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Transcript
HIGH SCHOOL SCIENCE Physical Science 3: Forces WILLMAR PUBLIC SCHOOL 2013-2014 EDITION C HAPTER 3 Forces In this chapter you will: 1.Describe force. 2.Explain how the motion of an object is affected when balanced and unbalanced forces act on it. 3.Compare and contrast the four kinds of friction. 4.Describe the path of a projectile. 5.Describe Newton’s Three Laws of Motion. 6.Describe Momentum. S ECTION 3.1 Forces L OREM O BJECTIVES I PSUM : 1. Define Lorem ipsum force, and dolor give sitexamples amet of forces. 2. Consectetur Describe howadipisicing forces combine elit, sed and doaffect eiusmod tempor incididunt ut labore et dolore magna motion. aliqua. 3. Ut enim ad minim veniam, quis exercitation Vocabulary: ullamco laboris nisi ut aliquip ex commodo force consequat. spring scale 4. Duis aute irure dolor in in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Newtons balanced force unbalanced force net force Suppose your teachers asked you to move a heavy desk in your classroom. The desk will not move by itself. How would you move it? You might get on one side of the desk and start pushing. Or you might grab the legs and start pulling. Either way, you would be using a force. A force is needed to put the desk in motion. A force is a push or a pull that acts on an object. A force can cause a resting object to move, or it can accelerate a moving object by changing the object's speed or direction. Forces differ in size and direction. You could use a small amount of force or a lot of force. A force can be left or right, or up or down. Forces are often easy to measure. In fact, if you've ever shopped at a grocery store, you may have measured forces using a spring scale. A spring scale is the tool used to measure force. The stretch of the spring in the scale depends on the amount of weight (a type of force) acting on it. As more fruit is placed on the scale, the spring is stretched farther and the scale reading increases. Force is measured in Newtons, abbreviated as N. One Newton is the force that causes a 1-kilogram mass to accelerate at a rate of 1 meter per second each second (1 m/ s2). In fact, 1 Newton is equal to 1 kilogram-meter per second squared (1 N = 1 kg·m/s2). Even when an object is not in motion, forces are acting on it. The floor is actually pushing up on the desk, at the same time, gravity is pulling down on it. The size of the forces are the 2 same, but in different directions. Whenever forces are the same in size but opposite in direction, they are balanced forces. An object is not in motion will never move if balance forces act on it. How can you move the desk? You can move it because you unbalance the forces. The size of your force is greater than the force of air pushing on the other side of the desk and the friction. Unbalanced forces act in the opposite direction but differ in size. If unbalanced forces act on an object, the object will always more in the direction of the greater force. Force is a vector and is combined using vector addition. The net force is the overall force acting on an object after all the forces are combined. Section Review: 1. How do forces change the motion of an object? 2.Why does a heavy chair not move by itself? 3. What tool do you use to measure force? 4.What is the unit of force? 5. What is the difference between a balanced force and an unbalanced force? 6.If an unbalanced force acts on a ball, the ball will always move in which direction? When the forces on an object are balanced, the net force is zero and there is no change in the object's motion. Balanced forces have zero net force and no motion. When the forces on an object do not balance, the net force is unbalanced and the object accelerates. Unbalanced forces have net force that are not zero and accelerate. 3 S ECTION 3.2 Friction L OREM O BJECTIVES I PSUM : 1. Describe Lorem ipsum how dolor friction sitopposes amet motion. 2. Consectetur Identify types adipisicing of friction.elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Vocabulary: 3. Ut enim ad minim veniam, quis exercitation friction ullamco laboris nisi ut aliquip ex commodo consequat. static friction 4. Duisfriction aute irure dolor in in voluptate velit esse sliding cillum dolore eu fugiat nulla pariatur. rolling friction fluid friction air resistance Friction is the rubbing of one object against another. Friction is a force that resists, or works against, motion. It slows down moving objects and creates heat. Try rubbing your hands together; you will feel them getting warmer and warmer. Friction occurs because no surface is perfectly smooth. Even surfaces that look smooth to the unaided eye appear rough or bumpy when viewed under a microscope. All moving objects are subject to friction, a force that opposes the motion of objects that touch as they move past each other. Friction acts at the surface where objects are in contact. Friction can work for or against us. You could not walk without friction to keep your feet from sliding on the ground. Without friction, you could not pick things up and hold them. Too much friction between moving parts in a car engine can cause the parts to wear out. You know that friction produces heat. That’s why rubbing your hands together makes them warmer. But do you know why the rubbing produces heat? Friction causes the molecules on rubbing surfaces to move faster, so they have more heat energy. Heat from friction can be useful. It not only warms your hands. On the other hand, heat from friction can be a problem inside a car engine. It can cause the car to overheat. To reduce friction, oil is added to the engine. Oil coats the surfaces of moving parts and makes them slippery so there is less friction. There are four main types of friction: static friction, sliding friction, rolling friction, and fluid friction. 4 Static friction is the friction force that acts on objects that are not moving. Static friction always acts in the direction opposite to that of the applied force. Without this static friction, your feet would slip out from under you, making it difficult to walk. Sliding friction is a force that opposes the direction of motion of an object as it slides over a surface. Because sliding friction is less than static friction, less force is needed to keep an object moving than to start it moving. That’s why it’s easier to slide a piece of furniture over the floor after you start it moving than it is to get it moving in the first place. Rolling friction is the friction force that acts on rolling objects.Rolling friction is much weaker than sliding friction or static friction. This explains why it is much easier to move boxes on a wheeled dolly than by carrying or sliding them. Fluid friction is friction that acts on objects that are moving through a fluid. A fluid is a substance that can flow and take the shape of its container. Fluids include liquids and gases. If you’ve ever tried to push your open hand through the water in a tub or pool, then you’ve experienced fluid friction between your hand and the water. When a skydiver is falling toward Earth with a parachute, fluid friction between the parachute and the air slows the descent. Fluid pressure with the air is called air resistance. The faster or larger a moving object is, the greater is the fluid friction resisting its motion. The very large surface area of a parachute, for example, has greater air resistance than a skydiver’s body. Section Review: 1. Complete the sentences using: friction heat little motion resists a. The rubbing of one object against another is b. Friction is a force that motion. c. Every friction. . , or works against, that takes place on Earth is affected by d.Smooth surfaces such as ice provide very friction. e. Friction makes . 2.How is heat create with friction? 3. What are the four main types of friction? 4.Compare the strengths of static, sliding and rolling friction. 5 S ECTION 3.3 Projectile Motion L OREM O BJECTIVES I PSUM : 1. State LoremNewton’s ipsum dolor law of situniversal amet gravitation. 2. Consectetur Explain how adipisicing gravity affects elit,the sedmotion do eiusmod of tempor incididunt ut labore et dolore magna objects. aliqua. 3. Ut enim ad minim veniam, quis exercitation Vocabulary: ullamco laboris nisi ut aliquip ex commodo gravity consequat. law of universal gravitation 4. Duis aute irure dolor in in voluptate velit esse cillummotion dolore eu fugiat nulla pariatur. projectile Gravity is a force that acts between any two masses. ALL matter has gravity. Newton was the first one to suggest that gravity is universal and affects all objects in the universe. That’s why his law of gravity is called the law of universal gravitation. Universal gravitation means that the force that causes an apple to fall from a tree to the ground is the same force that causes the moon to keep moving around Earth. Universal gravitation also means that while Earth exerts a pull on you, you exert a pull on Earth. Anything that has mass, no matter how small, exerts gravity on other matter. Earth has a great amount of mass so the gravity of Earth itself is stronger than the gravity of anything on Earth. The force of Earth's gravity holds you on the ground. It is the pull gravity that gives weight to all matter on Earth. The moon has gravity. Since the moon has less mass than Earth, it has less gravity. Newton’s law also states that the strength of gravity between any two objects depends on two factors: the masses of the objects and the distance between them. • Objects with greater mass have a stronger force of gravity. For example, because Earth is so massive, it attracts you and your desk more strongly than you and your desk attract each other. That’s why you and the desk remain in place on the floor rather than moving toward one another. • Objects that are closer together have a stronger force of gravity. For example, the moon is closer to Earth than it is to the more massive sun, so the force of gravity is greater between the moon and Earth than between the moon and 6 the sun. That’s why the moon circles around Earth rather than the sun. When gravity pulls objects toward the ground, it causes them to accelerate. Acceleration due to gravity equals 9.8 m/s2. You might think that an object with greater mass would accelerate faster than an object with less mass. After all, its greater mass means that it is pulled by a stronger force of gravity. However, a more massive object accelerates at the same rate as a less massive object. The reason? The more massive object is harder to move because of its greater mass. As a result, it ends up moving at the same acceleration as the less massive object. Earth’s gravity also affects the acceleration of objects that start out moving horizontally, or parallel to the ground. A cannon shoots a cannon ball straight ahead, giving the ball horizontal motion. At the same time, gravity pulls the ball down toward the ground. The combination of an initial forward velocity and the downward vertical force of gravity causes the ball to follow a curved path. In the picture, part A shows how gravity acts on falling objects. Although their masses are different, the large and small balls fall at the same rate. Part B shows that the yellow ball is a projectile, following a curved path. Gravity causes objects to accelerate downward, whereas air resistance acts in the opposite direction and reduces acceleration. A curved path of a falling object is projectile motion. Air resistance and gravity are the only forces acting on a projectile. 7 Section Review: 1. What two factors affect gravity? 2.How is weight related to gravity? 3. Why does the moon have less gravity than Earth? 4.On Earth, how fast do object accelerate due to gravity? 5. Why don’t larger object fall faster? 6.Why does projectile motion follow a curved path? 8 S ECTION 3.4 Newton’s Laws L OREM O BJECTIVES I PSUM : 1. State LoremNewton’s ipsum dolor first sit lawamet of motion. 2. Consectetur Explain inertia's adipisicing relationship elit, sed to mass. do eiusmod tempor incididunt ut labore et dolore magna 3. State Newton’s second law of motion. aliqua. 4. Identify the relationship between acceleration 3. Ut enim ad minim veniam, quis exercitation and weight. ullamco laboris nisi ut aliquip ex commodo 5. State Newton’s third law of motion. consequat. 4. Duis aute irure dolor in in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Vocabulary: inertia About 300 years ago, a scientist name Isaac Newton had some ideas about forces and motion. His ideas became known as Newton’s Laws of Motion. According to Newton's first law of motion, the state of motion of an object does not change as long as the net force acting on the object is zero. An object in motion stays in motion. An object at rest stays at rest. Thus, unless an unbalanced force acts, an object at rest remains at rest, and an object in motion remains in motion with the same speed and direction. Suppose you see a soccer ball on the grass. It is at rest. The force of ground pushing up on its is balanced by the force of gravity pulling down on it. The soccer ball with keep its state of motion, which in this case is at rest. Suppose you kick the ball. You have unbalanced the forces with the force of your kick. The ball will be in motion. The ball will stay in motion until some other force comes along. In this case, a force called friction will stop the ball after a while. Newton's first law of motion is sometimes called the law of inertia (in ur shuh). Inertia is the tendency of an object to resist a change in its motion. In other words, neither the direction nor the speed of the object will change as long as the net force acting on it is zero. The inertia of an object depends on its mass. Objects with greater mass also have greater inertia. Think how hard it would be to push a big box full of books. Then think how easy it would be to push the box if it was empty. The full box is harder to move because it has greater mass and therefore greater inertia. To change the 9 motion of an object, inertia must be overcome by an unbalanced force acting on the object. According to Newton's second law of motion, the acceleration of an object is equal to the net force acting on it divided by the object's mass. You might think that actions and reactions would cancel each other out like balanced forces do. Balanced forces, which are also equal and opposite, cancel each other out because they act on the same object. Action and reaction forces, in contrast, act on different objects, so they don’t cancel each other out and, in fact, often result in motion. acceleration = Force / mass or Force = mass x acceleration Do you sometimes talk about weight and mass as if they were the same thing? Although related to each other, mass and weight are not the same. Weight is the force of gravity acting on an object. An object's weight is the product of the object's mass and acceleration due to gravity acting on it. According to Newton's third law of motion, whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object. This means that forces always act in pairs. These two forces are called action and reaction forces. For every action, there is an equal and opposite reaction. Action and reaction forces do not act on the same object. When a rocket is launched, burning fuel makes hot gases come out of the bottom of the rocket down against the Earth. The Earth pushes back on the rocket. The rocket takes off, moving away from Earth. The gases pushing down is the action. The Earth pushing up on the rocket is the reaction. 10 Section Review: 1. What is Newton’s first law of motion? 2.What is another name for the Newton’s first law of motion? 3. What is Newton’s second law of motion? 4.What does the “F” stand for in Newton’s second law? What does the “m” stand for in Newton’s second law? What does the “a” stand for in Newton’s second law? 5. Tori applies a force of 20 newtons to move a bookcase with a mass of 40 kg. What is the acceleration of the bookcase? 6.What is Newton’s third law of motion? 7. How do action and reaction forces interact? 11 S ECTION 3.5 Momentum L OREM O BJECTIVES I PSUM : 1. Lorem Describe ipsum momentum dolor sitand amet the conservation of momentum. 2. Consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Vocabulary: 3. Ut enim ad minim veniam, quis exercitation momentum ullamco laboris nisi ut aliquip ex commodo consequat. 4. Duis aute irure dolor in in voluptate velit esse cillum dolore eu fugiat nulla pariatur. What if a friend asked you to play catch with a bowling ball? Hopefully, you would refuse to play! A bowling ball would be too heavy to catch without risk of injury — assuming you could even throw it. That’s because a bowling ball has a lot of mass. This gives it a great deal of momentum. Momentum is a property of a moving object that makes the object hard to stop. It equals the object’s mass times its velocity. Momentum is the product of an object's mass and its velocity. An object with large momentum is hard to stop. An object has a large momentum if the product of its mass and velocity is large. The momentum for any object at rest is zero. You can calculate momentum by multiplying an object's mass (in kilograms) and its velocity (in meters per second). Momentum is measured in units of kilogram-meters per second. Momentum = mass x velocity This equation shows that momentum is directly related to both mass and velocity. An object has greater momentum if it has greater mass, greater velocity, or both. For example, a bowling ball has greater momentum than a softball when both are moving at the same velocity because the bowling ball has greater mass. However, a softball moving at a very high velocity — say, 100 miles an hour — would have greater momentum than a slow-rolling bowling ball. If an object isn’t moving at all, it has no momentum. That’s because its velocity is zero, and zero times anything is zero. 12 In a closed system, the loss of momentum of one object equals the gain in momentum of another object. When an action and reaction occur, momentum is transferred from one object to the other. However, the combined momentum of the objects remains the same. According to the law of conservation of momentum, if no net force acts on a system, then the total momentum of the system does not change. Section Review: 1. What is the momentum formula? 2.What is needed for an object to have a large momentum? 3. If you double the velocity of a moving object, how is its momentum affected? 4.What is the law of conservation of momentum? 5. Create a diagram to illustrate the transfer and conservation of momentum when a moving object collides with a stationary object. 13