. Apply Which has more friction between them: two pieces of sandpaper or two pieces of plastic? 6. Categorize Is air resistance a contact force or a noncontact force? Explain your answer. Reading Essentials Motion and 139
7. Predict what would happen if the dog s force increased to 60 N while the girl s force stayed at 50 N. What would be the net force? Key Concept Check 8. Describe What can happen when forces combine? Visual Check 9. Draw an arrow to show the acceleration that would result if the pull force on the right side of the figure was reduced to 150 N while the friction force stayed the same. What would be the net force? Balanced Combining Suppose you need to pull your desk away from the wall to get something that fell behind it. When you pull, the desk will not move, so you ask a friend to help you. With both of you pulling, the desk moves. You each applied a force to the desk, and you applied it in the same direction. The combined force was great enough to move the desk. When more than one force acts on an object, the forces combine and act as one force. The sum of all the forces acting on an object is called the net force. When forces act in the same direction, they add together to form one net force. Sometimes forces act in opposite directions. Imagine watching a dog pulling on its owner s leash. The dog applies a force to the left. The owner pulls the leash to the right, applying an opposite force. When two forces act on the same object in opposite directions, you must include the direction of the forces when you add them. The positive direction is usually to the right. The girl s force on the dog s leash is +50 N. The dog s force on the leash is the same size as the girl s but in the negative direction. The dog s force is -50 N. The net force on the leash is 50 N + (-50 N) = 0 N. The dog doesn t move. Balanced If the net force on an object is 0 N, the forces acting on the object are called balanced forces. Look at the figure below. The net force on the left side of the figure is 0 N. The forces acting on the object are balanced. Unbalanced When the net force on an object is not 0, the forces acting on the object are unbalanced forces. The net force on the right side of the figure below is 100 N to the right. The forces acting on the object are unbalanced. Acceleration is in the direction of the larger force. Combining Unbalanced 50 N 50 N Acceleration Friction = 100 N Pull = 200 N The forces are balanced. They are equal in size and opposite in direction. 50 N + (-50 N) = 0 The forces act in opposite directions, but they are not balanced. 200 N + (-100 N) = 100 N 140 Motion and Reading Essentials
Unbalanced and Acceleration When you kick a soccer ball, its motion changes. The forces on the ball are unbalanced. When unbalanced forces act on an object, the object s velocity changes. Unbalanced forces can change either the speed or the direction of motion. 10. Describe how unbalanced forces affect an object. Change in Speed Look at the figure below. In the top left image, the train is pulling away from the station. The force of the engine pulling the train forward is greater than the force of friction holding it back. Visual Check The forces on the train are unbalanced, so it accelerates. The train speeds up. The train accelerates in the direction of the larger force, which is the forward pull of the engine. 11. Analyze What would happen if the friction in the bottom right image was suddenly greater than the force of the engine? Change in Direction When the train goes around a curve, as shown in the bottom left image of the figure, the track exerts a sideways force on the train s wheels. These unbalanced forces change the train s motion by changing its direction and its velocity. The train accelerates. The Effect of Balanced and Unbalanced Unbalanced = Acceleration Balanced = Constant Motion Changing speed Resting Gravity Friction from rails Force of engine Force of track Changing direction Constant velocity Force of engine Force from track on wheels Reading Essentials Friction from rails Force of engine Motion and 141
Make a small four-door shutterfold to record what you learn about forces. Contact Balanced Non- Contact Unbalanced Key Concept Check 12. Contrast How do balanced and unbalanced forces affect motion? 13. Explain What is inertia? 14. Infer Before the crash, why does the car move with a constant velocity? Balanced and Constant Motion How do balanced forces affect an object s motion? The forces acting on a train that is sitting still on a track are balanced. The force of gravity pulls the train down toward Earth. The track pushes in an upward direction with an equal force. If the forces were not balanced, the train would sink into the ground or float upward, depending on the direction of the greater force. When a train moves along a straight track, the force from the engine moves the train forward. The force of friction between the wheels and the track is equal in size to the engine s force, but in the opposite direction. The forces acting on the train are balanced. The train does not accelerate, but it moves at a constant velocity. When balanced forces act on an object, the motion is constant. The object is either at rest that is, completely still or it is moving at a constant velocity. and Newton s Laws of Motion Isaac Newton was an English scientist who lived in the late 1600s. He developed three important rules about motion called Newton s laws of motion. Newton s First Law of Motion As you just read, when balanced forces act on an object, the object s motion is constant. According to Newton s first law of motion, if the net force acting on an object is zero, the motion of the object does not change. Newton s first law of motion sometimes is called the law of inertia. Inertia is the tendency of an object to resist a change in its motion. Newton s first law explains why safety belts can protect passengers in a car from danger. Engineers who study car crashes often use crash-test dummies as models of car passengers. Because of inertia, crash-test dummies without safety belts keep moving forward after a car crashes into a wall. Before the crash, the car and the test dummies move with a constant velocity. When the car crashes into the wall, unbalanced forces act on the car, and it stops. However, the dummies, which are not attached to the car, continue to move with a constant velocity because of their inertia. They continue to move until an unbalanced force changes their motion. If that unbalanced force does not come from a safety belt, it could come from the windshield. 142 Motion and Reading Essentials
Newton s Second Law of Motion As you have read, when an unbalanced force acts on an object, the object accelerates. Newton s second law of motion states that acceleration of an object is equal to the net force exerted on the object divided by the object s mass. This means that the greater the mass, the greater the force must be to accelerate the object at the same rate. Acceleration is in the same direction as net force. You can write Newton s second law of motion as an equation: force (f ) acceleration (a) = mass (m) For example, think about a bowler exerting a force by throwing a 4-kg bowling ball. A force of 80 N is needed to cause a mass of 4 kg to accelerate to 20 m/s every second. The greater the force is that the bowler exerts on the ball, the faster the ball will accelerate as long as the mass of the ball remains constant. If the bowler wanted to accelerate a mass of 8 kg at the same rate (to 20 m/s every second), the bowler would need a force of 160 N. 15. State What effect does an increase in an object s mass have on the acceleration of the object if net force remains the same? Newton s Third Law of Motion Imagine a runner on a starting block. The runner s feet exert a downward force on the block. The block exerts an upward force on the runner s feet. When the runner pushes on the block to start the race, the block pushes back, helping the runner get a fast start. Newton s third law of motion says that for every action there is an equal and opposite reaction. In other words, when one object exerts a force on a second object, the second object exerts a force of equal size in the opposite direction on the first object. These equal and opposite forces are called force pairs. The runner s feet exert a downward force on the starting block, and the starting block exerts an upward force on the runner s feet. This is the force pair. Force pairs are not the same as balanced forces. Balanced forces act on the same object. Recall the figure of the train, shown earlier in the lesson. The force from gravity and the force from the train track are balanced and they act on the same object the train. Think about Newton s laws as you move through your day. Together, the laws help you understand why objects move as they do. 16. Contrast force pairs and balanced forces. Reading Essentials Motion and 143
Mini Glossary air resistance: the frictional force between air and objects moving through it contact force: a push or a pull one object applies to another object that is touching it force: a push or a pull on an object friction: a contact force that resists the sliding motion of two surfaces that are touching gravity: an attractive force that exists between all objects that have mass Newton s first law of motion: the law that states that if the net force acting on an object is zero, the motion of the object does not change Newton s second law of motion: the law that states that acceleration of an object is equal to the net force exerted on the object divided by the object s mass Newton s third law of motion: the law that states that for every action there is an equal and opposite reaction noncontact force: a force that one object applies to another object without touching it 1. Review the terms and their definitions in the Mini Glossary. Write a sentence that illustrates an example of one of Newton s laws of motion. 2. Use terms from the lesson to fill in the missing bubbles in the concept map below. What do you think electrical force Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? Types of magnetism contact kick ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON 144 Motion and Reading Essentials