3 Friction: A Force That Opposes Motion

Transcription

1 CHAPTER 1 SECTION Matter in Motion 3 Friction: A Force That Opposes Motion BEFORE YOU READ After you read this section, you should be able to answer these questions: What is friction? How does friction affect motion? What are the types of friction? How can friction be changed? National Science Education Standards PS 2c What Causes Friction? Suppose you are playing soccer and you kick the ball far from you. You know that the ball will slow down and eventually stop. This means that the velocity of the ball will decrease to 0. You also know that an unbalanced force is needed to change the velocity of objects. So, what force is stopping the ball? Friction is the force that opposes the motion between two surfaces that touch. Friction causes the ball to slow down and then stop. What causes friction? The surface of any object is rough. Even an object that feels smooth is covered with very tiny hills and valleys. When two surfaces touch, the hills and valleys of one surface stick to the hills and valleys of the other. This contact between surfaces causes friction. If a force pushes two surfaces together even harder, the hills and valleys come closer together. This increases the friction between the surfaces. STUDY TIP Imagine As you read, think about the ways that friction affects your life. Make a list of things that might happen, or not happen, if friction did not exist. READING CHECK 1. Describe What is friction? TAKE A LOOK 2. Explain Why can t you see the hills and valleys without a close-up view of the objects? Interactive Textbook 11 Matter in Motion

2 SECTION 3 Friction: A Force That Opposes Motion continued STANDARDS CHECK PS 2c If more than one force acts on an object along a straight line, then the forces reinforce or cancel one another, depending on their direction and magnitude. Unbalanced forces will cause changes in the speed or direction of an object s motion. 3. Identify Two identical balls begin rolling next to each other at the same velocity. One is on a smooth surface and one is on a rough surface. Which ball will stop first? Why? What Affects the Amount of Friction? Imagine that a ball is rolled over a carpeted floor and another ball is rolled over a wood floor. Which surface affects a ball more? The smoothness of the surfaces of the objects affects how much friction exists. Friction is usually greater between materials that have rough surfaces than materials that have smooth surfaces. The carpet has greater friction than the wood floor, so the ball on the carpet stops first. What Types of Friction Exist? There are two types of friction: kinetic friction and static friction. Kinetic friction occurs when force is applied to an object and the object moves. When a cat slides along a countertop, the friction between the cat and the countertop is kinetic friction. The word kinetic means moving. The amount of kinetic friction between moving surfaces depends partly on how the surfaces move. In some cases, the surfaces slide past each other like pushing a box on the floor. In others, one surface rolls over another like a moving car on a road. There is usually less friction between surfaces that roll than between surfaces that slide. Static friction occurs when force applied to an object does not cause the object to move. When you try to push a piece of furniture that will not move, the friction observed is static friction. TAKE A LOOK 4. Describe When does static friction become kinetic friction? a b c There is no friction between the block and the table when no force is applied to the block. If a small force (dark gray arrow) is applied to the block, the block does not move. The force of static friction (light gray arrow) balances the force applied. When the force applied to the block is greater than the force of static friction, the block starts moving. When the block starts moving, kinetic friction (light gray arrow) replaces all of the static friction and opposes the force applied. Interactive Textbook 12 Matter in Motion

3 SECTION 3 Friction: A Force That Opposes Motion continued How Can Friction Be Decreased? To reduce the amount of friction, you can apply a lubricant between two surfaces. A lubricant is a substance that reduces the friction between surfaces. Motor oil, wax, and grease are examples of lubricants. You can also reduce friction by rolling, rather than sliding, an object. A refrigerator on rollers is much easier to move than one that just slides. Another way of reducing friction is to smooth the surfaces that rub against each other. Skiers have their skis sanded down to make them smoother. This makes it easier for the skis to slide over the snow. Critical Thinking 5. Infer How does a lubricant reduce the amount of friction? If you work on a bicycle, you may get dirty from the chain oil. This lubricant reduces friction between sections of the chain. How Can Friction Be Increased? Increasing the amount of friction between surfaces can be very important. For example, when the tires of a car grip the road better, the car stops and turns corners much better. Friction causes the tires to grip the road. Without friction, a car could not start moving or stop. On icy roads, sand can be used to make the road surface rougher. Friction increases as surfaces are made rougher. You can also increase friction by increasing the force between the two objects. Have you ever cleaned a dirty pan in the kitchen sink? You may have found that cleaning the pan with more force allows you to increase the amount of friction. This makes it easier to clean the pan. TAKE A LOOK 6. Identify Why is it important to put oil on a bicycle chain? READING CHECK 7. Identify Name two things that can be done to increase the friction between surfaces. Interactive Textbook 13 Matter in Motion

4 Section 3 NSES PS 2c SECTION VOCABULARY friction a force that opposes the motion between two surfaces that are in contact 1. Describe What effect does friction have when you are trying to move an object at rest? 2. Compare Explain the difference between static friction and kinetic friction. Give an example of each. 3. Compare The figure on the left shows two surfaces up close. On the right, draw a sketch. Show what the surfaces of two objects that have less friction between them might look like. 4. Analyze Name three common lubricants and describe why they are used. 5. Analyze In what direction does friction always act? 6. Identify A car is driving on a flat road. When the driver hits the brakes, the car slows down and stops. What would happen if there were no friction between the tires and the road? Explain your answer. Interactive Textbook 14 Matter in Motion

5 M Forces, Motion, and Energy Answer Key Chapter 1 Matter in Motion SECTION 1 MEASURING MOTION 1. When an object changes position over time in relation to a reference point, the object is in motion. 2. the mountain 3. Owen s desk 4. one positive unit on the y-axis and then one positive unit on the x-axis, or one positive unit on the x-axis and then one positive unit on the y-axis 5. Average speed does not mean constant speed. At times the bird flies faster, and at times it flies more slowly. 6. average speed total distance total time average speed 10 m 2.5 s 4 m/s 7. a straight diagonal line from 0,0 to the point 10 s, 7 m 8. No, velocity needs a direction. 9. Situation What changes Raindrop falling faster and faster Runner going around a turn on a track Car taking an exit off a highway Train arriving at a station Baseball being caught by a catcher Baseball hit by a batter Answers will vary. speed direction speed and direction speed speed speed and direction speed and direction 10. Yes, his speed is increasing. 11. It slows down. 12. positive for increasing speed, negative for decreasing speed 1. Speed is the rate at which an object moves, but velocity is the rate at which an object moves in a particular direction. 2. From left to right: at rest, positive acceleration, constant velocity, deceleration 3. The skateboard starts at rest and speeds up for 15 s. It stops for 5 s. It speeds up for 10 s. Then it turns around and speeds back to its starting point. 4. average speed total distance traveled total time average speed 80 m 40 s 2 m/s SECTION 2 WHAT IS A FORCE? 1. direction and size 2. balanced forces 3. Arrow pointing right is the size of both small arrows combined. 4. The rope would not move. The forces would be balanced. 5. The motion of the object will not change. 6. It will change its motion. 1. Net force is the total force acting on an object. If all of the forces balance each other out, there is no net force. 2. You push down on pedals, you push the handle bars in either direction, and your weight pushes down on the seat N to the right, 13 N down 4. The object on the left will move or accelerate to the right. The object on the right will move or accelerate down. 5. Balanced forces have a net force of 0 N, and the object will not change its motion. If the net force is other than 0, the force is unbalanced and the object will change its motion. SECTION 3 FRICTION: A FORCE THAT OPPOSES MOTION 1. Friction is a force that opposes the motion between two surfaces that touch. 2. Usually, the hills and valleys are very tiny. 3. The ball on the rough surface will stop first because of greater friction, or a greater unbalanced force. 4. when the force that is applied to an object at rest causes the object to move 5. A lubricant makes the surfaces smoother by filling up the valleys and coating the hills. 6. to reduce friction 7. Make surfaces rougher and increase the force between two surfaces. Interactive Textbook Answer Key 81 Forces, Motion, and Energy

6 M Forces, Motion, and Energy Answer Key continued 1. Friction makes it harder to get objects moving and keep them moving. 2. Static friction is friction between two surfaces that are not moving. Kinetic friction is between moving surfaces. Possible examples: Friction between the floor and a refrigerator that is being pushed but is not moving is static friction. Friction between the floor and a refrigerator that has started to slide is an example of kinetic friction. 3. Sketch should show smoother surfaces; peaks and valleys have less relief. 4. Grease, oil, and wax are lubricants. They reduce the friction between surfaces that touch. 5. Friction always acts in a direction opposite the direction of motion. 6. The car would not stop. Friction between the tires and the road is needed to start or stop. 7. The forces must be unbalanced, because the object starts moving (it is accelerating). SECTION 4 GRAVITY: A FORCE OF ATTRACTION 1. force of attraction between objects caused by their masses 2. Their masses are small, so they don t have much gravity. 3. All objects in the universe attract each other because of gravitational force. 4. masses of objects, distance between objects 5. gravity 6. Gravitational force is greater between objects with large masses N 8. We are on Earth, but the sun is very, very far away. 9. As the distance between two objects increases, the gravitational force decreases. As the distance between two objects decreases, the gravitational force increases. 10. Mass is the amount of matter in an object, but weight is the amount of gravitational force on an object ,588 N on Earth, 271 N on the moon 12. Gravity pulls down, and the shelf pushes up. 1. Gravity is a force of attraction between objects. It is determined by the masses of objects and the distance between them. 2. It will get larger. 3. The astronaut has the same mass on Earth and the moon, but the astronaut has a greater weight on Earth than the moon. The mass does not change, because it is the amount of matter in the astronaut. The weight is greater on Earth because Earth s gravity is greater. 4. Earth Planet X Mass of astronaut Weight of astronaut 80 kg 80kg 784 N 1,568 N 5. Student should circle the first pair. The reason given should be that the spheres are the largest pair of masses separated by the smallest distance. Chapter 2 Forces and Motion SECTION 1 GRAVITY AND MOTION 1. The cannonballs hit the ground at the same time. 2. Golf ball should be drawn at the same heights as the table tennis ball m/s; after the third second, the ball is moving 9.8 m/s faster than it was at the end of the second second. Then, 19.6 m/s 9.8 m/s 29.4 m/s. 4. final g t final 9.8 m/s s 2 s 19.6 m/s 5. t final g t 98 m/s 10 s 9.8 m/s s 6. the flat paper 7. The crumpled paper has a greater net force acting on it. 8. when the force of air resistance equals the force of gravity Interactive Textbook Answer Key 82 Forces, Motion, and Energy